Department ofHorticultural Science

Horticulture Information Leaflet 645645November 1992

WEED MANAGEMENT FOR WILDFLOWERS

Lena Gallitano, Research AssistantW. A. Skroch, Extension Horticultural SpecialistD. A. Bailey, Extension Horticultural Specialist

The use of wildflowers in the land-scape has increased since Lady BirdJohnson first promoted them in thelate 1960’s and early 1970’s. Wild-flowers were further popularized bythe "Meadow in a Can" seed collec-tions that were marketed in theearly 1980’s. A number of bookshave been written that describemethods for planning and plantingwildflowers, however, few recom-mendations are available regardingmaintenance and long-term weedmanagement. In wildflower plan-tings, weed management is a com-plex system that requires knowledgeof the specific wildflowers andweeds, environmental conditions,and control methods. Therefore, theobjective of this leaflet is to discussweed management strategies thatcan be applied to the planning,establishment, maintenance andrenovation stages of a naturalizedwildflower planting.

In North Carolina, extensiveplantings on the highways havegiven wildflowers high visibility andfurther promoted the concept for usein other landscapes including golfcourses, office parks and privateresidences. Naturalized wildflowerplantings such as meadows arebeing recognized and appreciated aslandscape designs. Wildflowersattract birds, butterflies and otherwildlife as well as provide colorwhen planted in lieu of largeexpanses of turf grass. Whenallowed to naturalize, they canreduce the maintenance required bya more formal landscape design.

The ultimate objective in planting anaturalized wildflower area is todevelop an attractive, permanentplanting that will provide flowersyear after year with self-seedingannuals and perennials. However,wildflower plantings that are leftunmanaged will eventually revert tothe composition of plant species inthe original plant communitythrough a process called succession.Succession is a gradual change overtime in the species composition ofthe plant community. The speciesthat appear/disappear during thistime change can generally be predic-ted based on historical knowledge ofthe planting site. For example, it iswell documented that in NorthCarolina’s Piedmont, the followingsequence of successional species willoccur in a cultivated area that isabandoned and not properly man-aged or maintained after the initialgrowing season:

Years AfterAbandonment Major Plant Species Present

0-2 Crabgrass (Digitaria spp.)Horseweed (Conyza canadensis)Common ragweed (Ambrosia

artemisiifolia)Asterspp.

3-5 Broomsedge (Andropogonspp.)Loblolly pine (Pinus taeda)

10-15 Loblolly pine15-75 Loblolly pine with hardwood

understory75+ Mixed forest eventually changing to

mature hardwoods

Wildflower plantings will essentially followthis successional process because they aregiven very little maintenance after the initialestablishment period. In addition to succes-sional changes, other weed problems can beanticipated, based on knowledge of the siteselected. A primary source of weed contami-nation is from seed that have been depositedin the "soil seed bank" over a long period oftime. These seed often remain dormant formany years but will germinate when tillagelifts them to the soil surface where moisture,air, and light are favorable for germination.Research has found as many as 98 to 3,068viable weed seed per square foot in the topsix inches of soil. This quantity of seed, ifallowed to germinate, would present a for-midable weed population in a new planting.Therefore, in selecting a site for a wildflowerplanting, knowledge of previous weeds andpast use of the land will allow insight intopotential weed problems in the future.

I. PLANNING

A. Analyze the site for potential weedproblems. Assessing the wildflower site forpotential weed infestations from the weedspresent, the soil seed bank, the surroundingarea and anticipated successional changes, isthe first step toward developing a weedmanagement strategy. The current weeds onthe site should first be identified. If it isinfested with difficult-to-control perennialweeds such as nutsedge (Cyperus spp.) orwhite clover (Trifolium repens), it may bebest to select another site for the wildflowerplanting or weed pressure may be too greatfor wildflower establishment. If the sitemust be used, it may be advisable to spendseveral seasons dedicated to controlling andremoving as many perennial and reseedingannual weeds as possible before plantingwildflowers. Weed species and populationsin surrounding areas should also be consid-ered in the site selection process. Seeddispersal mechanisms that allow wind andwater to bring in seeds from outside theplanting area will add to the site’s weedpopulation. Germination times, such as fallor spring for annual weeds, will also affectwildflower growth if weeds are establishedand large enough to compete when thewildflowers germinate. After potential weed

species and their sources have been identi-fied, their growth habits and characteristicsshould be studied. Knowledge of theseweeds is essential in determining methods ofcontrol. (Weed identification references areincluded in the Reference Section.)

B. Select the desired wildflower species.The second consideration in the planningphase is the selection of the wildflowerspecies. The species planted will affect weedmanagement at the site. The objective inselecting a variety of wildflowers is todevelop a plant community that will beattractive and compatible in terms ofgrowing requirements, that will flower overextended periods of time, that will reseed thesite for generation of new plants, and thatcan be held and maintained at the floweringsuccessional stage. In order to be successfulin a particular location, the wildflowerspecies must be compatible with and adapt-able to growing conditions at the site. Theymust also be competitive with other speciespresent, including weeds. Native speciesgenerally are most adaptable to localgrowing conditions and are usually the mostcompetitive with native weeds that will bepresent.

In determining species that are to beincluded in a wildflower seed mix, consider-ation should be given to the germinationtimes and growth characteristics of thespecies. Most wildflower mixes are acombination of annuals for flower color thefirst year and reseeding annuals or peren-nials for flowers in the second andsucceeding years. The objective in using amix is to develop a wildflower community ofcompatible species. Species that germinateand emerge rapidly after planting willbecome better established, provide groundcover and, consequently, will help reduce thenumber and growth of weeds. Early estab-lishment allows the wildflowers to developbetter root systems and to capture availableresources such as water and nutrients sothey can successfully compete with weedsthat germinate at later times. A carefulselection of wildflower species will help withweed management at the site. (The Refer-ence Section includes resources for selectingwildflower species.)

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II. SITE PREPARATION ANDESTABLISHMENT

The second stage of weed management inwildflowers is site preparation and estab-lishment of the planting. In North Carolina,fall (October/November) is the recommendedplanting time for wildflowers. Generally, thesoil is cool (40 to 60°F) and moist at thistime of year which promotes seed germi-nation and establishment. Winter annualsand some perennials will germinate andoverwinter as small seedlings. In addition,the soil is cool enough to hold summerannual seed in a dormant state for earlyspring germination. The most effective weedmanagement during site preparation is tokill as many weeds and viable weed seed aspossible to prepare the site for wildflowerplanting. There are essentially two methodsthat are effective: 1) the use of a systemicherbicide and 2) fumigation.

A. Site Preparation with Herbicides.Initial weed control with a systemic, non-residual herbicide such as glyphosate(RoundupTM and other trade names), is theapproach most often used in establishingwildflowers. Glyphosate is often thought tokill all plants, however, it has selectivity forsome plants depending on the time of year itis applied and the growth state of the plant.Proper timing of the application is, therefore,very important to achieve maximum weedcontrol results. (N.C. Cooperative ExtensionService Publication AG-427, "Weed ControlSuggestions for Christmas Trees, WoodyOrnamentals, and Flowers" and the glypho-sate label contain details on applicationrates and timing for specific weeds.)

In order to plant wildflowers in October orNovember, site preparation must begin inlate summer (August/September). Totalpreparation time is approximately four to sixweeks using the following method:

1. Be sure the area has NOT beenmowed so grass and weeds will be theproper size at the time of spraying asspecified on the glyphosate label.

2. Spray only when the plants are dry.When the site meets these conditions,

uniformly spray the area for coveragebut do not wet the plants to runoff.

3. Allow at least six hours of dryingtime for maximum plant kill with gly-phosate. The timing of this first applica-tion should coincide with active weedgrowth because the optimum suscepti-bility to glyphosate for perennial weedsis when they are actively growing. Inaddition, they are most susceptible whenthey are not stressed.

4. Seven to ten days after the initialspray when the grass and weeds aredying, till the site and prepare it forfinal planting. This should be accom-plished by mid-September for planting inearly October.

5. If weeds emerge, re-treat withglyphosate two to four weeks after thefinal tilling and bed preparation. Tillingwill bring new weed seed to the surface.The soil will be warm and if sufficientmoisture is present, many annual weedseed may germinate. These seedlingswill die within seven days after treat-ment and the site will be ready forplanting.

B. Site Preparation with Fumigation. Asecond technique for site preparation andestablishment of wildflowers is the use offumigation. Fumigants kill most weeds anddormant weed seed except those with hardseed coats such as Carolina geranium(Geranium carolinianum), white clover, andnutsedge. Fumigation is a temporary weedcontrol method and new weed seed willgerminate as they are introduced into theplanting area. The advantage of fumigationis that wildflowers become better establishedprior to weed emergence. Research hasshown that fumigation can result inincreased plant size over non-fumigatedsites. Although increased size can benefitwildflower growth, the cost of fumigation ishigh and must be considered.

There are several fumigants available foruse in wildflowers including methyl bromide,metham and dazomet. Methyl bromide is agas and for large areas requires the use of

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specialized equipment and a licensedapplicator. Metham (Vapam and Sectagon)are water soluble liquids and are less activethan methyl bromide, but they are easier touse. Dazomet (Basamid Granular) is agranular product that is most effective incool soil temperatures. The metham anddazomet fumigants remain in the soil forlonger periods of time than methyl bromide.To ensure the soil is safe for planting a soilbioassay should be conducted after anyfumigation. However, because of thepotential for prolonged soil activity withmetham and dazomet, a bioassay is essentialwhen these fumigants are used. If the soilstill contains the fumigant, planting wild-flower seed will result in the death of theseed. Extension publication AG-427 containsadditional fumigation information and acomplete procedure for conducting a soilbioassay.

If fumigation is chosen for preparation of awildflower planting site, the site should betilled and prepared for planting in latesummer to early fall (August/September).Fumigation requires adequate soil moisturefor movement in the soil and is generallyvery effective in September to early Octoberwhen soil temperatures are still high. Thesite can be planted when the bioassay showsthe soil is safe for planting (late October/November).

C. Planting. At planting time, loosen thesoil surface slightly to enhance seed contact.At this time the soil surface should bedisturbed as little as possible because mostannual weed seed that germinate are withinthe top one-fourth inch of soil. If the soil isdeeply disturbed, additional weed seed thatare capable of germinating and competingwith the wildflowers will be moved to thesoil surface.

Distribute the wildflower seed evenly in theplanting area to give each seed adequatespace and resources for germination. Evenseed distribution allows maximum coverageof the soil by new seedlings and provides themost benefit in weed suppression. Aftersowing, either tamp the soil or water thearea to ensure good seed-soil contact. Alight, seed-free mulch, such as wheat straw,

can also be used for seedling protection. Themulch should be lightly and evenly dispersedwith no heavy clumps. A heavy, unevenmulch reduces wildflower growth anddevelopment and results in areas with noplants.

III. MAINTENANCE AND RENOVATION

The third and final stage in the developmentof a weed management strategy for wild-flowers is to develop both short and long-term maintenance programs. The bestapproach for developing these programs isto: 1) anticipate weed problems, such assuccessional changes, before they occur; 2)identify seedlings quickly as they grow; and3) take corrective weed control action as soonas possible. In general, weed seedlings areeasier to control before they mature andestablish good root systems. Optimum sizefor best control is usually four inches or less.Early control will reduce weed competitionwith young wildflower seedlings giving themmaximum growth conditions.

After a wildflower planting is established,there is no single approach or "magicformula" to manage encroaching weeds. Theproblem is further compounded by the factthat most wildflower plantings are a mix ofspecies, thereby reducing the number ofherbicides that can be safely used for weedcontrol. Similar plants, such as those in thesame family, generally have similar toler-ance to selective herbicides. For example,many popular wildflowers are in theCompositae family and so are many of thetroublesome weeds in North Carolina. As aresult, if an herbicide is safe for the wild-flower, it will probably have no effect incontrolling closely related weeds. With amix of wildflowers from various families, it isunlikely that one herbicide will be complete-ly effective for weed control without damag-ing some wildflowers. Therefore, weed man-agement programs must be developed to usea combination of cultural, chemical andmechanical weed control techniques.

Most wildflower plantings are not static butchange annually and seasonally due to themix of species. A good mix will provideflushes of seasonal flowering from spring to

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fall. Annual changes will occur based on thenumber of perennial plants and those speciesthat have reseeding potential and the extentto which that potential is realized. Reseed-ing is essential for natural regeneration ofplants in order to perpetuate and extend thelife of the planting. Weed control techniquesshould be carefully selected in order tooptimize wildflower reseeding potential atthe site.

An integrated weed management approachthat incorporates multiple weed controltechniques and is site specific will be themost effective for wildflowers. Avoidance ofthe weed problem by preventing regener-ation is often easier than trying to establishweed control at a weed infested site. Inselecting the techniques to use, rememberthat small seedlings are the easiest tocontrol. Also, control of weeds at the site willprevent weed seed dispersal and the develop-ment of other weed propagation structuressuch as rhizomes and tubers. Good controlwill minimize future weed pressure. A rangeof weed control techniques are discussedbelow. The manager must evaluate the siteon a continuing basis and select those tech-niques that will be most effective on a longterm basis for a given location.

A. Competition. One method of weed con-trol that is often overlooked is competition.Seedlings that emerge first are often able tocapture more space and resources and thisgives them a competitive edge over latergerminating seed. Young plants are the mostsusceptible to damage from weed competi-tion. Therefore, if wildflowers are planted ina properly prepared, weed-free site, they willbe quicker to germinate and establish thanweeds. Early development of the wildflowercanopy in the spring will also help suppressweed growth. An even distribution of seed-lings at this stage will allow maximumgrowth of the wildflowers. If the wildflowersare heavily planted, they will compete withthemselves. This will cause dieback andopen areas within the site which promotesweed establishment. In North Carolina,horseweed can be expected to encroach inthe first two years based on successionalchanges. It is difficult to control in wild-flowers because it is a fall germinating

composite. Horseweed, however, can be sup-pressed with a good cover of fall wildflowersbecause it requires bare ground to germi-nate. Understanding weed and wildflowerbiology is essential in order to make maxi-mum use of growth characteristics for weedsuppression.

B. Mowing. Most wildflower plantings aremaintained with an annual mowing. Mowingprevents development of pines and hardwoodtrees and arrests the successional develop-ment at the herbaceous plant stage. Mowingshould be timed to meet three objectives:1) to remove weeds before they flower anddevelop viable seed; 2) to disperse wildflowerseed for reseeding within the site; and 3) toremove dead plant material and improve theappearance of the planting. Mowing is animportant management tool and timing isessential to maximize weed control andwildflower reseeding.

C. Hand pulling. Hand pulling is a viablealternative for weed control in small wild-flower planting sites. Any weeds that arepulled from the site before seed mature anddisperse will contribute to future overallweed control. One weed can produce thou-sands of seeds. Therefore, even on thesmallest scale, the contribution of handpulling should not be overlooked.

D. Chemical control. Chemical weed con-trol programs are based on herbicide selec-tivity and plant tolerance. The objectivewith chemical control is to find an herbicidethat wildflowers can tolerate but which isdetrimental to the weeds present. As dis-cussed earlier, similar plant species havesimilar tolerance to herbicides. For thisreason, herbicides are often classified asbroadleaf and grass (graminicide) herbicidesbased on the weeds they control. Herbicidelabels list the weeds that are controlled aswell as tolerant, desirable species. Atpresent, there are very few wildflowers inclu-ded on broadleaf herbicide labels. Therefore,there are no broadleaf herbicides that areknown to be safe for a wide range of wild-flower species. On the other hand, mostweedy grasses can be controlled in wild-flowers with a graminicide such as sethoxy-dim (Vantage, Poast) or fluazifop (Ornamec,

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Grass-B-Gon) without damaging the broad-leaf wildflowers. However, use of a gram-inicide will also control any native orornamental grasses that may be included inthe wildflower planting.

In addition to the broadleaf and grass classi-fication, herbicides are also categorized bytheir activity as either preemergence orpostemergence herbicides. For the purposesof maintaining established wildflowerplantings, preemergence herbicides would beapplied to the soil prior to the emergence ofthe target weeds. Postemergence herbicidesare applied after the emergence of the targetweeds.

Preemergence products are applied uniform-ly to the soil to prevent germination andgrowth of the weeds controlled by theherbicide. This will require tolerance by allthe wildflowers in the planting or it maydamage susceptible species. In an estab-lished bed, these herbicides may be safe forexisting plants. However, there is littleknown regarding the effects of preemergenceherbicides on the reseeding potential ofwildflowers. If the preemergence herbicideis detrimental to the wildflower seeds,regeneration of new wildflower plants wouldbe reduced. In a planting with a good,established wildflower population and agrowing weed population, the use of apreemergence may be warranted to reducethe weeds, even though reseeding will bereduced for a period of time. Managersshould consider these possibilities whenmaking the decision to use preemergenceherbicides.

Postemergence herbicides are usuallysprayed on actively growing weeds. One wayto circumvent damage to the wildflowerplanting is to select a method for applicationthat directs the herbicide to the targetedweeds and avoids the wildflowers. Thefollowing application methods can be usedfor this purpose:

1. Spot spray - A small sprayer is usedthat can be directed to individual or smallgroups of weeds so the spray will not contactdesirable wildflowers.

2. Wipe-on application - An applicationdevice is used to wipe the herbicide on weedsthat are taller than the wildflowers. Equip-ment may be either a rope wick or rollertype applicator. This technique may beparticularly useful for reducing tall weedssuch as horseweed. Inexpensive hand wicksare available.

3. Clip-cut - Hand-held clippers thatdispense a thin layer of herbicide onto theblade can be used to selectively cut weeds.This method applies the herbicide to the cutsurface of the stem so it is translocated intothe weed to kill the total plant.

When planning any chemical controlprogram, the user has the responsibility todetermine the most effective herbicide touse, timing of the application, and the properrates. Herbicide labels must always becarefully read and followed for the bestand most cost effective results.

E. Allelopathy. Some plants producechemical compounds that inhibit the growthof other nearby plants (allelopathy). Thetoxic substances may be released from theroots or leaves and through plant residueson the surface that are leached into the soilwith rainwater. Weeds are known to haveallelopathic (detrimental) effects on somecrops and vice versa. Some of the weedsthat are known or suspected to be allelo-pathic are bermudagrass (Cynodon dactylon),johnsongrass (Sorghum halepnese), yellowand purple nutsedge, pigweed (Amaranthusspp.) and sunflower (Helianthus spp.).Horseweed is known to be allelopathic toitself. This most likely is the reason itdeclines in the successional changes in NorthCarolina’s Piedmont. However, even thoughit will decline over several years, if it is leftin most wildflower plantings, it will severelyreduce the wildflower population by out-competing more desirable plants.

Knowledge of allelopathic effects will neverbecome a total weed control method for wild-flowers. However, by observing differencesin wildflower plantings and surroundingareas, managers may see differences in plantgrowth that can be effectively used indeveloping a weed management strategy.

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F. Renovation. Wildflower plantingsdecline over time because many herbaceousperennial plants often have a limited lifespan. Decline may also be attributed toweed pressure, allelopathy, site, and envi-ronmental growing conditions. Renovation ofan existing planting may be more cost effec-tive than starting with a new bed if the weedpressure is first brought under control.

New seeds can be planted into existing wild-flower plantings that are relatively weedfree. The soil should be disturbed as little aspossible to prevent additional weed seedgermination. This can be accomplishedeither by raking or using a slot seeder.Since the seed cannot be tamped withoutdamaging existing plants, water them in toprovide adequate soil contact. Renovationshould be completed following the sametimeframes used for initial seeding.

Weed management in wildflowers is an on-going challenge. As wildflowers becomemore popular, additional knowledge andexperience will generate improved manage-ment techniques. The primary objective forwildflower plantings should be a naturalizedplanting with seasonal changes and interestrather than a weed-free groomed area. Thiscan be achieved with properly applied weedmanagement techniques.

References and Resources

Baldwin, F.L. and E.B. Smith. 1981. Weedsof Arkansas: A Guide to Identification.Cooperative Extension Service, Universityof Arkansas, Fayetteville, AK.

Corley, W.L. and A.E. Smith, Jr. 1990.Evaluation of Wildflower Plant Species andEstablishment Procedures for Georgia RoadSites. GDOT Research Project No. 8604Final Report. University of Georgia, GA.

Martin, L.C. 1986. The Wildflower MeadowBook. East Woods Press, Charlotte, NC.

North Carolina Dept. of Transportation.1989. Wildflowers on North CarolinaRoadsides. N.C. Department of Transpor-tation Landscape Unit, Raleigh, NC.

North Carolina Botanical Garden. TheUniversity of North Carolina. Box 3375,Totten Center. Chapel Hill, NC 27599.

Phillips, H.R. 1985. Growing andPropagating Wild Flowers. The Universityof North Carolina Press, Chapel Hill, NC.

Skroch, W.A. and J.F. Derr. 1992. WeedControl Suggestions for Christmas Trees,Woody Ornamentals, and Flowers. N.C.Cooperative Extension Service PublicationAG-427. N.C. Coop. Ext. Serv., N.C. StateUniversity, Raleigh, NC 27695-7609.

Stuckey, J.M., T.J. Monaco and A.D.Worsham. 1989. Identifying Seedling andMature Weeds Common in the Southeast-ern United States. AG Extension Bulletin208. N.C. Agricultural Extension Service,N.C. State University, Raleigh, NC 27695-7603.

Smith, S.W., Editor. 1984. Landscaping withWildflowers and Native Plants. OrthoBooks, San Francisco, CA.

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