Grower 101: Managing Weeds in Outdoor Cut Flowers By Tina Smith

Don't let weeds take much-needed water, nutrients and light from your annuals, biennials and perennials. Find out how to stop them.

Weeds compete for water, nutrients and light, resulting inreduced flower yield and increased threat of serious insect and diseaseproblems. A successful weed management program utilizes cultural practices suchas cultivation and mulching, or a combination of cultural and chemicalmeasures, taking into consideration labor costs and the cost and availabilityof materials.

Weed management begins with a survey of the site. Weedsshould be identified, and the level of weed pressure noted. Weeds can beclassified according to their life cycles; knowing the weed life cycle isimportant in determining the optimal timing for cultural management practicesor herbicide applications. Summer annuals emerge in the spring, flowerthroughout the summer and set seed before the first frost. In cultivatedfields, summer annuals tend to predominate as the primary weed type. Winterannuals germinate at the end of the summer or early fall, overwinter, thenflower and fruit in the summer. Biennials need at least two seasons to completetheir life cycles, and like annuals, biennials die after flowering. Perennials,which survive more than two seasons, can propagate by seed or vegetatively.Vegetative reproductive organs such as tubers, rhizomes (underground stems),stolons (above-ground creeping stems), bulbs and corms are often resilient toboth cultural and chemical control measures and should be targeted for controlbefore planting the field. It is also important to scout weed populationsduring and after the growing season to assess the success of the weed controlprogram.

Herbicides are available that may be safely used to controlweeds in cut flowers. However, in many situations, herbicides cannot be used orare not effective in controlling all the weeds. Even if effective herbicidesare available, growers should utilize cultural practices that reduce weedinfestation and spread. These practices will be especially important whereherbicides cannot be used.

Cultural Practices

Field Preparation.Minimize weeds by preparing the field the year prior to planting. Begin byfrequently tilling the soil 2-3 times early in the season until mid-July.Frequent tillage reduces weed seed populations by bringing weed seeds to thesurface where they will germinate, grow a little and then be tilled under.Allow weeds to germinate and grow from July until fall, then apply glyphosate(Roundup-Pro) over the area. This will kill many perennial weeds that are best controlledin the early fall when nutrients are being stored in the root system. The fieldis now ready for planting.

Cultivation and Spacing.Some growers have minimized weed competition with a combination of earlycultivation and narrow, between-row spacing. This can be effective if the cropgets a head start on the competition. If the crop’s growth is impeded in anyway, the weeds will take over. Regular cultivation can limit weed competitionbetween rows. However, cultivation can injure the roots of some cut flowers,contribute to erosion and result in some water losses due to increasedevaporation. Also, deep cultivation tends to bring additional weed seeds to thesurface where they will germinate, grow and compete with crops. For bestresults, cultivation should always be shallow and directed at young weeds tolimit the destruction of crop roots. In-row cultivation is particularlydifficult and typically requires hand-hoeing or hand-weeding.

Mulches. Mulches caneffectively control most annual weeds from seed. According to Dr. AndrewSenesac of the Long Island Research Laboratory, black plastic or a geotextilefabric such as Weedblock can control most annual weeds around field-grownherbaceous perennials. In some studies, however, due to the physical restrictionof the spreading shoots, these mulches reduced Achillea and Stachys flowerproduction. Spreading perennials that do not produce adventitious roots did notshow any significant yield differences from the controls. Organic mulches suchas bark, straw and composted yard wastes effectively control many annual weeds.Some growers use rotted sawdust, wood chips, spoiled hay and straw. If notcomposted properly, sawdust and wood chips will rob the soil of nitrogen. Barkmulches can be used but are often too costly. Hay generally contains too manyweed seeds and often increases the weed pressure. Clean, weed-free straw isoften the most cost-effective mulch available, but some growers may find othereconomical alternatives. Organic mulches should be applied to weed-free, warmsoil soon after planting. To be most effective, they should be applied in alayer 2-3 inches deep.

Chemical Control — Herbicides

Herbicides can be classified into two general-usecategories: pre-emergent and post-emergent. Pre-emergence herbicides areapplied before the weeds germinate but after the crop has been planted.Post-emergence treatments are applied after the weeds have emerged. Herbicidesmay also be classified based on their selectivity. Non-selective herbicideswill control any herbaceous plants that they contact. Selective herbicides willcontrol or suppress only certain types of plants or weeds.

Herbicides are available in several formulations. Usually,the sprayable formulations (emulsifiable concentrates, wettable powders, dryflowables and water-dispersible granules) are less expensive than granularformulations. But granular formulations are often safer on transplanted cutflowers than are the sprays (due to reduced foliar absorption). Sprayableformulations can be applied through a tractor-mounted sprayer or by hand-heldbackpack sprayers equipped with a spray boom. With a backpack sprayer, maintaina constant foot pace, even spray pressure and uniform nozzle sizes. Regardlessof the formulation or equipment used, it is important to apply herbicides asuniformly as possible.

When applying an herbicide, the square footage of the areato be treated and the calibrated sprayer/spreader output (amount per area)should be known. Misapplication of the chemical can result in poor weed controlor injury to the crop. Read and follow all label directions before applying anychemical. A sprayer that is to be used for herbicides should be labeled as suchand used only for that purpose.

The following is a partial listing of herbicides that can beused in cut flower production. Because of the wide variety of cut flowerspecies grown, it is difficult to recommend any one herbicide that will coverall crops. Due to labeling restrictions, possibility of crop injury, limitedmarket and difficulty in obtaining new labels, many chemical companies do notactively pursue cut flower labels. It is the user’s responsibility to followlabel instructions, especially prior to purchasing a product. Some labelsprovide scientific names and common names; some list common names; and someprovide a combination.

Nonselective, post-emergence herbicides

Nonselective herbicides can be used to control weeds in afield prior to planting or to spot-spray weeds growing between crop rows. Careshould be taken in selecting an herbicide to ensure that there will be noresidual chemical present that could damage the crop. Chemicals that would beused for this purpose include glyphosate (Roundup-Pro), glufosinate (Finale),paraquat (Gramoxone), diquat (Reward) and pelargonic acid (Scythe). Do notapply these herbicides over the top of cut flowers; plants will be injured orkilled.

Glyphosate (Roundup-Pro).Glyphosate is absorbed bygreen tissue and translocated to the root system of the plant. Since there isno residual soil activity, a crop can be seeded or transplanted into the fieldsoon after application. Actively growing weeds are much more susceptible to theherbicide. Many perennial weeds are best controlled in the early fall whennutrients are being stored in the root system. Glyphosate is effective forfield preparation to control perennial, broadleaf and grass weeds.

Glufosinate (Finale).Glufosinate is also a translocated herbicide, but not as well-translocated as glyphosatein perennial weeds. Like glyphosate, it has no soil residual activity and canbe used as a site preparation treatment or as a spot spray to control escapedweeds. Since glufosinate is not as well-translocated as glyphosate, completespray coverage is essential to obtain maximum control, and good control ofperennial weeds may not be achieved.

Diquat (Reward), Paraquat (Gramoxone) and pelargonic acid(Scythe). These are contact herbicides(i.e, they kill foliage on contact but are not translocated in the plant) andhave no residual soil activity. They will suppress many species of annualgrasses and some broadleaf weeds. Repeated applications may be needed to weakenand suppress perennial weeds. Complete coverage is essential.

Selective, post-emergence herbicides

Sethoxydim (Vantage), clethodim (Envoy) and fluazifop-p(Fusilade II) control most annual and perennial grasses, whilefenoxaprop-p-ethyl (Acclaim Extra) controls summer annual grasses. They can beapplied over the top of many broadleaf crops when grasses are actively growingand before they reach maximum size. When applied to a labeled cut flower crop,all open flowers should be harvested before application to avoid injury. Do notuse spray adjuvants with Vantage. With Envoy and Fusilade II, use only thespray adjuvants specified on the labels. Use of non-labeled spray adjuvants mayresult in contact burn on cut-flower foliage and buds. Additionally, to avoidover-dosing and associated crop damage, these herbicides should be applied onan area basis, not a spray-to-wet basis.

Some flowers on the Vantage label include antirrhinum,centaurea, chrysanthemum, dahlia, Dianthus barbatus, Dianthus deltoides,gladiolus, gypsophila, iris, physostegia, rudbeckia and tagetes; plus somevarieties of aster, celosia, coleus, gerbera, lavandula, limonium, salvia andzinnia.

Some flowers on the Fusilade II label include achillea,ageratum, Alcea rosa, antirrhinum, calendula, campanula, coleus, Coreopsisverticillata, Dianthus barbatus, iris, heuchera, liatris, sedum, Staticesinuata, tagetes and zinnia.

Envoy is labeled for over-the-top applications to achillea,ageratum, antirrhinum, chrysanthemum, coleus, dahlia, dianthus, gazania,heuchera, iris, pansy, phlox, salvia, tagetes and some varieties of zinnia. Envoyis the only post-emergence selective grass herbicide that controls annualbluegrass (Poa annua).

Acclaim is more effective on young, actively growing grassyweeds than on large grassy weeds. Some flowers on the Acclaim label includeachillea, antirrhinum, aquilegia, astilbe, campanula, centaurea, chrysanthemum,coreopsis, cosmos, delphinium, dianthus, dicentra, doronicum, echinacea,gazania, gypsophila, iberis, iris, liatris, oenothera, paeonia, papaver,rudbeckia, Statice sinuata and zinnia.

Pre-emergence herbicides

To prevent seedling weeds from emerging in a crop, apre-emergence herbicide may often be used. Several pre-emergence herbicides areavailable for controlling annual grasses and small-seeded broadleaf weeds, butlarge-seeded broadleaf weeds are not as easily controlled. Careful weedscouting can identify hard-to-control species and facilitate the selection ofthe most effective herbicide for the crop. If pre-emergence herbicides are tobe used, be sure they are labeled for use on the crop to be grown. Also, in amixed field of cut flowers, all species being grown should be listed on theherbicide label.

Cut flowers are usually started from transplants, divisionsor tubers, but sometimes they are grown in the field from seed. Generally, pre-emergenceherbicides should be applied after transplanting. Most direct-seeded flowersare more susceptible to damage from pre-emergence herbicides than transplantedseedlings. To achieve the same level of safety, the herbicide usually Áshould not be applied until after plants emerge and are established. Each ofthe herbicides described below should be watered-in to “activate,” ormove, the herbicide into the soil where it can be absorbed by germinating weedseeds.

Bensulide (Bensumec 4LF). This controls crabgrass, annual bluegrass, other annual grasses and afew broadleaf weeds for 3-4 months. Ornamentals need to be well-establishedbefore the application of bensulide. Some labeled flowers include aster,bachelor’s button, calendula, campanula, coral bells, daffodil, dahlia, daisy,freesia, gazania, gladiolus, marigold, pansy, primrose, ranunculus, stock,sedum, sweet pea, tulip, wallflower and zinnia.

Dithiopyr (Dimension).It is primarily used to control crabgrass in turf but is labeled for annualgrass and small-seeded broadleaf weed control in several flowers, includingachillea, antirrhinum, aquilegia, celosia, centaurea, coleus, coreopsis,delphinium, dianthus, iberis, iris, monarda, narcissus, osteospermum, pansy,rudbeckia, salvia, sedum, tagetes, tulip, zinnia and others.

Napropamide (Devrinol).This will control certain annual grasses and annual broadleaf weeds. Flowers onthe label include African daisy, aster, chrysanthemum, dahlia, daisy,gladiolus, narcissus and zinnia. In field trials, high rates caused yieldreductions in zinnia and marigold. For effective control, the chemical must bewatered-in after application.

Oryzalin (Surflan A.S. Specialty). This chemical controls most annual grasses and manyannual broadleaf weeds and should be applied only to established plants.One-half-inch of rainfall or irrigation is needed to activate oryzalin. Flowerson the label include aster, Callistepheus chinensis, campanula, digitalis,doronicum, gaillardia, heuchera, iberis, limonium, osteospermum, sedum, achillea,antirrhinum, coreopsis, dianthus, dicentra, echinacea, gladiolus, gypsophila,hyacinth, iris, liatris, narcissus, ranunculus, rudbeckia, salvia, tagetes,tulip and zinnia. However, severe injury has been observed on transplantedcelosia, begonias, gomphrena, salvia, phlox and several other species. SurflanXL is a granular formulation containing oryzalin plus benefin that, in researchtrials, has been safer for transplanted herbaceous ornamentals thanspray-applied Surflan.

Trifluralin (Treflan 5G). This herbicide controls annual grasses and a few broadleaf weeds forabout 6-8 weeks. It is volatile and must be incorporated by irrigationimmediately after application. The granular formulation is more often used toreduce vapor losses. Treflan is probably the safest herbicide on transplantedcut flowers discussed herein; however, it is the weakest on broadleaf weeds.Flowers on the Treflan label include : achillea, ageratum, antirrhinum, aster,calendula, centaurea, chrysanthemum, coreopsis, dahlia, dianthus, dicentra,digitalis, echinacea, gaillardia, gypsophila, heuchera, iris, lavandula,liatris, limonium, matthiola, monarda, oenothera, papaver, phlox, rudbeckia,salvia, scabiosa, sedum, tagetes, zinnia and more.

Metolachlor (Pennant MAGNUM). This is another pre-emergence herbicide that controls annual grasses,but its main use is for pre-emergent control of yellow nutsedge (Cyperusesculentus) from tubers. Pennant does not control purple nutsedge and iscurrently only available as an emulsifiable concentrate formulation that canburn tender foliage. Pennant is labeled for use on achillea, ageratum, allium,antirrhinum, aquilegia, artemesia, aster, campanula, chrysanthemum, coreopsis,delphinium, dianthus, gaillardia, gladiolus, hyacinthus, iris, lilium, lythrum,oenothera, ornithogalum, phlox, physostegia, pedum, Statice sinnata, tagetes,tulips, zinnia and a few other species. Injury to gladiolus and zinnia has beenreported.

Herbicide labels can be obtained from the Web siteswww.cdms.net, www.bluebooktor.com/index.html and www.greenbook.net.

This article was reprinted with permission from FloralNotes.

The author would like to thank Joseph C. Neal, NorthCarolina State University, and Richard Bonanno and Randall G. Prostak,University of Massachusetts Extension.



Tina Smith

Tina Smith is an outreach educator in the Extension Floriculture Program at the University of Massachusetts, Amherst, Mass.



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