em 8823 • april 2003 $3.00 weed control in container crops · 2008-02-08 · 2 • weed control...

Weed Control in Container Crops

EM 8823 • April 2003 $3.00

Weed Control in Container CropsA Guide to Effective Weed Management through Preventive Measures

J. Altland

James Altland,Extension agent, NorthWillamette Researchand Extension Center,Oregon StateUniversity.

W eeds compete with commer-cial crops for nutrients, water,and light. Competition from

weeds is particularly important incontainer crops due to the crops’limited root volume. Weeds also harborinsects, disease organisms, and verte-brate pests (Figure 1). Marketablenursery crops must be free of weed,insect, and pathogen pests.

Effective weed management involvesa combination of sound sanitary andcultural practices along with proper useof preemergence herbicides. Thispublication is separated into threesections:➤ Using sanitary and cultural practices

to minimize weed populations

➤ Selecting the most appropriatepreemergence herbicides

➤ Timing of herbicide applications

Sanitary andcultural practicesSanitation

Weed control in container produc-tion must be preventive. In field pro-duction, where crops are planteddirectly in soil, weeds often can beefficiently controlled after they germi-nate with directed sprays of herbicides

or mechanicalcultivation. Incontainer crops,where directedsprays are notfeasible, there arefew alternatives tocostly hand-weeding toremove weedinfestations. Thus,a successfulcontainer weedmanagementprogram shouldprevent weedgermination.

The first step to effective weedmanagement is sanitation. A commoncharacteristic of weeds is their abilityto produce prolific numbers of seed.Practices that minimize the number ofweed seeds in the production systemwill improve weed control. The moreweed seed allowed to contaminatecontainers, the higher the probabilitythat weeds will germinate in areaswhere the herbicide barrier has beenweakened (see page 6).

The following are some sanitationpractices that should be considered forreducing weed seed numbers incontainers.

Figure 1.—Annualsowthistle (Sonchusoleraceus), shownabove, is a weedtypically found in andaround nursery crops.It can serve as a hostfor aphids.

2 • Weed Control in Container Crops

��weeds, such as eclipta (Eclipta alba),can establish in drainage holes andcompete for water and nutrients(Figure 3).

When beds are empty between cropcycles, remove or chemically controlexisting weeds. Install fresh stone ornew weed fabric if necessary, and sweepthe area to remove all debris. Herbi-cides can be applied directly on thestone or weed fabric. Products contain-ing prodiamine are an excellent choicefor this application due to theherbicide’s low solubility (0.013 ppm).Current products containingprodiamine (Table 1) are labeled forthis use, and research has shown theyprovide weed control in gravel beds forup to 8 months (Briggs et al., 1998).Other preemergence herbicides also aresuitable for this application, but useonly those with low solubility (lessthan 1 ppm).

Weed control in noncrop areasEliminating weeds in noncrop areas

such as roadways, drainage ditches,between hoop houses, etc., will drasti-cally reduce production of weed seedsand improve weed control (Figure 4,page 4). This can be accomplishedwith regular mowing to prevent weedsfrom setting seed.

Mechanical removal such as hoeingor plowing can be used, although thesemethods make the area susceptible tosoil erosion. Herbicides provide effec-tive control. Postemergence herbicidescan be used to eliminate existingweeds, and preemergence herbicidescan be used to prevent regrowth.Maintaining weed-free noncrop areas isprobably the easiest and most effectivesanitary practice for reducing weedseeds in containers.

Figure 3.—Ecliptaoften infest containersthrough drain holes,and their rootsaggressively competewith crops for nutrientsand water.

Figure 2.—Weeds(bittercress is shownabove) can germinatein small piles of debrisspilled on weedfabrics.

Weed control under containersContainers should be placed on

covered ground, using either gravel,plastic, or woven weed-fabric to coverthe ground surface. Weeds under thecovered area usually are suppressed.However, weed seeds often germinatein debris on top of the surface cover,and their roots penetrate down throughthe cover.

Weed controland sanitationunder andaround contain-ers is almost asimportant asweed control inthe containers.Weeds growingbetween con-tainers are animmediatesource of weed

seeds. Bittercress (Cardamine hirsuta)and oxalis (Oxalis spp.) growing indebris spilled on weed fabric can propelseeds several feet, rapidly infestingnearby containers (Figure 2). Other

Weed Control in Container Crops • 3

��Table 1. Preemergence herbicides commonly used in nursery production.

WeedActive Chemical Solubility control

Herbicide ingredient family (ppm) Formulation strengths

Gallery isoxaben benzamide 1.0 spray broadleafs

Goal oxyfluorfen diphenyl ether 0.1 spray broadleafs

Princep simazine triazine 6.2 spray broadleafs

Devrinol napropamide acetamide 73.0 spray or granular grasses

Lasso alachlor chloroacetamide 242.0 spray grasses, sedges

Pennant metolachlor chloroacetamide 530.0 spray sedges, grasses

Kerb pronamide chloroacetamide 15.0 spray winter annuals

Casoron dichlobenil nitrile 21.0 granular broadleafs

Surflan oryzalin dinitroaniline 2.5 spray grasses

Treflan trifluralin dinitroaniline 0.7 granular grasses

Pendulum, pendimethalin dinitroaniline 0.3 spray or granular grassesCorral

Regal Kade, prodiamine dinitroaniline 0.013 spray or granular grassesFactor,Barricade

Ronstar oxadiazon oxadiazole 0.7 spray or granular grasses

Rout oxyfluorfen diphenyl ether 0.1 granular grasses and broadleafsoryzalin dinitroaniline 2.5

Snapshot isoxaben benzamide 1.0 granular grasses and broadleafstrifluralin dinitroaniline 0.7

XL 2G benefin dinitroaniline 0.1 granular grasses and broadleafsoryzalin dinitroaniline 2.5

OH2 oxyfluorfen diphenyl ether 0.1 granular grasses and broadleafspendimethalin dinitroaniline 0.3

Pre Pair napropamide acetamide 73.0 granular grasses and broadleafsoxadiazon oxadiazole 0.7

Regal O-O oxyfluorfen diphenyl ether 0.1 granular grasses and broadleafsoxadiazon oxadiazole 0.7

Weed control on bark pilesKeep bark piles weed-free. Not only

will weeds growing on bark pilesgenerate seeds that can be blown intocontainers, but they also deposit seedand/or vegetative propagules (tubersfrom nutsedge (Cyperus spp.), rhizomesfrom oxalis, etc.) directly into themedia that will be used for potting

(Figure 5, page 4). When bark piles arekept weed-free, they generally are not asource of weed seeds (Cross andSkroch, 1992).

Steam pasteurization, solarization,composting, and fumigation are sometreatments that will kill seed and otherpropagules in bark piles. These treat-ments are too expensive to be used


��demonstrated that by simply washingpropagation pots with pressurizedwater, the number of germinated weedscan be reduced up to six-fold(Bachman and Whitwell, 1995). Thesame study showed that when dirtypots were used, weeds (bittercress)germinated around the edges of thepots, compared to few or no weedswhen new or cleaned pots were used.

Wind breaksEven if weeds are effectively con-

trolled throughout your nursery, itmight be impossible to affect weedcontrol on neighboring property. Yetseed from neighbors likely will blowinto your containers (Figure 6, page 5).Use of a wind break, such as a tall,weed-free hedgerow or fence, to mini-mize the amount of seed entering thenursery will improve weed control(Figure 7, page 5). The hedgerow orfence need not be permanent. Onenursery lined a side of its property(bordering a weedy lot) with large15-gallon plant material (mostly treesand large shrubs). This solution worksdoubly well by providing additionalproduction area and by blocking weedseed from entering the nursery.

Start with weed-free linersUse of weed-free liners is critical,

especially when dealing with weedsthat have extensive root systems suchas oxalis, liverworts (rhizoids instead ofroots), and pearlwort (Sagina procum-bens). Because roots from these plantscan generate new plants, thoroughhand-weeding to remove shoots androots is necessary. A single escapedweed can generate new plantlets at analarming rate. One bittercress plant(Figure 8, page 6) can produce up to5,000 seeds in just 5 weeks (Bachmanand Whitwell, 1995).

regularly in most nursery operations.Simple sanitation will avoid the needfor treatment.

Clean potsUse of clean or new pots for propa-

gation and/or potting will also reducethe number of weeds in containers.A common characteristic of containerweeds is their small seed size. Seedscling to the side of containers alongwith other debris. Research has

Figure 4.—Weeds innoncrop areas, such asthose shown heregrowing next tocontainers, disseminatelarge numbers of seed,making weed controlefforts less effective.

Figure 5.—Weedsgrowing in bark pilesadd seeds andvegetative propagulesto the bark, resulting ingreater weed numbersin containers.


��Cultural practices

Research currently being conductedat Oregon State University (OSU) isdemonstrating how fertilizer placementaffects weed establishment. Growerstypically apply fertilizers by eithertopdressing (applying fertilizer to thecontainer surface after potting), incor-poration (mixing fertilizer with barkprior to potting), or dibbling (placingfertilizer directly under the plant rootball while potting). By dibbling fertiliz-ers, nutrients are not available at thecontainer surface. Thus, weeds oftenfail to germinate, and those that germi-nate grow poorly. Control of commongroundsel (Senecio vulgaris), oxalis(Oxalis corniculata), and prostratespurge (Chamaesyce prostrata, syn.Euphorbia prostrata) is greatly improvedby dibbling fertilizers compared totopdressing or incorporating. Researchhas demonstrated that most crops growas well and nitrogen leaching often isreduced by dibbling fertilizers com-pared to topdressing or incorporating(Meadows and Fuller, 1984). If dib-bling is not an option, incorporatingreduces weed growth compared totopdressing.

Other OSU research has evaluatedthe influence of cultural practices onliverwort (Marchantia polymorpha)control (Svenson, 1998; Svenson et al.,2001). Liverworts thrive in moistenvironments with high levels ofavailable nitrogen and phosphorus(Figure 9, page 6). Thus, any practicethat allows the container surface to dryquickly, or removes nitrogen andphosphorus from the container surface,results in improved liverwort control.Cultural practices that reduce liverwortinfestations include increased aircirculation, use of a mulch, use ofcoarse container media (containersurface dries more quickly), and

dibbling or incor-porating fertilizerinstead oftopdressing.Herbicides such asRonstar(oxadiazon) andoryzalin alonefailed to provideadequate liver-wort control inOSU research. Itwas concludedthat a combina-tion of herbicidesand cultural practices was necessary forliverwort control.

Herbicide useFor instructions on safe herbicide

use, consult your local Extension agent,or refer to the Oregon Pesticide Applica-tor Manual: A Guide to the Safe Use andHandling of Pesticides (EM 8532).

Maintaining a chemical barrierSanitation is the first step in effec-

tive weed control; proper herbicide useis the second. Herbicides form a chemi-cal barrier over the container surface.

Figure 7.—Use ofpermanent ortemporary hedgesaround the perimeterof the nursery willprevent weed seedsfrom blowing in.

Figure 6.—Seeds ofsome weeds, such asthe common groundselshown above, have anattached white pappusthat aids in winddispersal. These andsimilar weed seedsoften are blown infrom nearby properties.


��Weeds should be pulled before they goto seed. However, soon after removingweeds from an area, apply an herbicideto create a complete chemical barrierand prevent germination of moreweeds.

Sufficient and uniformherbicide application

To create an effective chemicalbarrier over the container surface,always apply herbicides at the ratespecified on the label. If rates are toolow, the chemical barrier may not besufficient to prevent weed growth. Ifrates are too high, the herbicide maycause crop injury. Also, it is importantto apply herbicides uniformly. Whenherbicides are not applied uniformly,weeds will emerge in areas with insuffi-cient herbicide. Use properly calibratedequipment that is functioning cor-rectly. Pay careful attention to equip-ment calibration and applicationuniformity, and take steps to ensurethat proper herbicide rates are applied.

Even with correct application,however, flaws in equipment engineer-ing may result in nonuniform applica-tions. It has been documented thateven with trained staff using properlycalibrated equipment, the actualamount of applied herbicide may varythroughout the treated area from 0.5 to2.2 times the intended rate (Dardenand Neal, 1999). Collection pansplaced throughout the application areacan be used to measure uniformity ofapplication and identify regions withweak herbicide barriers. This method issimilar to using rain gauges to measureirrigation uniformity.

In summary, the most effective weedmanagement is provided by usingsanitation to reduce weed seed num-bers in your production system, whilemaintaining an effective chemicalherbicide barrier to prevent growth of

Figure 9.— Liverwortsinfesting thesecontainers thrive onhigh nutrient levels,moist soil, highhumidity, and shade.

Figure 8.—Bittercressproduce seed at analarming rate. Onebittercress plant canproduce up to 5,000seeds in just 5 weeks.

Though eachherbicide con-trols weedsdifferently,preemergenceherbicides pro-vide control atthe point wheregerminatingseeds emergethrough thechemical barrier(Figure 10,page 7).

If the chemical barrier is incomplete,there will be a gap where weed seed cansuccessfully germinate and grow. Severalcommon practices can disrupt thechemical barrier, including but not

limited to the following: poking holes inthe barrier with fingers or hands whilemoving containers, dropping containers,and container blow-over. All of theseactivities should be minimized to pre-vent disruption of the chemical barrier.Instruction and explanation of thisconcept to the work crew is necessary, asthey typically are responsible for movingand working around the containers.

Pulling uncontrolled weeds alsocreates gaps in the chemical barrier.


��the few seeds that elude sanitationefforts. If herbicide use is over-empha-sized while neglecting sanitation,increased numbers of weed seeds willfind a place to germinate where thechemical barrier has been weakened.On the other hand, if sanitation ismade a priority while herbicide appli-cations are neglected or implementedimproperly, the few weed seeds that arepresent will emerge and quickly gener-ate more seeds.

Selecting preemergenceherbicides

Preemergence herbicides are appliedbefore weed emergence to preventweed growth, in contrast topostemergence herbicides, which killweeds after they have emerged and areactively growing. Base your selection ofherbicides primarily on three criteria:the crop to which the herbicide will beapplied, weed species to be controlled,and herbicide solubility. Other consid-erations include the importance ofrotating herbicide chemistry and thechoice of granular versus spray-appliedherbicides.

Crop tolerance to herbicidesSelecting an herbicide based on the

crop being grown is critical. Everyherbicide label describes how theproduct should be used and to whichplants it can be applied (Figure 11).The plant list is a compilation of plantsto which the herbicide has beenapplied safely in experimental tests.Every effort is made by chemicalmanufacturers to ensure that plantslisted on labels can be treated safely.However, not every environmental orcultural situation can be predicted oraccounted for when testing products.Therefore, prior to using a new

Figure 11.—Everyherbicide has a labelthat describes how theproduct should be usedand to which crops itcan be applied. Readeach label carefullyprior to use.

Figure 10.—Herbicides form achemical barrier overthe container surface.As weeds germinatewithin the chemicalbarrier, they either arekilled or are inhibitedfrom growing.

herbicide, or usinga familiar herbi-cide on a newcrop, conduct asmall trial toensure the plantand herbicide arecompatible underconditions specificto your productionsystem (regardlessof whether theplant is listed onthe label).

Some labelsspecify whether the product can beapplied to container crops, field crops,or both. Most labels specify whichspecies can be treated; some list thegenus only. The issue is further compli-cated for nurseries because the horti-culture industry frequently introducesnew cultivars, species, and even genera.In Oregon, morethan 2,000 spe-cies, and manymore varieties orcultivars, aregrown commer-cially. For cropsnot listed on thelabel, some manu-facturers allowyou to conducttrials on a smallnumber of plantsto determine foryourself whetherthe herbicide issafe. While this clause is helpful tonursery growers, in Oregon you stillmust obtain an experimental usepermit by the Oregon Department ofAgriculture (ODA).

Chemical barrier

Media


��Dinitroaniline herbicides

There is some concern over usingproducts containing dinitroaniline(DNA) herbicides at potting. Thisclass of chemicals (see Table 1)includes trifluralin, prodiamine,pendimethalin, benefin, and oryzalin.DNA herbicides are root-inhibitingchemicals used in many herbicideproducts. While most crops are tolerantof DNAs regardless of when they areapplied, some crops are sensitive atpotting (azaleas, herbaceous perennials,and some ornamental grasses, forexample) and are subject to lodging,poor root development, and stunting.Use products that do not contain aDNA if previous experience or priorknowledge does not provide you withinformation on crop safety at planting.

Weed susceptibility toherbicide types

Every herbicide has both active andinert ingredients. The active ingredientis the chemical component that actu-ally provides weed control. The inertingredient is a carrier (for granularherbicides) or a substance to facilitateherbicide solubility or coverage (forspray herbicides).

No single preemergence herbicideprovides complete control of all weedspecies. Herbicides are generally effec-tive on either broadleaf weeds or grasses.

The following herbicides are effec-tive in controlling emerging broadleafweeds but provide poor control of mostgrasses: Goal (oxyfluorfen), Gallery(isoxaben), and Princep (simazine).

Herbicides that are effective atcontrolling emerging grasses and somesmall-seeded broadleaf weeds (but notall broadleaf weeds) include: Surflan(oryzalin), Pendulum (pendimethalin),and Factor, RegalKade, or Barricade(prodiamine).

Ronstar (oxadiazon) does not fallneatly into either category. It controlsmany broadleafs and grasses, but whenused alone, it fails to control someweeds, most notably common chick-weed (Stellaria media) and pearlwort.

When a single chemical is used, poorweed control may result due to infesta-tion of uncontrolled species. Table 1lists the types of weeds controlled bypreemergence herbicides used innursery production.

The herbicides mentioned abovethat are effective against grasses andsmall-seeded broadleafs are among themost effective herbicides for contain-ers. This is because many weedsprevalent in container crops are “small-seeded” broadleafs. Bittercress, oxalis,pearlwort, and phyllanthus (Phyllanthustenellus) are small-seeded broadleafs.Nonetheless, using an herbicide combi-nation that contains an active ingredi-ent effective against broadleafs and oneeffective against grasses provides bettercontrol of common container weedsthan any single herbicide alone.

Selecting a granular herbicide basedon prevalent weed species is easybecause most of these products areformulated to provide broad-spectrumweed control. That is, they contain anactive ingredient for controlling grassweeds and one for controllingbroadleafs. For example, Snapshot TGcontains the active ingredients triflura-lin and isoxaben. If using granularpreemergence herbicides, it is a goodidea to use those that contain twoactive ingredients, one for controllinggrasses and one for controllingbroadleafs.

Granular herbicides are loaded ontoa dry carrier. The carrier is the inertingredient used to make applicationeasier. For example, Ronstar 2G


��contains 2 percent of the active ingre-dient oxadiazon and 98 percent inertmaterial. Inert carriers can be clayparticles, crushed corn cobs, a recycledpaper product called biodac, or granu-lar fertilizers.

Spray-applied preemergence herbi-cides are sold with just one activeingredient. Currently, they cannot bepurchased as combination products.Herbicide manufacturers do not supplycombination spray-applied preemer-gence herbicides for nursery cropsbecause blends can be made simply byadding multiple herbicides to a spraytank. A common question, however, is“which herbicides can be tank-mixedtogether?” In terms of chemical com-patibility, herbicide labels eitherprovide this information or explain asimple procedure for testing compat-ibility (mixing the products in a smalljar to observe the reaction). In terms ofeffectiveness, mixing a chemical fromeach of the two groups previouslymentioned generally provides excellentcontrol, superior to either productapplied alone.

The active ingredient for everyherbicide is listed near the top of thelabel or on the front of the bag/con-tainer. Check the active ingredients tomake sure the product you use providescontrol of the weeds prevalent in yournursery. Table 1 lists commonly usedherbicides, their active ingredients, andthe weed groups best controlled byeach product.

Solubility of preemergenceherbicides

Herbicide solubility is a measure ofhow readily an herbicide goes intosolution. Herbicides with high solubil-ity are more likely to go into solution.Herbicides in solution can have nega-tive consequences on plant growth andweed control.

Preemergence herbicides are appliedto the container surface, where theyform a chemical barrier. As weed seedsgerminate, the roots or shoots growthrough the chemical barrier, wherethey contact and/or absorb the herbi-cide. Because container productionutilizes high volumes of water applieddaily, herbicides with a high solubilityare likely to go into solution and beflushed away from the container sur-face. If herbicides are moved away fromthe container surface, the chemicalbarrier will be weakened. Herbicidesthat are flushed through the media alsomay contact crop roots and causeinjury.

Herbicides for container productionshould have a solubility of less than3 ppm. (Table 1 lists the solubility ofeach active ingredient in commonpreemergence herbicide products.) Useherbicides with higher solubility onlywhen specific circumstances require it.For example, metolachlor has a veryhigh solubility, but is currently the onlyherbicide that provides effectivepreemergence control of nutsedge (ornutgrass, Cyperus spp.). Usemetolachlor when this weed is ofconcern (providing the crop beinggrown is on the label).

Rotating herbicide chemistryRotating herbicides is accomplished

by using herbicides from differentchemical families in successive applica-tions. Rotate herbicides as often aspossible, at least once per growingseason. Table 1 lists the chemicalfamilies of herbicides commonly usedin nursery production. With granularherbicides that contain two activeingredients, at least one of the activesshould be from a different chemicalfamily than those in the previouslyapplied herbicide.


��Failure to

rotate herbicidesmay result inweed shifts(Figure 12).A weed shiftoccurs whenconstant use of asingle herbicideresults in gradualbuildup of aweed species

tolerant of that chemical. For example,constant use of Ronstar alone mayresult in a gradual buildup of commonchickweed.

Also, herbicide resistance may occurif herbicides are not rotated. Resistancedevelopment is uncommon, but notimpossible. It has been documentedthat common groundsel is triazine-resistant (Princep is a triazine herbi-cide) due to frequent use of this chemi-cal class in agronomic crops. Therehave been undocumented reports ofcommon groundsel developing resis-tance to dinitroaniline herbicides. Thisresistance could have serious implica-tions in the nursery industry, becausemany of our herbicides are at leastpartly composed of dinitroanilines(including Rout, OH2, Snapshot,Surflan, Pendulum, Factor, Barricade,and Treflan).

Granular vs.spray-applied herbicides

Herbicides are available in either agranular or sprayable formulation.Some are available in both; forexample, pendimethalin can be pur-chased as Corral (granular form) orPendulum 65DG (sprayable form).Table 1 lists common herbicides andthe formulations in which they areavailable.

Granular herbicides generally aresafer for container production because

they fall to the container surfacewithout injuring crop foliage (assumingfoliage is dry at the time of applica-tion). However, granular herbicidesgenerally are more expensive thanspray herbicides (especially shippingcosts).

Because of lower costs, and in somecases a nursery’s equipment and layout,growers may prefer to use spray herbi-cides. Some spray preemergence herbi-cides cause injury when applied tofoliage, some do not. Through carefulexperimentation and record keeping,some nurseries have determined whichcrops can tolerate spray herbicides.Without this type of experience, usegranular herbicides when treatingcontainer crops, as they generally aresafer.

In conclusion, crop tolerance, weedspecies, and herbicide solubility shouldbe considered first when choosing aherbicide. Other factors, such as price,availability, and time of year (seepage 11) also may affect herbicidechoice. Selecting the correct preemer-gence herbicide, in conjunction withusing sound sanitation and culturalpractices, will result in a more effectiveweed management program. As always,read and follow pesticide labels prior toapplication.

Timing preemergenceherbicide applications

Timing of preemergence herbicideapplications cannot be described interms of calendar dates. Each nurseryhas its own schedule and procedures forpotting, overwintering, pruning, etc. Itis more appropriate to discuss herbicidetiming relative to events such aspotting and overwintering, which canbe applied to any nursery regardless ofits production schedule.

Figure 12.—Consistent use of thesame herbicide mayresult in a weed shift orweed resistance.Common groundsel,shown above, isresistant to triazineherbicides and has beenreported to be resistantto dinitroanilineherbicides.


��At a minimum, apply herbicides in

the spring as plants are removed fromoverwintering (or soon after potting),and make another application prior tooverwintering. One or two additionalapplications in the middle of thegrowing season may be necessary. Somegrowers have shown that, with propersanitation and a little hand-weeding,only two applications each year arenecessary. Most nurseries, however,routinely make three or four applica-tions each year.

Apply herbicides priorto weed seed germination

The most important rule for herbi-cide application, the rule that trumpsall others, is that preemergence herbi-cides must be applied prior to weedseed germination. Preemergenceherbicides will not control weed plantspresent at the time of application(Figure 13). One notable exception isspray-applied Goal (oxyfluorfen), whichwill kill weeds less than 4 inches tall.However, it is limited to field use andsome container-grown conifers.

Existing weeds in containers must behand-weeded prior to herbicide appli-cation. Weed plants present at the timeof herbicide application will continueto grow and produce seed, thus per-petuating the problem. Applyingpreemergence herbicides to containerswhere weeds are growing is a waste ofthe herbicide and labor needed toapply it (a costly mistake).

Herbicides at pottingHerbicides can and should be

applied soon after potting. Whenpotting liners or shifting plants intolarger containers, allow two or threeirrigation events to occur prior toapplying an herbicide in order to settlethe substrate (media). For best results,

apply herbicidesafter recentlypotted crops havereceived approxi-mately 1 inch ofirrigation orprecipitation. Forsome herbicides,you should wait2 to 4 weeks priorto applying herbi-cides when potting bareroot plants intocontainers. (Check labels for specificinstructions.)

If herbicides are applied immediatelyafter potting, before settling hasoccurred, macropores in the substratemay allow herbicides to channel andmake contact with plant roots, thuscausing injury or stunting. If herbicidesare withheld for too long after potting,weed seed germination may occur.

Herbicide applicationsin early spring

Crops being removed from overwin-tering structures, held over from theprevious growing season, require apreemergence herbicide application.Herbicides cannot be applied in cov-ered structures (hoop houses, retract-able roof structures, etc.). Eitherremove plants from the covered struc-ture, or remove the covering from thestructure prior to herbicide application.Hand-weed all containers prior toapplication. Preemergence herbicidesare not effective against emergedweeds.

The best time to make this applica-tion is when plants are spaced pot-to-pot. Consider the following example,which compares herbicides applied tocontainers spaced pot-to-pot to thosethat have been spaced apart.

Example: Seventy-five thousand(75,000) 1-gallon shrubs (each pot is

Figure 13.—Preemergenceherbicides will notcontrol weeds presentat the time ofapplication. Evensmall weeds, such asthe oxalis shownabove, will continueto grow and generatenew seed.


��6 inches wide) spaced pot-to-potoccupy roughly 0.4 acre, requiring 80 lbof herbicide (assuming a rate of 200 lb/acre). Spaced just 3 inches apart(assuming no aisle ways), the plantswill occupy roughly 1 acre, requiring200 lb of herbicide. By spacing thecontainers, the herbicide cost jumps250 percent!

Now consider the environmentalimpact of applying herbicides to con-tainers pot-to-pot versus those that arespaced. When the containers in thisexample are pot-to-pot, roughly79 percent of the herbicide will fallinto the containers while 21 percentfalls between the containers and ontothe ground, where it may be washedaway with irrigation or rain. Whenspaced 3 inches apart, roughly35 percent of the herbicide falls intothe containers, while 65 percent fallsbetween the containers (Figure 14)!

These numbers are based on calcula-tions of the container surface area.Actual field measurements have beenmade that verify their accuracy(Gilliam et al., 1992), although theactual percentage of herbicide fallingbetween containers varies based onplant height, canopy shape, etc. None-theless, it is clear that application ofherbicides to containers prior to spac-ing makes sense both financially andenvironmentally.

Herbicide degradationand reapplication

Herbicides degrade over time. Howlong an herbicide persists depends onseveral factors, including light, tem-perature, and substrate moisture.Several processes are responsible forherbicide degradation, includingphotodegradation, chemical degrada-tion, microbial degradation, leaching,and volatilization. Photodegradationoccurs when ultraviolet (UV) lightbreaks chemical bonds of theherbicide’s active ingredient. Second-ary molecules resulting from thecleavage of the parent molecule gener-ally are less effective in providing weedcontrol.

Microbial degradation occurs whensoil microorganisms use the herbicidesas a food source. Virtually all pesticidesare organic compounds made up mostlyof carbon, hydrogen, nitrogen, andsulfur. These compounds are a foodsource for microorganisms. When thesame chemical is applied repeatedly,microorganisms that preferentially feedon the chemical increase in number,consuming the chemical and thusreducing its efficacy and longevity ofcontrol. This factor is particularlyimportant with preemergence herbi-cides, which are designed to havelongevity in container media and soil.

Chemical degradation occurs when anonbiological chemical reactioncleaves the herbicide’s active ingredi-ent into nonactive secondary mol-ecules. The most common form ofchemical degradation is hydrolysis(chemical cleavage through reactionwith water).

Volatilization is the process by whichchemicals go from a solid or liquid stateinto a gaseous state. If herbicidesvolatilize to a gas and are released into

In Out In Out

Pot-to-pot Spaced 3 inches Spaced 5 inches

In Out

Figure 14.—Herbicides fall either inor out of (between)containers dependingon the pots’ spacing.


��the atmosphere, they no longer provideweed control and may even causeinjury to desirable crops. Of preemer-gence herbicides labeled for nurserycrops, trifluralin is most subject tovolatilization.

Herbicides are most prone to volatil-ization and photodegradation immedi-ately after application, another reasonwhy incorporation of the herbicide byirrigation is critical. Most preemer-gence herbicide labels recommendirrigation immediately after applicationwith approximately 0.5 inch of water.Incorporation with irrigation will movethe herbicide into the container; thus,little herbicide will be lost to volatiliza-tion and photodegradation, and aneffective chemical barrier will improveweed control. Without incorporationof the preemergence herbicide, efficacyof the product and subsequent weedcontrol will be reduced. (Note thatlabels on foliar-applied postemergenceherbicides, in contrast, recommend adry period after application to allowthe herbicide to be absorbed by weedfoliage.)

Over the course of the growingseason, microbial and photodegra-dation are the primary means of herbi-cide degradation for the products mostcommonly used by container growers.Because of degradation, herbicidesshould be reapplied every 60 to 90 daysif the plant canopy is not sufficient toexclude weed growth.

Application of preemergenceherbicides prior tooverwintering

If containers will be covered or setinside an enclosed structure for thewinter, apply preemergence herbicides2 to 4 weeks prior to covering

Figure 15.—Whetherplants are placed in apermanent structure orunder a plastic film,apply herbicides atleast 2 to 4 weeks(check label) prior tocovering withoverwinteringstructures.

(Figure 15). Check the label forspecific recommendations. Again, allherbicides volatilize (turn to gas) atsome level, and they are especiallyprone to volatilization soon afterapplication. The major problem withusing herbicides inside an enclosedstructure is that the volatilized herbi-cides will injure plants by directabsorption of vapors or herbicidecondensation on the foliage.

Weed growth while plants areoverwintering can be devastating.Winter annuals such as bittercress,oxalis, fireweed (Epilobium angusti-folium), and common groundsel canoverwhelm a crop while overwinteringunless an herbicide or some other formof weed control is used.

ConclusionHerbicide timing is critical, both in

terms of providing effective weedcontrol and minimizing potential cropinjury. Always check herbicide labelsto make sure you are following manu-facturer recommendations regardingapplication timing.

Weed control is most effective whenherbicides are applied at the properrate, the proper time, and used inconjunction with good sanitation.


��Literature citedBachman, G. and T. Whitwell. 1995.

Hairy bittercress seed production,dispersal, and control. Proc. South.Nurs. Res. Conf. 40:288–290.

Briggs, J., T. Whitwell, G. Legnani, andM. Riley. 1998. Preemergentbittercress control on gravel beds.Proc. South. Nurs. Res. Conf.43:407–410.

Cross, G.B. and W.A. Skroch. 1992.Quantification of weed seed con-tamination and weed developmentin container nurseries. J. Environ.Hort. 10:159–161.

Darden, J. and J.C. Neal. 1999. Granu-lar herbicide application uniformityand efficacy in container nurseries.Proc. South. Nurs. Res. Conf.44:427–430.

Gilliam, C.H., D.C. Fare, andA. Beasley. 1992. Nontarget herbi-cide losses from application ofgranular Ronstar to container nurs-eries. J. Environ. Hort. 10:175–176.

Meadows, W.A. and D.L. Fuller. 1984.Plant quality and leachate effluent asaffected by rate and placement ofOsmocote and SREF on containergrown woody ornamentals. Proc.South. Nurs. Res. Conf. 29:75–79.

Svenson, S. 1998. Suppression ofliverwort growth in containers usingirrigation, mulches, fertilizers andherbicides. Proc. South. Nurs. Res.Conf. 43:396–402.

Svenson, S., J. Paxson, and K. Sanford.2001. Composts and shading influ-ence Marchantia infestations incontainer grown nursery crops. Proc.South. Nurs. Res. Conf. 46:445–447.

Wilson, C, T. Whitwell, and M. Riley.1994. Ground covers influenceisoxaben and trifluralin content insimulated nursery runoff water. Proc.South. Nurs. Res. Conf. 39:53–57.

For more informationOregon Pesticide Applicator Training

Manual: A Guide to the Safe Use andHandling of Pesticides, EM 8532(Oregon State University, Corvallis,reprinted 2003).

How Herbicides Work: Uptake, Translo-cation, and Mode of Action, EM 8785(Oregon State University, Corvallis,2001).Many OSU Extension Service

publications may be viewed or down-loaded from the Web. Visit the onlinePublications and Videos catalog athttp://eesc.oregonstate.edu

Copies of many of our publicationsand videos also are available from OSUExtension and Experiment StationCommunications. For prices andordering information, visit our onlinecatalog or contact us by fax(541-737-0817), e-mail ([email protected]), or phone(541-737-2513).


��

Use herbicides safely!

➤ Wear protective clothing andsafety devices as recommendedon the label. Bathe or showerafter each use.

➤ Read the herbicide label—evenif you’ve used the productbefore. Follow closely theinstructions on the label (andany other directions you have).

➤ Be cautious when you applyherbicides. Know your legalresponsibility as a pesticideapplicator. You may be liable forinjury or damage resulting fromherbicide use.

© 2003 Oregon State University.

Trade-name products and services are mentioned as illustrations only. This does not mean that the Oregon State University ExtensionService either endorses these products and services or intends to discriminate against products and services not mentioned.

Produced and distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. Extension work is a cooperative program ofOregon State University, the U.S. Department of Agriculture, and Oregon counties. Oregon State University Extension Service offerseducational programs, activities, and materials—without discrimination based on race, color, religion, sex, sexual orientation, nationalorigin, age, marital status, disability, or disabled veteran or Vietnam-era veteran status. Oregon State University Extension Service is anEqual Opportunity Employer.

Published April 2003.

em 8823 • april 2003 $3.00 weed control in container crops · 2008-02-08 · 2 • weed control...

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