21THE HORTICULTURIST APRIL 2010

Japanese knotweed (Fallopia japonica) hadbeen minding its own business in Japanand China for millennia. On volcanoes in

Japan the dwarf form of the plant (F. japonicavar. compacta) had the habitat to itself, andwas the primary coloniser of volcanic ashfields. Long association with sulphurousemissions and sudden depositions of hot ashendowed it with certain tenacity. The highaltitudes meant a much restricted growingseason, early flowering and dwarf habit. Keyto its survival was the woody rhizome thatwent vertically downwards, where storednutrients and dormant buds were beyondthe reach of the next deposit of hot ash. Insuch habitats, plants are characteristicallyseparate clumps not connected byunderground rhizomes.

The tall lowland forms of the plant (F.japonica var. japonica) faced a different set ofproblems. In Japan they were just onecomponent of a giant herb community,either competing with such plants asMiscanthus and bamboo, or trying to find aspace in a forest clearing or riverside. Herethe rhizome may have acquired thesecondary function of seeking out newresources of space, light and evasion ofpredation.

Siebold’s plantsIn order to find out how Japanese knotweedcame to end up in Britain and cause suchproblems, one must delve into the currentlyrather unfashionable area of the history ofbotany. Philipp von Siebold was a mostcolourful character, whose name is attachedas the authority to many of the Asiaticspecies that we now take for granted in ourgardens. As a surgeon he was able to travelto Japan with the Dutch trading delegation ata time when Japan was effectively closed offfrom the rest of the world. In spite of beingrestricted to an artificial island in theharbour at Nagasaki, his knowledge ofadvanced western surgical procedures wonhim many friends in Japan, who in returnprovided him with choice Japanese plantsthat gardeners in the West were keen topossess.

After an incredible sequence ofmisadventures he was finally reunited withwhat remained of his plant collections inLeiden, The Netherlands, in 1842. There heset about forming his Royal Society forEncouragement of Horticulture. Preserved inits journal the Jaarboek van de KoninklijkeNederlandsche Maatschappij tot Aanmoedigingvan den Tuinbouw, are a complete series ofdated and priced catalogues of Von Siebold& Co. Anyone who has ever worked on thehistory of plant introductions will appreciatewhat a stroke of fortune this was, sinceplant catalogues of this date are extremelydifficult to come by.

In the 1848 edition of the Siebold & Cocatalogue, the category ‘plants newlyimported from Japan’ includes a range ofspecies with prices from 300-600 francs.Polygonum sieboldii (Japanese knotweed)was available only as a collection of onemother plant and 25 strong plants for thesum of 500 francs. There is a seven-linefootnote extolling the virtues of thiswonderful plant, which can do practicallyanything, short of bringing back the dead!Given that 500 francs spent in the samecatalogue would have filled the barouchelandau to overflowing with choiceornamentals - this was an extremelyexpensive plant.

Siebold was, of course, nobody’s fool. Inorder to make money on a plant as readilypropagated as this, you needed a high initialprice. Indeed by 1856 24 plants could be hadfor only 6 francs!

The next step in 1850, was to send apackage of plants, including Japaneseknotweed to Kew Gardens in the hope ofextracting further novelties in exchange.This is all duly recorded in copperplatehandwriting in the Kew Inwards Book, alongwith a terse note saying that on account ofbad selection only six of them were new tothe gardens.

Contemporary illustrations show that theSiebold plant is actually male-sterile, with

Invasive Aliens

Opening Pandora’sseed packetJapanese knotweed arrived as a sought-after exotic plantbut is now a proscribed alien. John Bailey explores someof the unpredictable outcomes that have arisen for theseindestructible plants.

John Bailey is Principal ExperimentalOfficer in the Department of Biology at theUniversity of Leicester, where he has beenworking since 1977. A colleague in thedepartment, Ann Conolly, introduced himto Japanese knotweed, and after herretirement he carried on from where sheleft off. After completing a PhD on it, hemaintained a strong interest in the plantand established many internationalcollaborations. The adoption of molecularapproaches in the 1990s and the hard workof two PhD students greatly extended thescope and impact of the research, resultingin his contribution to more than 30scientific papers on the subject. It is hisproud boast that he is more interested inthe sex lives of these plants than he is ineradicating them. Invitations to lecture onthe subject have taken him as far afield asJapan and the US. He now has his sightsset on New Zealand and is currentlylooking for funding to take this researchfurther.

Manuscript RBG Kew Inwards book 1848-1858showing the list of plants sent by Siebold in 1850.Item 34 ‘Polygonum Sieboldii’ is more commonlyknown as Japanese Knotweed (Fallopia japonica

var. japonica).

Photo: Royal Botanic Gardens Kew

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cordate leaf bases. The fact that only thefemale form of a dioecious species wasoriginally imported has had a number ofimportant consequences. Firstly, it could notreproduce itself by seed. Not a major issue,when as little as 0.8gm of rhizome couldgive rise to a new plant. Secondly any seedcollected from it would inevitably be hybrid,so later on when male plants of the relatedgiant knotweed (F. sachalinensis) becameavailable, seed could be collected from

Japanese knotweed and distributed asJapanese knotweed seed. This seed was ofcourse the hybrid now known as Fallopia xbohemica (after its original description in theCzech Republic), and is a widespread, ifunder-recorded, component of Japaneseknotweed populations throughout the West.

We now enter the ‘Dark Age’ of Japaneseknotweed knowledge where the onlyevidence of its existence comes from datedherbarium specimens, and the occasionalpublished note of where it had becomeabundant. The early history of itsintroduction was carefully pieced together bythe painstaking research of Ann Conolly in

the 1960s and 1970s, and displayed in aseries of dated distribution maps. Some ofthe earliest non-garden records came fromWelsh coal-mining valleys and it is possiblethat, rather than escapes, these wereexamples of deliberate planting, since one ofmany claimed benefits of the plant, wasstabilisation of loose soil; coal barons nodoubt keen to try anything that allowed theirspoil heaps to be more vertiginous.

Cause for concernBy the late 1970s the true extent of thespread of Japanese knotweed in the UK wasbecoming apparent, and clearly someinfluential people were beginning to getrattled. Accordingly, when the wide-reachingWildlife and Countryside Act was publishedin 1981, provision was made for theproscription of alien plants and animals thatit was felt were particularly harmful to theenvironment, Japanese knotweed being oneof two land plants so listed. I will not dwellon the almost 30 years it has taken for anyof the many other deserving contenders tobe added to this list, but examine the strangeconsequences of Japanese knotweed beingon it in the first place.

The offence to ‘plant or otherwise causethe plant to grow in the wild’ was longdeemed to exclude movement of viablerhizome fragments by road works andredevelopment schemes and was perhapsmore suited to evil-doers planting it innature reserves at the dead of night! This allchanged with the Environmental ProtectionAct of 1990, when the pendulum swungabruptly to the other extreme. It became anoffence to move as much as an atom ofJapanese knotweed from one site to anotherunless it was in specially sealed anddecontaminated lorries delivering it toofficially designated deep landfill sites.

Eradicate…Overnight the industry of Japanese knotweederadication was born, and suddenlydevelopers took a serious interest in botany,with knotweed-infested sites selling atserious discounts. Guaranteed eradicationwas an expensive business, with developersfearful equally of prosecution and thepossibility of any of the ‘armour-piercing’rhizome being left on their sites. Majorsporting venues such as Wembley and manyof the London Olympic venues have been hitby these costs. Costs, incidentally, that areincurred only in the UK - other Europeancountries apparently being able to survivewithout such legislation, which is, Imaintain, an entirely unintendedconsequence of including Japaneseknotweed in the 1981 Act.

In the UK press Japanese knotweedregularly attracts epithets, such as ‘monster’,‘barbarian’ and ‘triffid’, but in spite of themythology surrounding it, if caught earlyenough, it is readily eliminated by the use ofa range of herbicides. Problems arise in thetreatment of long established stands, wheredecade after decade, the network of woody

A nymph of Aphalara itadori, the approved bio-control agent for Japanese knotweed.

Contemporary illustration of Siebold’s Polygonumsieboldii (Japanese knotweed).

F. x conollyana at its type locality in Haringey North London, with Ann Conolly in attendance.

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avid

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ABI

Bio

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“It became an offenceto move as much as an

atom of Japaneseknotweed from one siteto another unless it wasin specially sealed anddecontaminated lorriesdelivering it to officiallydesignated deep landfill

sites.”

THE HORTICULTURIST APRIL 2010

Photo: Royal Botanic Gardens Kew

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underground rhizomes has increased in girthand range, effectively producing an‘underground tree’.

Above ground growth and re-growth willbe decimated by the appropriate systemicherbicides, but die-back does not extend farenough into the rhizome to eliminate it. Onecan soon reach a stalemate with suchstands, whereby only small mal-formedshoots show, presenting little surface areafor absorption of further systemic treatment,yet the plant is not dead. Improvements inthe timing and method of application ofherbicide, such as stem injection in theautumn, are helpful but other approachesare also necessary.

On redevelopment sites, which do not havethe luxury of several seasons of treatment,other solutions, such as on-site burial sealedunder geo-textile membranes, arepreferable. But even here we are verging onthe metaphysical. I readily admitted my

ignorance to a textile manufacturer as to thenecessity of incorporating expensive copperfilaments in such membranes!

Japanese knotweed’s struggle to survive inits native habitat is in stark contrast to theprofligate vigour it exhibits in the West. Thisis down to a more than suitable climate,absence of competition in ruderal areas anda more or less complete absence of attentionfrom native invertebrates. In Japan it is hostto a range of invertebrate predators, with thedifferent groups targeting different plantorgans. The other obvious method of controlis to reintroduce the plant to some of theenemies that have been left behind in thearea of origin - biological control.

Biological controlCABI Bioscience and its predecessors have adistinguished history in this area, and in thelate 1990s, whilst planning a collecting tripto Japan we were approached by Dick Shawof CABI. Work at Leicester by MichelleHollingsworth had already established that

all the Japanese knotweed (Fallopia japonicavar. japonica) was a single clone, and thatuseful variation in the chloroplast DNAexisted in plants sent directly from Chinaand Japan. Catherine Pashley then came onthe scene with the intention of comparingthe chloroplast patterns found in theEuropean clone with those of plantscollected from a wide survey of Japan,thereby establishing the origin of theimported plant.

It is one of the basic tenets of biologicalcontrol that you should understand thetaxonomy and reproductive biology of theorganism to be controlled, and also knowwhere the particular genotype evolved. ThatJapanese knotweed was already known tobe a clone, was a bonus, since once asuitable control organism was found, theplant would not have any reserves ofresistance to it. It is necessary to know thearea of origin, so that the appropriatecontrol agents can be sought in this area.Historical and morphological data hadalready ruled out China as an origin of ourclone, but although Japan is not a largecountry, it is extremely long, with variedhabitats and dissected by mountains.

After much hard work, a matchinggenotype was found in Nagasaki, whichperhaps unsurprisingly was near whereSiebold was based during his Japanesesojourn. After a considerable amount oftesting, CABI announced in 2009 that asuitably specific control agent had beenidentified, a pysllid sapsucker Aphalaraitadori, which in greenhouse trials haddevastated the knotweed plants. Whilstcontinuing to work on some promisingmicro-organisms, CABI applied forpermission to release the psyllid last year.Permission was granted in early 2010. Therelease, due in spring/summer 2010, will beon a phased basis, at two or three closely-monitored sites. As with all bio-controlprogrammes, the aim is to weaken ratherthan eradicate the target organism, andexisting control measures and publiceducation will remain an essential part ofthe campaign against Japanese knotweed.

Unpredictable outcomesWe are all familiar with the potentialdangers to the environment posed by alieninvasive species, which freed from theconstraints of the area in which theyevolved, are often capable of out-competingthe local species and having a majornegative impacts on biodiversity. There ishowever, a less well-known dangerassociated with plant introductions - thecreation of new species by allopolyploidy.This can occur when two distantly relatedplants hybridise to produce a sterile F1 -sterility arising from the failure of thechromosomes of the two genomes to pair atmeiosis. If followed by chromosomedoubling, at a stroke fertility is restored anda new allopolyploid species is created.

The most important case of allopolyploidspeciation was in the evolution of thehexaploid bread wheat - without which - I

THE HORTICULTURIST APRIL 2010

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Eroded surface on Mount Aso, Japan, showing thedeep and extensive woody rhizomes of the montane

F. japonica.

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would not be sitting here writing this article!In historical times we have witnessed twocases of amphiploid speciation in the wild inBritain, Spartina anglica and Seneciocambrensis. The key similarity is that bothinvolved hybridisation between a nativeBritish species and an introduced species,the outcomes of the speciation events arerather different. Spartina anglica was sosuccessful at reclaiming mudflats, that inaddition to out-competing its British parent,it has been widely distributed round theworld for this purpose, and considered analien invasive in many regions. Seneciocambrensis on the other hand, was producedat a period when wartime bomb damage,followed by the wide-scale redevelopment ofthe 1960s and 1970s, created the habitat thatit favoured. But its numbers are nowdeclining with the reduction of derelict land.

The key requirement for allopolyploidy isthe bringing together and hybridisationbetween, distantly related plant species thathave long been kept apart by variousgeological events. The seemingly relentlessquest for horticultural novelties is constantlybringing such species together. Whilst thechances of this happening are not very high,and the outcomes not always necessarilybad, the pre-requisite for this is the presenceof sterile F1s between distantly relatedspecies.

Work at the University of Leicester over theyears has revealed some extraordinaryfeatures of the Japanese knotweed sensu lato(s. l.) population structure. Firstly, in spite ofits clonal nature it has successfully spread to2,761 of the 3,859 10km recording squares inthe British Isles. As a female plant it canonly produce seed by hybridisation - in this ithas been extremely successful. F. xbohemica, the hybrid with giant knotweedoccurs at four different chromosome ploidylevels (44,66,88 & 110), and is capable ofbackcrossing to its female parent. This iswhy I use the term Japanese knotweed s.l. tocover F. japonica, F. sachalinensis, F. xbohemica and any back-crosses. So in spiteof its initial clonal nature, extensivehybridisation has not only created newvariation, but also produced more vigorousindividuals that are more difficult to control.

Russian vineHybridisation between Japanese and giantknotweed, is of course nothing remarkable,since both species are closely related andboth occur in Japan. Far more extraordinaryis the hybrid between Japanese knotweedand Russian vine. Collect seed from anyJapanese knotweed plant in an urban areaand the chances are that it will have beenpollinated by Russian vine. The presence inthe West of large areas of female japaneseknotweed actually constitutes a hugeinadvertent breeding experiment, andanything that can possibly pollinate it willhave done so. Apart from their extremevigour and garden origins, these two plantswould appear to have little in common.

Russian vine is a diploid woody climberwith 20 chromosomes and Japaneseknotweed is a rhizomatous herb with 88chromosomes, and they originate fromdifferent parts of Asia. They are of coursedistantly related, and members of thePolygonaceae, and if the differences ingrowth habit are ignored they have a lot offloral and morphological characters incommon. Some years back, I well recall thehorror of Beth Chatto, when I offered hersome of this hybrid (now known as F. xconollyana 2n=54), in exchange for some of

her F. japonica var. spectabile Japaneseknotweed. Indeed the thought of combiningthe vigour of the two parents and adding apinch of heterosis (hybrid vigour) does ratherconjure up a triffid like scenario!Fortuitously, the opposing over-winteringstrategies tend to cancel each other out, andwhilst F. x conollyana is a perfectly viableplant, and can with time, establish largestands, it is not a particularly aggressiveplant.

New ZealandJust when I thought (not for the first time),that my work on Japanese knotweed had runits course, I received seed collected fromsome New Zealand Japanese knotweedplants. It germinated well and once again Iwas presented with some twining plantswith 54 chromosomes. Russian vine,however, was not known from that area,and it appeared that we actually had hybridsbetween an indigenous Muehlenbeckia andJapanese knotweed!

From an academic viewpoint this isremarkable in terms of the length of timethat has elapsed since these two plants hadlast shared a common ancestor. In practicalterms, this is a much more compatiblehybridisation than the one producing F. xconollyana, since both parents bringcomplementary features to the hybrid.Considering that Muehlenbeckia australis onits own is able to produce impenetrablethickets, Pandora’s seed packet may yethave some triffid-like surprises waiting forus!q

Useful linksCornwall Knotweed Forum(http://projects.exeter.ac.uk/knotweed)

Knotweed biological control(www.cabi.org/japaneseknotweedalliance)

Child L, Wade M. (2000). The JapaneseKnotweed Manual. Packard PublishingChichester (ISBN 1853411272).

Typical monoclonal stand of Fallopia japonica var.japonica along the River Wnion at Dolgellau, Wales.

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Japanese knotweed is not always a blot on the landscape, here in North Ledaig (Scotland), F. japonica var.compacta makes an attractive scene.

THE HORTICULTURIST APRIL 2010