The Cambridge University Botanic Garden Undergraduate Certificate of Higher Education

in Practical Horticulture and Plantsmanship

Dry Shade in the Temperate Garden

Problems and Potentials

Giulio Veronese Trainee Horticultural Technician

2014 – 2015

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The Cambridge University Botanic Garden Undergraduate Certificate of Higher Education

in Practical Horticulture and Plantsmanship

Dry Shade in the Temperate Garden

Problems and Potentials

Giulio Veronese Trainee Horticultural Technician

2014 – 2015

Limitation imply possibilities. A problem is a challenge.

Russell Page

© Giulio Veronese 2015

Printed: Le Cottage at the Cambridge University Botanic Garden, March 2015

Photographs by Abramo De Licio, expect where other source is given. Front cover photograph by Giulio Veronese, picturing dry shade planting at the Cambridge

University Botanic Garden. Diospyros lotus and Zanthoxylum americanum are underplanted with Danae racemosa, Galanthus nivalis ‘Flore Pleno’ x G. plicatus, Epimedium perralderianum and Ruscus aculeatus.

Digital modification of pictures by Giulio Veronese.

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Contents

Introduction 7 Woodlands: an Ecological Background 8 Morphological and Phenological Adaptations 12 Degrees of Shade 15 Dry Shade: Problems and Potentials 16 Horticultural Practices 17 The Case of the Woodland Garden at the Cambridge University Botanic Garden

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Conclusion 37 References 39 Acknowledgments 40

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Dry Shade in the Temperate Garden Problems and Potentials

Introduction

An ecological background is presented, arguing the temperate mixed woodlands as the key ecosystem where hardy shade-tolerant plants can be observed for use in dry shade gardening. Woodland microclimates and plant communities are mentioned, with particular attention to the eastern, drier part of the British Isles. Relevance is also given to the stratified arrangement of the woodlands, as a structural model for horticulture. Attention is then drawn into the botanical adaptations of plants that have evolved in shade, illustrating their morphological and phenological features and behaviours. Preference is given to European species for the examples. Focus is gradually moved to the garden environment and horticulture. Firstly, different degrees of shade are examined, their advantages and limitations discussed. Secondly, problems and potentials of dry shade are debated; the aim is to reveal the possibilities, expertise and enjoyment which can be derived from gardening under difficult conditions. The main body of the essay is then illustrated, with the description of horticultural practices. Emphasis is given to the relevance of soil and light improvement and use of the right plants, but opportune planning, planting out and establishment are also discussed. Tables are used listing hardy dry shade plants and their characteristics. The Woodland Garden at the Cambridge University Botanic Garden is taken as a case study. The present situation is discussed and described through tables and suggestions are provided. Finally, conclusions are given and the cornerstone thesis of the potential of dry shade is restated. Suggestions are also made on dry shade gardening as an increasingly important study in view of future water shortage scenario.

Dry shade planting at the Cambridge University Botanic Garden. Quercus trojana is underplanted with Carex pendula, Euphorbia

amygdaloides var. robbiae, Hedera helix and Mahonia x wagneri ‘Hasting Elegant’

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Woodlands: an ecological background

The woodland biome is where shade-bearing plants naturally grow. It has been described as “a small area of trees with an open canopy (often defined as having 40% canopy closure or less, i.e. 60% or more of the sky visible) such that plenty of light reaches the ground, encouraging other vegetation beneath the trees” [Thomas et al., 2007]. The amount and quality of available light varies according to the spacing and the density of the dominant canopy. The underlying species have adapted to very different habitats from stretches in deep shade to open, well-lit clearings. Temperature is more moderate than in the open meadows, cooler in summer and milder in winter. Because of the protection from the winds, the humidity at the ground layer is higher than above, even if the canopies intercept considerable amount of the rainfall. Consequently, the ground surface is quicker to warm up. Because of the consistent leaf fall, accumulation and the decomposition of plant remains, soils tend to be acidic, even if they overlie bed rock consisting of limestone, chalk or chalky boulder clay, with the pH decreasing towards the surface level.

Modulating effect of the vegetation cover. The fate of visible solar radiation (wavelengths 0.4-0.7 µm) received by a temperate forest on a clear day in summer. Most of the radiation absorbed at and beneath the canopy is used in the evaporation of water or in

transpiration. About 6-16% of visible radiation (mainly green 0.5 µm), some orange and red (0.6-0.75 µm) as well as about 70% of the infrared (0.75-1 µm) is reflected. At ground level the distribution of radiation is

spatially very uneven. Large canopy gaps may be well lit for part of the day. Beneath small gaps sunflecks may reach particular spots on the ground briefly (changing continuously during windy conditions). Other

places under the densest part of the canopy will receive only weak transmitted and diffuse (reflected) light, possibly reduced to 2% of full sunlight or less, and with spectral composition very low in the portion (0.4 and

0.7 µm) useful for photosynthesis, somewhat higher in the green (0.55 µm) and high in far-red (0.75 µm).

[Figure and legend reproduced from Packham et al., 1992]

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The term forest is also used in the common language for describing wooded landscapes. However, forest is usually reserved in ecology for a larger area presenting a closer, denser canopy (20% closure is accepted). The identification between forest and woodland is beyond the aim of this essay and only the term woodland will be used, especially in consideration of its relevance to horticulture. Temperate deciduous and broad-leaf woodlands will be focused on, excluding other wooded biomes such as northern coniferous forests, tropical rainforests or chaparrals that present vegetation not suitable for dry shade gardening in temperate climates. Temperate mixed deciduous woodlands occour in eastern North America, western and central Europe and eastern Asia, with pockets in South America and New Zealand. This results in most hardy, shade-loving species being native to the Northern Hemisphere, originating from three distinctive areas that were once part of a continuous belt and still present many closely related genera, dominated by trees such as Fagus, Quercus, Tilia, Fraxinus and Acer species [Elliott, 1998]. North American regions are a source of admirable woodlanders, especially spring geophytes such as Erythronium and Trillum species. Species from Himalaya and China are florally superb but need cool damp conditions and usually flower late, especially when from monsoon type climates. Japan harbours some of the best woodland dwellers, such as many liliaceous species and a range of magnificent shrubs. European woodlanders might lack the glamour and diversity of their exotic cousins, but are easier to grow and more adaptable to dry shade conditions. A great representation of useful spring ephemerals such as Helleborus, Pulmonaria and Primula species is offered from European woodlands and mountains.

Terrestrial biomes of the world [Lincoln et al., 1998. Digitally modified picture]

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The terms communities have been suggested in ecology for indicating assemblages of plants correlated with particular conditions of soil, moisture and shade. Studies have identified twenty-five different communities of woodland and scrub in Britain [Rodwell ed., 1991]. The eastern regions remain considerably drier than western and central ones, with the lowest areas of rainfall in East Anglia and some pockets of Lincolnshire and Yorkshire. Prolonged periods of drought can affect here even lowlands with poorly-draining, high water tables. Consequently, a lack of surface and atmospheric humidity in summer leads to prohibitive conditions for vast the component of field and ground floras, such as ferns and bryophytes. The analysis of the driest woodlands and their vegetation supplies inspiration for successful dry shade gardening where conditions are more difficult. Similarly, the observation of the woodlands’ natural structure is of great horticultural interest, as it affects the type of understorey which can be introduced in the dry shade garden. Temperate deciduous woodlands typically present vertical structure of four distinct layers.

VERTICAL STRUCTURE OF A TEMPERATE DECIDUOUS WOODLAND

Layer

Description

Canopy (at 5+ meter)

This is composed of the woodland climax species that dominate the underlying vegetation. Deciduous woodlands are distinctive in the British Isles, interspersed with evergreen and coniferous woodlands and scrubs. Climax species in Britain are typically Fagus, Quercus, Betula, Fraxinus, Pinus, Taxus baccata and Acer pseudoplatanus. On the canopies, epiphytes can be present, such as mosses and lichens and, in damp conditions, polypody ferns.

Understorey (at ˂ 5 meter)

This presents the underlying shrubs, including the woody climbers. The shrub layer is not always represented in the British woodlands, while it is characteristic in the temperate woodlands of Eastern Asia. Also in Britain there are only five species of native woody climbers, being Lonicera periclymenum, Hedera helix, Clematis vitalba, Rosa canina and Solanum dulcamara [Rackham, 2006]. Climbers are more representative of the temperate North-American woodlands, where are referred as vines.

Field (at ˂ 1 meter)

This largely consists of herbaceous plants and subshrubs, including brambles. Here are found a great variety of emblematic woodland herbs, ferns, grasses and sedges, such as Hyacinthoides non-scripta, Melica uniflora, Millium effusum, Dryopteris filix-mas, Carex pendula and C. sylvatica. The composition of the field layer is more developed where significant illumination reaches the woodland floor, typically in glades or newly pollarded or coppiced areas.

Ground (surface)

This is formed by mosses and liverworts (ground living bryophytes), lichens and algae. As most of these species require constant high moisture levels, the ground layer tends to be poorly developed or absent in drier woodlands. The ground layer plays an important role in the wildlife within the woodland, supporting a multitude of small invertebrates, insects and fungi.

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Stratification of shoot systems in the Wyre Forest, Shropshire. The oak trunks have the basal curve characteristic of individuals which have regrown from coppice stools, while the mosses and liverworts of the bryophyte layer are too small o depict. The most acid area is on the left, while near the centre is an aspect society in which bluebell, creeping soft-grass

and bracken follow each other in a seasonal sequence (drawn by P.R. Hobson).

VW, bilberry (Vaccinium myrtillus); CV, heather (Calluna vulgaris); Q, oak (Quercus petrea or Q. robur); OA, wood sorrel (Oxalis acetosella); CA, hazel (Corylus avellana); RF, bramble (Rubus fruticosus); HN, bluebell (Hyacinthoides non-scripta); HM, creeping soft-grass (Holcus mollis); PA, bracken (Pteridium

aquilinum); LP, honeysuckle (Lonicera periclymenum) growing over oak coppice regenerations; TS, wood sage (Teucrium scorodonia); EA, wood spurge (Euphorbia amygdaloides); B, silver birch (Betula pendula).

[Figure and legend reproduced from Packham et al., 1992]

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Morphological and Phenological Adaptations

In the drier temperate woodlands, the combined inconsistency of water and light can seriously affect the growth and survival of plants, especially within the lower layers. As few nutrients are available, plants need to maximise their energy efficiently, specialising their adaptations in accordance to the natural conditions.

Storage Organs and Vegetative Expansion Woodland plants often feature root systems adapted to store water and nutrients. This is common in the herbaceous field layer, which competes in a strenuous battle with the dominant species of the canopy layer. Fleshy rhizomes are modified underground stems that effectively store water, swelling in periods of rainfall and becoming dormant in periods of drought. They are characteristic of many tough woodlanders, such as Primula elatior, Anemone nemorosa and Ruscus aculeatus. Stolons are morphologically similar but creep overground, typifying the growth of Fragaria vesca, Poa nemoralis and several ferns. Bulbs are underground storage organs which are found in many iconic woodland dwellers, such as Allium ursinum, Hyacinthoides non-scripta and Galanthus nivalis. All these underground organs not only serve a storage function, but also permit the spread of plants. Many woodlanders tend to propagate themselves by vegetative means, creating clonal communities when conditions are favourable for encroachment and advance. Particularly representatives of the field layer are poor dispersers, often having no low seed production or mechanism for long-distance seed dispersal. The Potentilla sterilis is among the species that propagate themselves exclusively by vegetative means.

Vegetative multiplication: rhizome, corm, tuber, stolon, runner. Examples of natural cloning. a, b) separation of individual ramets in stoloniferous plant by death of intervening

connections; c, d) disintegration of rhizomatous plant, plan view, by death of proximal components; e-h) production of daughter corms on parent corm which subsequently rots; i-k) persistence of stem tuber after

death of remaining plant; l, m) production of daughter bulbs in parent bulb which subsequently rots.

[Figure and legend reproduced from Bell, 1991. Digitally modified picture]

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Foliage Adaptations Woodland flora is adapted for maximum photosynthetic production in a lower light intensity than plants of the open fields. An extreme adaptation to shade is displayed by sciophilous species, which depend on a degree of heavy shade that would kill most of other flora. However, sciophilous shade-loving behaviour is mainly found in tropical rainforests (typically in the Araceae family), while shade-tolerant plants characterize the ground and field layers of the temperate woodlands. Hardy shade plants usually present broader and thinner leaves than sun plants, often horizontally orientated, able to catch more sunlight and maximise the production of energy. Foliage of Lysichiton americanus, Diphylleia cymosa and Hosta species act like natural solar collectors. Shade plants also have larger chloroplasts with more chlorophyll in the leaves, making them more sensitive to sunlight; too-direct and prolonged exposure would destroy the intricate structure of chloroplasts and stunt their growth. Furthermore, stomata are larger and less dense than in sun-loving plants, as wind and exposure occour very little in the woodland and moisture retention is not critical. However, plants such as Actaea racemosa or some Geranium and fern species have more dissected, wind-resistant blades.

A comparison of photosynthetic structure and function in sun and shade plants. Major characteristics are described for cells, leaves and whole plants. Original drawing by S. A. Robinson

[Figure and legend reproduced from Plants in Action, 2014]

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Shade Evasion and Germination In the woodland sunlight is in limited supply for most of the growing season, with lowest levels in summer when the tree leaf out. Underlying species can evade shade by starting their annual life cycle early in the spring and then completely die back above ground in summer. This is the reason why temperate deciduous woodlands (particularly North American and European) are floristically at their best in the spring. This adaptation is facilitated by the microclimate within the woodland, where ground tends to be more sheltered and temperatures more stable than in farmland, so that underlying plants are less likely to get late frosts, while it is still hazardous for trees to break bud. Also light wavelength plays a role in germination. The pigment phytochrome in the seeds of many woodland plants inhibits germination in short-wave red light, which is cut out by the tree canopy. In this way dormancy is preventatively maintained under dark conditions. Shade evasion is typical of many spring ephemerals such as Hyacinthoides non-scripta, Anemone nemorosa, and Ranunculus ficaria. Wintergreen species also evade shade by keeping some green leaves in winter (usually a basal rosette) and starting their growth with spring conditions, such as in Oxalis acetosella, Lamiastrum galeobdolon and Primula species. In contrast, the understorey layer tends to remain evergreen, because of the necessity to make the most of available light at any time of the year; classic examples are Hedera helix and Ilex aquifolium.

Biotic Pollination and Distribution As on the woodland floor the wind speed is drastically reduced, the vast majority of underlying species are adapted to biotic pollination, especially by insects. The problem consists in successfully attracting and guiding pollinators through shady conditions. The commonest strategy is to flower before the canopy leafs out and closes, but other solutions also exists, typically the production of pale or white flowers as in Sanicula europaea, or fragrant flowers as in Teucrium scorodonia. Wind pollination sometimes occour too, typically in species flowering early in the season when wind speeds are relatively high, as in the case of Mercurialis perennis. Seed dispersal also depends upon animals, with fruits either covered in hooks as in Galium aparine and Circaea lutetiana, or edible and thus likely to be transported considerable distance as in many nut-producing species including Corylus, Prunus and Berberis species.

Parasitic Behaviour In the strenuous competition for light, some species have given up the battle completely and adapted to need no chlorophyll at all. They derive their substances from parasiting host plants instead, usually a specific or restricted range of woody species. Parasitic plants act either directly, as seen in Lathraea and the Orobanche species, or via mycorrhizal fungi, as in orchid such as Neottia nidus-avis and Corallorhiza trifida. The relationship between vascular plants and fungi in myco-heterotroph species is crucial in terms of woodland habitat and contribute a lot more to the environment that has been realised yet. Its study and introduction in horticulture will definitely be a decisive step for the future management of our woodland gardens.

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Degrees of Shade

In the woodlands, the movement of the canopy permits considerable illumination at ground level and provides humidity and shelter against winds. This condition of dappled, moist shade is ideal for many woodland dwellers and can be replicated on the edges of the shade garden or under a solitary, light branched tree. However, shade can derive not only from natural planting, but also from structural features and permanent barriers, as often seen in human landscapes and town gardens. The shade cast by walls, fences or overhanging roofs can be solid and permanent, so presenting extremely challenging conditions, comparable to those of the densest and driest woodlands. In a typical urban garden, a low level of humidity is found in the air and near the ground, with airless and cramped conditions. The soil can be starved, dusty and root-ridden; also the pH tends to be more alkaline than in the woodland, because of the legacy of building works. Depending to the conformation of the overhanging structures and the direction of the prevailing winds, rain-shadow effect can be also present. Together with the sun-to-shade continuum, the light intensity is a crucial factor. This is very complex and variable, depending on a multitude of elements such as time of the day, aspect, season, altitude and latitude. Most woodland plants are adapted for spring sun and summer shade; a good garden design will replicate this asset. In terms of latitude, equatorial and mountain regions have the highest solar intensity. In Britain, the sun shines most intensely from June through September and is stronger in the southern than in the northern regions. Thus, a plant that thrives in light shade in Aberdeenshire may require partial or full shade in Devon. Morning light is often regarded as one of the most beneficial rewards for plants. This setting can be delineated by dense canopy or a structure to the east and north. However, especially in frosty weather, morning shade represents a protective shroud for camellias and new spring growth, as sudden sunlight cause plants to thaw too quickly, which can lead to cell rupture. Midday and afternoon shade is a reward for both sun and shade loving plants, providing a beneficial cooling effect during the summer heat. It is also a precious condition for strong-scented white flowers which attracts pollinators, especially moths; also flower and foliage may be more brilliant when plants are shielded from intense sunlight. A full southern and windy exposure without midday or early afternoon shade can be unbearable for many woodlanders. However, afternoon sun is more forgiving in the northern areas than in the south, where the heat can be very high. West- and east-facing areas can host a good variety of plants, as good light will be available for part of the day. In contrast, north-facing spots are often sunless in winter and spring, with difficulties for the snow to melt and the soil to thaw. In south-facing borders alongside walls and fences, drought can be exacerbated, especially in the case of stone or brick walls, when the heat retained increases the loss of moisture from the soil.

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Dry Shade: Problems and Potentials

In Britain, where sun comes in short supply, shade is commonly seen as a drawback and detrimental to successful garden design. However, in warmer climates, shade represents a fundamental asset in the landscape. It gives shelter and furnishes cool soothing oasis on hot summer days. Effective use of plants in shade can transmit a sense of restfulness and subtleness that is simply not possible in open sunny positions. Textures, forms and slight colour differences become more charismatic elements of the design and their contrast accentuate the drama. Many deep and pastel tones are enhanced in shade and variegated and golden-leaved plants can be particularly effective, whereas they are prone to scorching in full sun. Also blossoms tend to last longer and sometimes smell better in shade, taking a luminous and fragrant beauty that draws the eye in to linger. Rightfully it has been said that “a garden without shade is a dull place indeed” [Paterson, 1981]. One of the most popular and accepted adages of horticulture is “the problem of dry shade”. In garden literature, the matter is often consigned to a peripheral, hopeless appendix in books on shade; in the best case a short list of standard choices is suggested. In our town gardens, dry shady corners are left as a dark desert, or, in the best case, with only sorrel or annual meadow grass growing. The overall impression is that of pacific resignation. However, it is the positive recognition of a site that gives a good garden. This consists of the ability of working with, rather than against, those existing characters which cannot be altered. The dry shade garden can be inexpensive and not labour-intensive after full establishment. The difficult conditions will keep out aggressive, light-demanding grasses and weeds, so helping maintaining the desired planting. The limiting factor can also be seen as an advantage in terms of design, cutting down choice and producing a better-balanced composition. Most importantly, dry shade can represent the opportunity for the gardener to constructively understand and challenge difficult planting conditions. The premise lies in the skillful replication of the ideal growing conditions for shade plants to grow, so permitting the most natural transplanting of woodland plants into a human site. The obligations of a rather restricted palette of plants can then force one to experiment with a surprisingly vast number of solutions and combinations, with enormous personal enjoyment and development. Indeed one of the great pleasures of gardening lies in the continuous and alternate exercise in enthusiasm and experimentation. Sometimes “a triumph of hope over experience”, as Marina Schinz once said. In this way what was seen as a lost battle is turned into a joint venture with the plants; and the adage of “the problem of dry shade” can be rephrased into “the potential of dry shade”.

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Horticultural Practices

Suggestions of relevant horticultural practices follow, with emphasis on light and soil improvement, as well as the use of the right plants. It should be considered though, that the ultimate skill consists in the ability of integrating a wide range of different approaches in the most possible sophisticated and specialised way.

Consideration of the Canopy Layer Provision of the best possible shade is one of the essential requirements for successful results. From the initial planting and establishment, it is crucial to accurately and progressively select the permanent overhead planting, allowing time and space to observe and control the variation of shade in the garden due to maturing trees and shrubs. Too often newly established gardens are overplanted in order to have an instant effect. When the advantage of working with a tabula rasa is given, the background of the trees and larger shrubs needs to be planned ahead. When – oppositely – mature species are present on site, great consideration must be spent for the correct judgment of their impact; the time to survey is winter, when the tree’s outline is evident and examinable from all the angles. A totally evergreen overplanting should be avoided, as its shade is non-negotiable. Broadleaf and coniferous evergreen trees cast dense shade all year round and therefore should be rarely used. Also species developing natural low branching or spreading, weeping and dark crowns are problematic to plant underneath. Deciduous trees allow more seasonal light than evergreen, but careful thought is needed when considering the use of climax species, as they tend to form a dense canopy and eventually outcompete earlier species in succession. Also, early-leafing species and cultivars will restrict early-season light, drastically reducing the palette of plants to cultivate below. Trees cast shade that help less moisture evaporates from the soil surface and less water vapor transpires from the foliage of the understory plants. However, this is counterbalanced by the bigger quantity of moisture absorbed by their roots. From autumn to mid-winter, trees are relatively dormant (particularly deciduous) but will absorb more moisture as soon as winter slides into spring. Fast growing trees take more water than slower species of equivalent age and size. A young birch will absorb far more than a similarly-aged oak. During periods of drought, trees will also take much of the available water, especially on superficial soil layers. As a rule, deep-rooted trees are better than shallow-rotted, as it results in less competition for the understory planting. Trees are engaged in a continuous and active competition for resources. Some grow taller, catching light and shading their competitors; other develop aggressive roots that capture available water and nutrients. Allelopathic species defend themselves by producing chemicals that inhibit germination, so eliminating competition for space and resources in short supply. Allelochemicals are usually washed out of fallen leaves and stems or, in a few cases, excreted from roots [Capon, 1990].

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TYPES OF OVERHEAD PLANTING

Characteristic

Description

Species

DENSE SHADE (Evergreen)

Broadleaf and coniferous evergreen species allow no bright respite for the plants underneath and their use should be avoided or limited.

Fraxinus excelsior, Ilex aquifolium, I. x altaclarensis, Kalmia latifolia, Quercus ilex, Magnolia grandiflora, Rhododendron spp., Tilia spp. and Ulmus spp.

DENSE SHADE (Deciduous)

There are either woodland climax species or large-leaved trees. Climax trees outcompete most of the vegetation in the succession of woodland development, casting a heavy shade which is difficult to plant underneath.

Acer campestre, A. Pseudoplatanus, Aesculus hippocastanum, Carpinus betulus, Fagus sylvatica, Ilex aquifolium, Magnolia acuminata, M. grandiflora, Malus spp. and cvv., Parrotia persica, Platanus x hispanica, Prunus cvv., Q. rubra and Sorbus domestica.

LIGHT SHADE (Decidous)

These colonise the woodland earlier than climax species and are eventually outshaded by them. They allow considerable light and plant life underneath and are therefore good choices for the dry shade garden.

Acer griseum, Amelanchier spp., Betula spp., Celtis spp., Cornus florida, Fraxinus excelsior, Ginkgo biloba, Gleditsia triacanthos, Koelreuteria paniculata, Laburnum anagroides, Larix decidua, Sophora japonica and Sorbus aucuparia.

EARLY-LEAFING (Deciduous)

Trees that leaf out early get a long growing season, but are exposed to late frosts that can damage their vessel elements. These species are among the most challenging to plant underneath and should be generally avoided.

Acer platanoides, Aesculus flava, A. hippocastanum, Betula pendula, Carpinus betulus, Crataegus monogyna, Fagus sylvatica, Laburnum anagroides, Malus spp. and cvv., Populus trichocarpa, Salix alba var. vitellina and Tillia x europea.

LATE-LEAFING (Deciduous)

These are better choices for the understorey planting, as will allow considerable light levels early in the season. With climate change, the onset of lead-out seems to have changed as temperatures have increased.

Acer griseum, Carya cordiformis, Catalpa bignonoides, Cladrastis lutea, Cornus nuttallii, Diospyros lotus, Gymnocladus dioicus, Magnolia acuminata, Meliosma veitchiorum, Picrasma quassioides, Sophora japonica and Sorbus vestita.

BEST SHADE Species that leaf out late in the season and form open, thin canopies. Small leaves are best as they permit a modicum of summer light and respect the overstory.

Acer griseum, Betula spp., Broussonetia papifyra, Cornus nuttallii, Genista aetnensis, G. patula, Gymnocladus dioicus, Koelreuteria paniculata, Meliosma veitchiorum and Sophora japonica.

WORST SHADE Species as planes and horse chestnut have often leaves of such size to damage the overlying plants, especially semi-resting herbaceous or seedlings.

Acer platanoides, Aesculus hippocastanum, Carpinus betulus, Fagus sylvatica and cvv., Fraxinus excelsior and Malus spp. and cvv.

SHALLOW-ROOTED

These develop root system effective in getting water and nutrients from the top soil layers as well as in effectively stabilizing the tree.

Acer japonicum, A. rubrum, Betula spp., Celtis occidentalis, Cornus spp. and cvv., Liquidambar styraciflua, Morus nigra, Populus deltoides, P. nigra and Salix spp.

DEEP- ROOTED

They get less superficial moisture and nutrients from the top soil layer, but can cause cracks in foundations and damage underground structures.

Abies alba, Acer platanoides, Carpinus betulus, Carya spp., Nyssa sylvatica, Pinus sylvestris, Populus spp., Quercus alba, Sassafras albidum and Sophora japonica.

ALLELOPATHIC SPECIES

Allelopathy is a relatively new subject for science; different effects still need to be measured and demonstrated within the species. Proportion of allelopathy also appears to be variable, depending upon site, species and single specimen.

Acacia spp., Acer saccharum, Ailanthus altissima, Celtis occidentalis, Eucalyptus spp., Juglans spp., Pinus spp., Platanus occidentalis, Populus deltoides, Prunus cornuta, P. serotina, some Quercus spp., Robinia pseudoacacia, Sassafras albidum.

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Planting out and Establishment In the dry shade, correct planting out and subsequent establishment are crucial. Newly planted specimens require regular watering at first. This will settle the soil around the roots and help the plants move their roots out of the compost into the soil. For more successful establishment, small plants are desirable, as transplanted mature ones need more time (proportionately to their size and age) to recover and develop drought tolerance. It is a good practice to plant in autumn after the first rains have begun, allowing plants to make the most of the winter and spring rainfalls and prepare for the dry season. However, half-hardy species often dislike autumn planting and spring planting is preferred instead, ideally after the last frosts. When poor air circulation and low light levels occour, leaves can stay damp for a long time, increasing the risk of fungal diseases. Good planting spacing is important, as well as a watering regime that keeps the foliage dry and retains moisture in the ground, such as hoses or drip irrigation rather than sprinklers. Most importantly, it is crucial to avoid overwatering. Plenty of water will give vegetative but floppy plants, developing weak roots with reduced drought tolerance. Slightly stressed plants are more tolerant of dry conditions. Better to water infrequently, but profoundly to encourage formation of deeper roots.

Though they cast less shade, most birches, including the paper birch (Betula papyrifera), have shallow

root systems that draw moisture from the soil and so restrict the supply to other plants.

The most water-efficient and consistent way

to water plants in a shade garden is through the use of a soaker hose.

Conceal the hose with mulch.

[Figures and legends reproduced from Rice, 2013]

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Light Improvement When shade derives from overhead planting, light can be improved by either felling or selective pruning. Tree removal might open up new planting possibilities and settings, but it is not always desirable, especially with old specimens. There is no easy compensation for the lifetime’s growth of a rare tree in any environment. Crown thinning, lifting and removal are often more appropriate solutions. Thinning consists in selective pruning of individual branches that will open space to the light to filter. On deciduous trees, thinning is best done in winter as it is easier to gauge shape and appropriate branches. On the other hand, lifting is best done in late summer and consists in the removal of the lower branches of the overhead canopy, which can also improve the appearance of mature specimens. However, wind exposure and direction need consideration as understory plants will be exposed as a result. Crown removal can be achieved by pollarding and coppicing. It involves the drastic cutting at ground level (coppicing) or up two to three meters (pollarding) to produce a crop of new shoots. These techniques are the basis of historic management of woodland in Britain [Rackham, 2006] and are suitable to many native species, including willow, chestnuts and limes. The drawback is that underlying plants will need to cope with a sudden increase of direct sunlight. When the growing conditions are very restrictive and the tree is not particularly valuable, pruning or reduction of tree roots can be done. Only superficial, smaller roots can be removed, at the cost of compromising the trees stability. Finally, light levels can be increased by reflection. Water reflects light, as well as the use of bright colours in hard-landscaping materials, such as walls, fences or containers. Some died-in-the-wool gardeners might turn up their nose, but the use of containers is sometimes advisable. They can be permanent features or short-term installations, hidden in dense groundcovers perhaps, so optimising planting medium and watering regime.

Soil Improvement Irrigation is perhaps the quickest way for turning dry shade into moist shade, but another effective method is found in the improvement of the soil. This should always be carried out when starting a dry shade planting from scratch. It is done by double digging the ground and incorporating humus-rich, water retentive matter, such as well-rotted manure or compost, before backfilling. Perforated polythene can be lined at the bottom of the planting hole. Alternatively, soil can be raised by creating a raised bed, by using the same principles. One foot is sufficient, as some shallow-rooted trees such as Betula and some Quercus species are sensitive to changes in soil depth. Mulching consists in the application of a layer of usually organic matter on the soil surface, and can be seen as the creation of natural woodland conditions in a horticultural setting. Composted pine bark and leaf mould are among the best materials for woodland plants. Mulching is an ideal method of soil improvement, which helps retaining moisture, by both slowing the evaporation from the ground and stopping rainwater from soaking through. It will also vastly improve a garden in looks, performance and maintenance.

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Removing the lower branches on this fir, Picea, will brighten the planting area below and allow a wider range of plants to be grown.

Hostas are ideal sculptural container plants.

This study in blue combines a potted hosta with an azure urn in the corner of a sheltered patio.

Even a low stone wall can create enough extra depth of soil to encourage hostas,

pulmonarias, ferns, and other shade-loving plants to grown more luxuriantly.

Mulch really is a necessity for dry, shady beds.

The ideal, however, is to gradually “fill in the blanks” so that you are looking at plants rather than exposed bark mulch.

[Figures and legends reproduced from Rice, 2013]

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Use of the Right Plants Dry shade supports comparatively fewer plants than damp shade, with a more restricted palette of species available. Where conditions are constraining and difficult to improve, it is advisable to go for simple planting schemes. Only the most reliable bullies will thrive, and some layers might be lost in the vertical structure of the border. Plants can be concentrated singularly or in groups, creating bold statements or foreground features. Blankets of groundcover, masses of stout shrubs or even a scarce display of specimen species in a freshly-mulched bed are all possible options. Shrubs sometimes considered charmless and utilitarian, such as Aucuba, Berberis, Buxus, Ilex or Taxus can be effectively used in the dry shade border, their leaves contrasting in colour and texture with those of Epimedium, Helleborus and Hosta species. The gardens at Rousham are great examples of these solutions. Where conditions are easier to cope with, more complex palettes and intricacies of plants can be considered, with dramatic combinations of textures, colours and shapes. A classic association is between the foliage of hostas and ferns, but many juxtapositions are possible, for instance small-leaved box and voluminous ground-cover, or upright and dwarf conifers. The greatest success perhaps derives from the introduction of bulbous and low herbaceous perennials, which represent the ecological foundation of the woodland floor and add interest early in the season when little else is around. As a rule, light colours should always be used, as they stand out in shade. Deep tones tend to recede unless set off by a brighter contrasting color; the luminous flowering stalks of Adenophora lillifolia will be more effective if coupled with the blooms of bright varieties of hosta or perennial foxglove. The recourse to some evergreens in the understorey layer is also advisable, as it provides all-year-round structure and interest. Variegated selections can be considered and introduced, as they can enliven the darkest corners. However, in dense shade they might struggle photosynthesizing and need removal of all-green shoots, so avoiding reversion. Finally, a certain element of mystery and tension should always be pursued in the composition. The most attractive woodland gardens are naturally crossed with meandering pathways, winding through emerging plants and hidden retreats. Such a design helps enlarge the perception of the area, while secretly revealing the authentic, sinister characters of the dry shade garden. The inclusion of sitting areas will also give occasion for resting and immerging in the elements of the composition, with the additional shelter provided by shade.

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Ground cloaking Epimedium x perralchicum ‘Fröhnleiten’ efficiently smothers weeds around a

specimen hosta and phormium in a shady yard.

The small, even foliage and neat habit of boxwoods such as ‘Green Velvet’ contrast well with the texture

of larger-leaved epimediums and hostas.

The range of perennial suitable for dry and shady places offer a thoughtful choice of leaf shapes and

colours, and some also feature bright, graceful flowers..

Lamium maculatum flowers cheerfully around a hosta and helps camouflage the fading foliage

of daffodils that bloomed earlier in spring.

[Figures and legends reproduced from Rice, 2013]

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Plants for a dry shady island bed, approx 6 x 4 m. Plan and elevation by Beth Chatto. This design is for an area which is in shade, or partial shade, for most of the day.

Shade may be cast by tall buildings or from broadleafed trees further away.

1. Taxus baccata ‘Fastigiata; 2. Euphorbia robbiae; 3. Acanthus mollis or A. spinosus; 4. Iris foetidissima ‘Variegata’; 5. Hedera helix ‘Manda’s Crested’; 6. Tellima grandiflora ‘Purpurea’; 7. Alchemilla mollis; 8. Pulmonaria saccherata; 9. Cyclamen neapolitanum; 10. Epimedium x ‘Sulphureum’; 11. Thalictrum

aquilegifolium; 12. Polystichum setiferum ‘Acutilobum’; 13. Millium effusum ‘Aureum’; 14. Vinca minor ‘Aureovariegata’. 15. Lonicera nitida ‘Baggesen’s Gold’; 16. Brunnera macrophylla; 17. Hosta lancifolia; 18. Lamium maculatum ‘Roseum’; 19. Pulmonaria rubra; 20. Hosta sieboldiana; 21. Helleborus corsicus; 22. Iris foetidissima; 23. Liriope muscari; 24. Ajuga reptans ‘Burgundy Glow’; 25. Euphorbia cyparissias;

26. Tiarella cordifolia; 27. Ajuga reptans ‘Atropurpurea’; 28. Saxifraga x urbium ‘Aureo Punctata’; 29. Pulmonaria azurea ‘Munstead Blue’; 30. Hedera helix ‘Lutzii’; 31. Bergenia cordifolia ‘Purpurea’;

32. Veronica prostata ‘Trehane’; 33. Iris innominata hybrids; 34. Vinca minor ‘Bowles’ Variety’; 35. Arum italicum ‘Pictum’ (of gardens); 36. Apera arundinacea; 37. Valeriana phu ‘Aurea’; 38. Polygonatum x

hybridum; 39. Digitalis ciliata; 40. Geranium endressii; 41. Viola labradorica.

[Figures and legend reproduced from Chatto, 1992]

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HARDY PLANTS FOR THE TEMPERATE DRY SHADE GARDEN

Type

Species

Horticultural Interest

SHRUBS Aucuba japonica For an unfashionable shrub, Aucuba has some real excellent qualities. It is a bold, resilient evergreen, with an extensive range of available cultivars, providing a variety of foliage form and pattern, as well as fruit color. A. japonica ‘Salicifolia’ is a rather dashing selection.

Berberis

Among the 600 species, there are adaptable and resilient selections for the dry shade garden. Evergreen species can provide structure, while deciduous species often have fantastic autumn colour (if some illumination is given). Recommendations are B. darwinii, B. julianae, B. sanguinea, B. x stenophylla, B. verruculosa and B. wilsoniae.

Buxus

Box are not only unmatched plants for structural planting, topiary and hedges; they can be important stalwarts for the dry shade garden. Generally speaking, slow-growing species are better in terms of management and moisture intake. B. microphylla and B. sempervires are the most suitable species, with many attractive (even variegated) selections.

Danae racemosa This is a most resilient and dependable shrub, growing in almost any possible conditions apart from dry soil in full sun. Good as ground cover as well as a background plant for more flamboyant planting.

Daphne laureola The spurge laurel is a superb British native for the calcicolous woodland garden. Its tolerance of poor, heavy soils makes it an incredibly rewarding plant for demanding conditions. D. pontica is suitable for dry shade too, but only in milder regions of the British Isles.

Euonymus Most are deciduous, but the more common evergreen species are mainly from shaded habitats. There are a great number of available selections with a range of growth habit, size and colouration. Dependable species are E. fortunei, E. kiautschovicus, E. japonicus and their numerous cultivars. E. fortunei ‘Silver Queen’ can be used as a climber, perhaps enveloping a tree trunk or in combination with ivy.

Ilex

Hollies are true woodland dwellers, which have been once memorably described as “sombrely cheerful’ [Jekyll, 2011]. They need careful establishment in dry conditions, but compensate with reliable growth and attractive, evergreen foliage and berries. The best species are Ilex aquifolium, I. cornuta and I. opaca.

Jasminum nudiflorum

Winter jasmine is a good plant not only for the winter garden, but for borders in dry shade too. Furthermore, it tolerates atmospheric pollution, so is useful for planting in town gardens. However, Jasminum nudiflorum will be truly comfortable only if the flowers can get sunlight.

Mahonia Mahonia might need extra water and mulching to establish, but will reward this hard work with great structure and coloration. Some species develop volcanic tones in cold conditions. Mahonia aquifolium is a suckering shrub good for dry shade, with many available cultivars and hybrids.

Paeonia Peonies are notoriously fussy plants but worth experimenting with. In her dry garden, Beth Chatto had success with P. lutea var. ludlowii and P. delawaii, with specimens developing attractive foliage but lacking the flowering achievable in sunnier spots [Chatto, 1978].

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SHRUBS Ruscus These are incomparably tough performers in dry shade, featuring recognisable and distinctive characters. Useful when evergreen structure, winter interest and low maintenance are desired. Not all species are hardy, but R. aculeatus and the intriguing R. hypoglossum can be grown in the British Isles.

Sarcococca Christmas box are adaptable evergreens with a powerfully scented winter blossom. They cope well with urban pollution and a wide range of soils, from chalky to acid. All the species do well in dry shade, but S. confusa, S. hookeriana and S. ruscifolia are particularly lovely. S. hookeriana var. digyna has extra appeal, with purple stems and excellent structure.

Sambucus Elders are common plants for damp shade situations, but their undeniable value makes them worth trying in dry shade, as long as proper care is taken during establishment. The cautious gardener might express concern, but Sambucus nigra ‘Pulverulenta’ can be a real addition in those areas of the dry shade border receiving more sunlight.

Symphoricarpos Sometimes considered utilitarian and charmless shrubs, snowberries actually show a degree of adaptability, tenacity and horticultural potential rarely equalled in this list. The distinctive, pulpy berries remain in good conditions for a long time and are good for the wildlife. Furthermore, the many cultivars give a chance to create attractive combinations and can be also used as specimen shrubs. Selections of Symphoricarpos albus, S. x doorenbosii and S. orbiculatus are all trustful choices.

Taxus x media Simply the most adaptable yew for dry shady situations. It is the cross between T. baccata and T. cuspidata, bringing together the vigour of the former with the additional hardiness of the latter. The numerous selections are all dioecious, so both male and female plants are necessary for fruit production.

Viburnum Among its over 150 versatile species, Viburnum offers some that thrive in dry shade. V. acerifolium, V. lantana, V. prunifolium and their cultivars are dependable choices. V. opulus is seen in cultivation and can be added to the dry shade garden, but need more moisture in the soil and in the air.

CLIMBERS Clematis They can be used in dry shade situations, but will need supplementary moisture. C. montana and all named hybrids can grow in deep shade, but its flowers need to reach towards the sun. C. x jouiniana ‘Praecox’ is perhaps the best species for dry shade, making a very attractive climber, perhaps to try in combination with Parthenocissus tricupidata.

x Fatshedera lizei

This is an odd, intriguing hybrid between Hedera hibernica and Fatsia japonica. Admittedly, it lacks the tenacity and resiliency to drought of ivy, but the leaves are a feature on their own and can give strong textural contrast in combination with other foliage plants. ‘Annemieke’ is a good variegated selection.

Hedera The ability of ivy to tolerate almost impossible conditions makes them invaluable choices for the dry shade garden. However, some water and encouragement might be need for the first year or two. H. algeriensis is a colorful large-leaved form, but not as hardy as H. helix.

Hydrangea anomala subsp. petiolaris

This is a supreme self-clinging flowering species from eastern Asia. It can be grown equally well on walls and on the base of woody plants, eventually reaching the top of mature trees. A drawback is that it can be slow to establish.

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CLIMBERS Lonicera Honeysuckles are great climbers for shady walls, but their additional ability to withstand dry conditions is not always fully recognised. Aphids will be less troublesome, but drought-stressed plants can get powdery mildew quite easily. This can be prevented by mulching at the base of the plants. Lonicera periclymenum and L. x tellmanniana and their selections will make good climbers once established.

Parthenocissus tricupidata

The glossy leaves shine in the gloom, and turn into a fiery flame in autumn. Foliage is also quite variable in size, which adds extra appeal. It can also be grown as a groundcover. In difficult conditions, it is much more resilient than its popular cousin P. quinquefolia (Virginia creeper).

HERBACEOUS PERENNIALS

Acanthus This is vigorous to the point that it can scare some people off. In truth, it can spread aggressively when too happy, but behaves commendably in constraining situations. The main drawback consists of the difficulty to remove unwanted plants from a site, as the chunky rhizomes will resprout for a long time. Also, plants are often prone to mildew. A. mollis and A. spinosus are both suitable dry shade plants. A. mollis Latifolius Group has greater tolerance as its larger foliage can make the most of the little light available.

Alchemilla mollis Lady’s Mantle’s ability to tolerate situations in dry shade is well-known. It is a true garden classic, giving downy pleated leaves and sprays of yellowish green flowers. Its colours can be used for softening harsher tones of orange and reds in the front of the border.

Anemanthele lessoniana

The Pheasant’s Tail Grass is a semi-evergreen ornamental grass, featuring a soft, arching habit which can soften effectively an herbaceous border. It combines well among shrub or lower groundcovers for textural contrast.

Arum

Often considered pernicious weeds in gardens, lords and ladies are more controllable in the unfriendly situation of dry shade. They are charismatic multi-season plants, with interesting foliage, flowers and fruits. Arum italicum and A. maculatum are the most common choices. A. maculatum ‘Pictum’ is a favourite, highly attractive.

Aruncus dioicus The giant goat’s beard is a hardy shrub from North America with the tendency to expand and form a beautiful dense screen in its maturity. For better establishment it need extra moisture and might appear a bit gangly for the first year or two. Excellent plant for the woodland garden.

Aspidistra elatior This old-fashioned Victorian plant has recently been relegated to kitchen windmills, but can be a good workhorse for dry shade in the milder gardens of the country. It is a foliage plant par excellence and its leaves have once been compared to “Agamemnon’s sword” [Orwell, 2011]. An increasing number of spotted and variegated cultivars have become available, mostly from Japan.

Bergenia Bergenia show perfect adaptations to shade with chunky rhizomes and large foliage. They are also among the few plants that will flower consistently in dry shade. Best to plant evergreen forms with broad leaves to catch sunlight. Many cultivars are available, turning the leaves in through shades of carmine, rose and red, depending on the exposure.

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HERBACEOUS PERENNIALS

Brunnera macrophylla

Another plant that has seen an increase in popularity among breeders and gardeners. Among the new-entries, there are attractive silver-leaved and bicoloured flowering forms, offering new possible combinations and juxtapositions. ‘Jack Frost’ is perhaps the most common selection, which can be useful as a bright edging plant.

Calamagrostis brachytricha

This is an attractive grass featuring an upright clump of plumes. Tolerates dry shady conditions well but some extra moisture might be desirable in periods of drought.

Carex Sedges are versatile and dependable choices, as their notorious aggressiveness is naturally constrained in dry shade. Variegated, golden-leaved forms can be planted for enlightening a carpet of deep-toned groundcovers, such as Hedera or Pachysandra. C. pendula offers a wide range of attractive cultivars, mostly from Germany and the Netherlands.

Convallaria majalis

This is a grand woodland colonizer, showing perfect adaptations to dry shade with chunky creeping rhizomes and white showy flowers that stand out in the dark. Various cultivars are available, including variegated, rose-colored and double-flowering ones.

Euphorbia amygdaloides var. robbiae

Its spiral arrangement of the foliage helps to maximize all the possible light available. Also the stout rhizomes ensure great adaptability and resilience. The dark, sinister appearance makes it a perfect ally for bright Lamium or Epimedium forms.

Galium odoratum Bright and endearing plant that can be used effectively at the front edge of a border, perhaps in combination with bolder characters, such as Helleborus and Heuchera species and cultivars. An additional bonus is the strong smell that derives from the flowers, especially when they are protected from direct exposure.

Helleborus

All hellebores like shade, but some species are best adapted for dry shade conditions. H. foetidus has a dark and lustrous appearance, which can perhaps be enlivened by growing the brighter, marbling H. lividus. Also the slender divided leaves of H. foetidus make it an ideal companion for heucheras and hostas. H. corsicus is possibly the best species for dry shade, but might need some shelter.

Heuchera Heucheras has become increasingly popular in gardens with many new selections recently introduced into the market. Their somewhat garish and colorful appearance is not always appreciated, but they are undeniably useful for adding drama and movement in the dry shade border. Cultivars deriving from the species H. maxima, H. pubescens and H. villosa show characteristic suitable for dry shady conditions.

Hosta Hostas are among the absolutely indispensable perennials for damp, as well as dry shade. However, different species have different shade requirements; generally blue-leaved hostas require deeper shade, while those with bright variegations can tolerate more sun. Hosta sieboldiana is a classic, showing great ornamental character and resistance to snails.

Luzula sylvatica Unlike the related Juncus, Luzula can withstand dry conditions. Their rhizomatous roots help them spread but not too aggressively. A great number of cultivars are available, permitting striking contrasts with the foliage of hostas and ferns.

Pachyphragma macrophylla

Start the season very early, sending up bunches of white cress-like flowers followed by shining round leaves. Good for filling room among dormant hostas.

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HERBACEOUS PERENNIALS

Paris Trillium are fantastic woodlanders but don’t cope with too dry conditions. Their relative Paris could be attempted instead, if a damp start is given. P. quadrifolia is a native of alkaline British woodlands; P. polyphylla is a Japanese cousin, highly ornamental and worth growing in our woodland gardens.

Polygonatum multiflorum

Solomon’s seal is an attractive workhorse, able to colonise large patches quickly due to its stout rhizomes. The abundant pendant white flowers can enliven the gloomiest areas under difficult conditions. Very good when a natural, woodland style of planting is wanted.

Pulmonaria The Boraginaceae family contains many good plants for dry shade, such as Brunnera, Pentaglottis, Symphythum and Trachystemon. Pulmonaria is another honorable representative, with a range of suitable species and cultivars. Believed to be suitable for moist shade only, pulmonarias are also good choices for dry conditions. It is advisable to get mildew-resistant varieties.

Rohdea japonica This is an uncommon plant in cultivation, but can be a good addition to mild woodland gardens due to its exotic and intriguing appearance. Good to associate with plants having more dissected leaves, such as maples and ferns. A softer but effective combination could be with plants from similar conditions, such as aspidistras. Cultivars are available, but generally without the hardiness and reliability of the true species japonica.

Smilacina racemosa

The false spikenard is closely related to Polygonatum, showing the same consistent degrees of adaptability and tenacity. However, it is larger than its cousin and produces dense, foamy spikes of fragrant flowers in late spring. Perfect as an understorey plant under the deep shade of trees, it can be a real showstopper.

Viola riviniana This is one of the species which can grow in dry shade. It is a good, efficient choice, able to cover densely and quickly a large area. Purpurea Group has extra appeal with purple leaves.

GROUND-COVERS

Epimedium Epimediums are among the best dry shade groundcovers. Get the most vigorous cultivars. They benefit from a good application of mulch. Plants tend to flower beneath the previous year’s foliage, so it is advisable to remove the old leaves early in spring, when the last frosts are gone. Good selections exist, mainly from species perralderianum, pinnatum, x versicolor and x warleyense.

Fragaria Ornamental strawberries are often underrated groundcovers. An incredible variety of species and hybrids is available. Due to their long stolons, they can quickly colonise a difficult patch in dry shade. Inspiration can also come from their cousins Potentilla.

Gaultheria shallon

Very good plant or dry shade. Great adaptation is shown by the combined effect of suckering habit and leathery foliage, which loses little moisture through transpiration.

Geranium Some of the most vigorous species and cultivars can self-seed in easier parts of the garden and get difficult to control. Many species are also interbreedable. They can be cut down to the ground after flowering. G. macrorrhizum is possibly the best adapted for dry shade; it is a hard working plant, featuring attractive deeply lobed leaves that are aromatic and turn orange in autumn. Other good herbaceous species are endressii, nodosum, phaeum and versicolor.

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GROUND-COVERS

Hedera

Ivies show a degree of adaptability and suitability rarely equalled from other dry shade great performers. They develop flowers only on the higher climbing parts; almost no flowers and fruits are produced on ground-covering plants. The north African species algeriensis is less hardy than helix, but still a good ornamental choice for the milder gardens of the country. The large-leaved species colchica is also a good choice.

Lamium

They are useful for adding light in the border, with silvery foliage and bright-coloured white, yellow and pink flowers. Best selections are derived from the species galeobdon and maculatum. Plants can be invasive if unchecked.

Liriope

One of the finest plants for dry shade, making bold statements with their dark, almost sinister appearance. They might be slow in their first years, but become tough and when established. The glossy grass-like foliage can associate well with Epimedium and fern species. The most popular species is muscari, but good selections come from spicata and exiliflora too.

Ophiopogon

Can be mistaken for Liriope, but has nodding flowers rather than erect. The most famous type is O. planiscapus ‘Nigrescens’, but many other selections also exist, with a surprisingly diverse range of, flowering colour, variegation and habit. Due to its rhizomatous roots, plants are easy to propagate quickly.

Pachysandra terminalis

Often described as a boring plant, this is actually a functional plant where an effective, lush green carpet is wanted. The fine leaves can associate well with those of Hosta or ferns. P. terminalis and its cultivars are the best for dry shade conditions; variegated forms exist.

Pentaglottis sempervirens

This is the forget-me-not and brunnera of dry shade. White, bicoloured and variegated selections also exist. Plants can be invasive when conditions are easier. It will also come back from roots left in the ground.

Rubus They are unquestionably among the great over-achievers. They are great plants for the woodland garden, naturally adapted to colonise the border. Most vigorous species can be overenthusiastic and need control. Rubus tricolor one of the best smoothers to grow under the dense shade of trees, with unusually large fruits.

Symphytum Another spreader that is sometimes invasive in happier conditions but better-behaved in dry shade. Plants are incredibly hardy. Its foliage can be used for making liquid feed. S. orientale is very attractive, featuring clusters of pendent white flowers.

Trachystemon orientalis

This elephantine plant is vigorous to the point that can overrun other less aggressive plants in the border. However, it can be used effectively as a weed-smoother, especially when large, low-maintenance coverage is needed. The combination of lush foliage and sky-blue, borage-like flowers will also add splendor to a gloomier corner. It needs water in the first years to get fully established.

Vinca

Vinca minor and V. major, the lesser and great periwinkles, are reliable groundcovers. V. minor is hardier and more vigorous and can spread if not checked. However, they mainly spread by vegetative means, so control can be done by cutting their stolons.

Waldsteinia

Originating from European, eastern-Asian and North American woodlands, Waldstenia show a great range of variety. They are similar in appearance to other low woodlanders of the Rosaceae family such as Fragaria and Potentilla, but lack stolons and have typically more rounded foliage. Good species for dry shade are W. fragarioides and W. ternata.

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FERNS Athyrium niponicum var. pictum

This is one a magnificent fern, featuring metallic-looking arching fronds on burgundy stems. It is an outstanding choice for the dry shade garden, and its character can contrast well with different-looking plants such as hostas and variegated grasses.

Dryopteris

Dryopteris are British natives that will tolerate dry shade once established. They are apparently untroubled with a heavy lack of water or light. Moisture is actually only needed for the prothallus reproduction stage, with plants thriving in dry, even root-cramped conditions. D. filix-mas is possibly the best dry-shade performer, rewarding the gardener with combined dependability and a naturalistic appearance. D. affinis and D. marginalis are also suitable species for dry shade.

Polypodium vulgare

Another showy and versatile fern. It can be grown along walls (and directly into their crevices), or as a real woodlander around trunks of deciduous trees. In damper areas of the country, it can spread but rarely becomes invasive. There are a number of reliable cultivated forms, all worth testing in the dry shade or woodland garden.

Polystichum This is a vast genus of cosmopolitan distribution, comprehending a few species suitable for dry shady conditions. P. setiferum is drought resistant, featuring good textural characters that can go well with other ferns such as Adiantum and Polypodium species. P. aculeatus is good too.

BULBS Anemone nemorosa

Wood anemones are among the most iconic spring flowering bulbs of the British woodlands, where they spread successfully with their small, skinny rhizomes. They complete their full growing circle in a matter of few weeks before the dominant trees leaf out. A. nemorosa ‘Robinsoniana’ is a real jewel, with in pale lavender blue flowers, with golden centred stamens.

Chinodoxa Dwarf bulbs related to Scilla but lacking the aggressiveness of their cousins. They display beautiful starry blue and white flowers early in spring. There is sometimes confusion in the market between the species. C. luciliae, C. forbesii and C. siehei have been sold under the same name but are distinct species. Fortunately, they are all good in dry shade.

Cyclamen

Cyclamens are beautiful dwellers of the European woodlands. They have adapted to dry shade by developing flat, slowly expanding tubers able to store water and nutrients. The two species that do well in dry shade are C. hederifolium and C. coum. Their bright flowers and patterned or silvery leaves can enliven the darkest woodland floor.

Eranthis Eranthis is a small genus of eight species in the Ranunculaceae family. The species familiar to British horticulture is E. hyemalis, featuring cup-shaped yellow flowers in early spring. Selections are available, mostly from Germany.

Galanthus Snowdrops are hardy and easy to grow in dry shade. Some extra moisture is desirable during periods of drought in summer, so avoiding the bulbs to dry out. Hundreds of different snowdrops are now available, but resilient selections are best, such as ‘Atkinsii’, ‘Dionysus’, ‘Zwanenburg’ and ‘S, Arnott’.

Narcissus Daffodils are not only unmatchable bulbs for open meadows and spring bedding schemes; they are also good performers in the dry shade border. Recommended selections are ‘Baby Moon’, ‘February Gold’, ‘Ice Wings’, ‘Jack Snipe’, ‘Jenny’ and ‘Sun Disc’.

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ANNUALS AND BIENNIALS

Corydalis sempervirens

This Native American should be seen more often in the British gardens. It develops bluish green rosette, bearing attractive bicoloured flowers in spring. Corydalis aurea is suitable for dry shade too, but very difficult to acquire.

Digitalis purpurea

Foxgloves are among the most sparkling and useful plants for a dry shade gardening, their colorful spikes enlivening the gloomiest spots. Different selections cross-pollinate very easily and it is advisable to buy new seeds each year if plants are treated as annuals.

Lunaria annua Honesty is a classic cottage garden plant, which can do well in shade too. It just needs a good start in sunny conditions, before to be transplanted and left in shade, where it will self-seed less profusely than in open, bright borders. Selections are available featuring white to purple flowers or variegated leaves.

PARASITES Lathraea Toothworts are a small interesting genus of holoparasitic protocarnivorous plants. Among the seven species, L. squamaria and L. clandestina are worth trying in the garden, the former growing on the roots of alder, beech and hazel, the latter on alder, poplar and willow (but also occasionally on maples and rhododendrons). Even if toothworts show distinctive characteristics to be suitable host plants growing in gardens, they are likely to remain curiosities rather than practical solutions, as establishment is tricky and there are very few commercial sources available [la Croix, 1978].

Orobanche Broomrapes are much numerous genus of over 200 species spread in the Northern Hemisphere, flowering usually from mid winter to late spring. Many species could be attempted, depending on the host plant. O. hederae parasites on ivy and is native to the British Isles, where grows in woodlands and hedgerows. It bears spikes of cream orchid-like flowers, which are veined with purple.

Dry shade can be challenging, but will still allow for stinking plant associations.

Here, the bold foliage of Helleborus x hybridus is surrounded by a miniature replica in Galium odoratum, which brings the added benefit of profuse starry blossom.

[Figure and legend reproduced from Rice, 2013]

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The Case of the Woodland Garden at the Cambridge University Botanic Garden

As most of the woodland plants suitable for our gardens are from Northern latitudes and high altitudes, successful shade gardening is difficult to achieve in the southern regions of Britain. Moreover, the South-East witnesses very dry conditions, as the Gulf Stream sheds most of its load over the western and central areas. Even when evaporation and convection is produced from Europe in late summer with consequent rainfall from west, winter rains occour along the coasts but the inland areas are missed. Cambridge in particular is one of the driest areas, the average rainfall being 570 mm p/a. The climate is characterized by prolonged periods of bright weather, light evenly-spread rainfall and strong drying winds. Hence the suggestion of the Woodland Garden at the Cambridge University Botanic Garden representing a fascinating proving ground for planting in dry shade. The site was firstly established in the mid-nineteenth century and covers today an area just under two acres bordering the Stream Garden, the Bambooserie, the Lake and the West and Main Walk. Unfortunately, much of the original planting and peripheral shelterbelt were damaged by the gale of 1987 and the storm of 2002. In 1998 the former curator Tim Upson wanted to give a phyto-geographical theme, with display of plants originating in North American, European and Eastern Asian woodlands. Unfortunately this arrangement never quite worked, as trees from different regions of the world were already present, so leading to difficult interpretation. The direction today is to establish a purely ornamental woodland garden, displaying solid backbone planting integrated by specimen planting.

The Woodland Garden at CUBG. Diagram showing some key-facts and natural conditions.

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THE WOODLAND GARDEN AT THE CAMBRIDGE UNIVERSITY BOTANIC GARDEN

Facts and Problems

Suggested Solutions

Lack or disproportion of representation of woodland layers Over the last few decades the planting within Woodland Garden has been seriously damaged by the southerly gale and storms. At the present day, its structure is characterized by an imbalance of tree canopy as well as a relatively poor representation of the overstorey layer. Climbers are also almost totally absent, and their introduction is problematic because of the lack of adequately supporting trees.

Integrated, long-term planting The existent large trees have to be saved and managed carefully, but no further dense canopy is desirable. Preference should be given for the planting of small- and medium-sized specimen trees. The introduction of climbers on the stronger trees is a fascinating hypothesis; suitable species can be Hydrangea hydrangeoides, Lonicera and Clematis spp. A suggestion could perhaps be to grow only native European species.

Absence of a declared planting theme As the phyto-geographical theme proved to be not successful, a direction is needed for the future planting of the Woodland Garden. Existing strong nucleus are represented from several species of Styracaceae, Araliaceae and Rutaceae families.

An ornamental woodland garden The most logical idea consists in gradually establishing a purely ornamental woodland garden, by working with a palette of reliable plants and gradually introduce some connoisseurs’ plants. Bulk up the existing successful planting, such as Galanthus, Helleborus orientalis and Hosta species. Suitable, closely-related species could be regularly tested, in order to give botanical connection to the planting. For instance, Ruscus aculeatus could be juxtaposed with R. hypoglossum.

Inadequateness of the present shelterbelt The exposure to drying speed winds (predominantly from south-west) aggravates the conditions within the Woodland Garden. Thus, the importance of a well-established shelterbelt to protect the area, allowing the establishment of an internal microclimate where woodland plants could be successfully cultivated. The existing shelterbelt needs incensement and replanting, as considerable sections of it have been damaged during the storms in the Eighties. Also several specimens show annually signs of water stress symptoms; Ilex species in particular have proved not to be suitable for the dry east Anglian climate.

Gradual introduction of long-term species surviving climate change Drought-resistant species should be chosen, perhaps considering suitable genera and species within the Sapindaceae family, which is already well represented in the shelterbelt by Acer and Aesculus.

Presence of invasive species The establishment and horticultural management of the desired planting is made more difficult due to the competition of a number of pernicious weeds, such as Allium paradoxum, Calystegia sepium, Circaea lutetiana, Geranium robertianum and Scilla bithynica. Arum maculatum and A. italicum are also present and prove to be difficult to control. Scilla bithynica is perhaps the major problem, as it retains leaves through summer, smothering the surrounding planting.

Integrated control with a degree of tolerance Because of the restricted staff on Section, as well as the size of the area, it is difficult to eradicate completely invasive species from the Woodland Garden and its proximities; a certain degree of tolerance has to be kept. The most possible effective and selective methods of weed control need to be pursued, such as chemical control with glyphosate. The removal of the larger patches should be undertaken only when a certain direction for the planting will be decided for the future Woodland Garden.

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Dry climate The considerably dry conditions lead to difficult establishment of plants as well as water stress in many mature species. The water stress influences infection and spread by honey fungus, which is already present in most of the Woodland Garden. Furthermore, the low-watering policy of the Botanic Garden complicates the management, drastically reducing the palette of suitable plants to work with.

Skilful combination of horticultural techniques As previously discussed in this essay, the combination of the most possible number of adequate horticultural practices is the best possible way to cope with difficult growing conditions. Dry conditions can be improved by increasing the level of moisture in the soil; the consistent application of mulch is among the best solutions (see box below). A hypothesis could also be to create of a canal crossing the central part of the Woodland Garden (where conditions are drier), so connecting the Stream Garden with the Lake. This will give the opportunity to grow marginal woodland plants.

Urgency of available mulching material The leafmould is currently used as a mulch for the Woodland Gardens beds. The heap lies out of the north-western border of the site. Correct mulching is critical, considering the combined factors of low moisture levels in the soil due to dry conditions, as well as general alkaline pH and generally acidophilic tendency of woodland plants. Moreover, in low-rainfall areas, mulching has shown to be more effective, as nutrients are not washed out.

Increase and differentiation of mulching applications Two different solutions can be taken in consideration. In the case of the leafmould heap kept in the present location, it would be desirable to have either more room available or, ideally, two heaps instead then one, so allowing a two-year-basis mulching regime; this will provide ready-to-use material, as well as half-rooted one, to be turned onto the empty space when the first has finished. Alternately, the leafmould could be stored in the compost yard, with the opportunity to an increase of production and the possible reorganisation and specialization of the compost production. Trainees’ blitzes could be organised if the material will eventually become abundant.

Presence of honey fungus One of the biggest problems in the long-term development of the Woodland Garden is the presence of honey fungus (Armillaria mellea) in virtually the entire surface of the site, especially along the western border. There are currently losses to each year among the woody and semi-woody species.

Cultivation of resistant species For the major permanent planting preference should be given to honey fungus resistant species. The Royal Horticultural Society provides advice service and a constantly updated list of suitable plants [RHS Advisory Service]. Mycorrhizal fungi could also be used for planting out and establishment of semi-woody species that are susceptible to honey fungus.

Damaged caused by pests Non-native animal species cause a lot of damage within the Woodland Garden. Muntjacs in particular are proving to be a serious problem, with an increasing population currently estimated around eleven individuals. They feed on vegetative and floral parts of many cultivated plants, favourite being Erythronium, Tricyrtis stolonifera and Trillium species. Badgers are also present but cause minimal damage.

Two possible ways Perhaps the best solution is to eradicate the muntjacs from the garden. As a matter of fact, they cause a lot of damage not only within the area of Woodland Garden, but also - and more importantly - in the entire Botanic Garden. Alternatively, deer-resistant species could be introduced, but this will further reduce the available palette of plants to cultivate. Many American studies exist on deer-resistant plants for the garden use.

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The Woodland Garden at CUBG. View of the access from the Stream Garden (above);

a meandering path bordered with logs and dressed with bark (below).

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Conclusion

It is beyond any doubt that dry shade is among the most difficult situations in horticulture. However, it can offer an engaging opportunity to challenge the natural limitations, with a surprising variety of plants and combinations. Great fun can derive for the horticulturalist through working with a multitude of unusual species and new selections. Ultimately, dry shade gardening at its best can convey a degree of restfulness and dynamism to challenge the brightness of the most pleasing sun borders and parterres. Observing woodland principles and structures is the essential requirement for successful dry shade gardening. Many horticultural and forestry technologies have already drawn inspiration and models from ecological studies. Forest gardening, a sustainable low-maintenance agroforestry system, is based on woodland layers [Hart, 1996]. Plant expeditions could also open the way to the introduction of new species, which would increase the available possibilities. Taiwan and Korea have been recently botanised and are increasingly giving new interesting woodlanders. The drier temperate woodlands of the Southern Hemisphere (noticeably Nothofagus woods of Argentina and Chile) could be sources of suitable species.

Finally, dry shade is an undeniably important subject for changing climate, especially in terms of the future water shortage. Recent years have witnessed an increasing interest in water-sensitive design in horticulture and town planning. Beth Chatto’s studies of gravel and woodland gardening offer inspiring suggestions [Chatto, 2000 and 2002]. Another interesting approach come from Nigel Dunnett and his work on rain gardens, where rainwater runoff from structural features is captured and then released back into the landscape in a controlled way with planting schemes to capture and slow it down [Dunnett, 2007].

A North-facing, dry shade bed at CUBG. Soil level has been raised to help establish plants in a very root-infested area.

Pterocarya fraxinifolia is underplanted with Daphne bholua ‘Jaqueline Postill’, Digitalis purpurea, Galanthus ‘Atkinsii’, Molinia caerulea ‘Strahlenquelle’, Phlox

paniculata, Polypodium interjectum, Polystichum setiferum Divisilobum Group, Pulmonaria longifolia, P. ‘Blue Ensign’ and Ruscus colchicus.

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References

Bell, A. D. (1991). Plant Form. An Illustrated Guide to Flowering Plant Morphology. 171. Capon, B. (1990). Botany for Gardeners. 33. Chatto, B. (1978). The Dry Garden. 56-60. Chatto, B. (2000). The Gravel Garden. Chatto, B. (2002). The Woodland Garden. Dunnett, N. (2007). Managing Water Sustainably in the Garden and Designed Landscape. Elliott, J. (1998). The Woodland Garden. A Guide to Shade-loving Plants. 7-10. Hart, R. (1996). Forest Gardening. Rediscovering Nature and Community in a Post-

Industrial Age. la Croix, J. F. (1978). Gardening in the Shade. 4-5 Lincoln R. et al. (1998). A Dictionary of Ecology, Evolution and Systematics. 332. Jekyll, G. (2011). Wood and Garden. Notes and Thoughts, Practical and Critical of a

Working Amateur. Ch. 2 “January”. 8. Orwell, G. (2011). Keep the Aspidistra Flying. Ch. 2. 34. Packham J. R. et al. (1992). Functional Ecology of Woodland and Forests. 1-57. Paterson A. (1981). Plants for Shade. 2. Rackham O. (2006). The New Naturalist Library (100). Woodlands. 1-17. Rice, G. (2013). Planting the Dry Shade Garden. 1-39. Rodwell J. S. ed. (1991). British Plant Communities. Vol.1. Woodlands and Scrubs. 20-26. Thomas P. et al. (2007). Ecology of Woodland and Forests. Description, Dynamics and

Diversity. 4-5.

Online sources Plants in Action (2014).

http://plantsinaction.science.uq.edu.au/edition1/?q=content/12-1-1-light-interception-and-utilisation

RHS Advisory Service (2014). https://www.rhs.org.uk/advice/pdfs/honey-fungus-host-list-march-2014

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Acknowledgments

The preparation and writing of this essay has depended heavily on the support and inspiration of some great persons and friends. I am most grateful to Jenny Sargent and Christine Alexander for the use of books from the Cory Lodge and Plant Science libraries respectively; to Pete Atkinson for the digital maps of CUBG; to Helen Seal and Simon Wallis for information and inspiration on the Woodland Garden at CUBG and its horticultural management; to John Kapor for information on the weather in the British Isles and its dry regions in particular; to Paul Aston for once again bravely proof-reading the first Italo-English draft of this essay and to Seth Ratcliffle for more late-night proof-reading and coffee supply. The beautiful photos of some dry shade planting at CUBG were taken by Abramo De Licio, while he was accidentally in Cambridge in between his connections from Amsterdam to Nice. I am especially grateful to him for his always friendly support and professionalism.