Postemergence Weed Control:Helping Turf On The Rebound

Dallisgrass. Photo courtesy: Max E. Badgley.

24 sportsTURF

To a sports turf manager, weeds in anew or renovated turf area are likethe first dent in a new car. After all

the time and work spent to obtain a flaw-less product, suddenly the glow of achieve-ment fades into the sobering prospect ofmaking payments for the next few years forsomething that isn't as special as it was theday it was new.

You can't keep a new car in the garageall the time and you can't keep athletes offsports turf. They are both built to be usedfor important purposes, and only regularmaintenance can keep them looking andperforming like new. The day you stop main-taining turf is the day the weeds move in.Without weed control, a field or golf coursethat could last forever falls apart in just oneseason. To make another automotive anal-ogy, we aren't talking about a little old lady'scar in sports turf, weare talking about a com-mercial vehicle with heavy use under toughoperating conditions. The turf is gouged andtrampled, mowed low, aerated, verticut,played on when injured or dormant, andgrows in soil that is often compacted. Yet,with proper care and use, it rebounds withindays for its next event.

Particular weeds find the worn patchesof high-use turf suited to their own estab-lishment and growth. They thrive on com-pacted soil where the turfgrass has beenopened up or thinned with play. Weed seedlies dormant in the soil waiting for a golfclub, cleats or elbow to clear its path to thesurface for germination and growth. Fre-quently, when temperature and moistureconditions are optimum for weed seed ger-mination, use of the playing surface is atits greatest.

Extension turf specialists will tell you thebest defense against weeds is a dense,healthy turfgrass. All plants compete forspace. Once established, the heathiest andmost aggressive plant wins the battle.

A significant challenge to the sports turfmanager occurs when a sports season takesplace while the primary turfgrass is eitherdormant or under stress from heat ordrought. For northern climates, this takesplace during the late fall and winter, whenKentucky bluegrass, ryegrass, tall fescueand bentgrass grow very slowly if at all.

High summer temperatures and droughtcan also hurt the competitiveness of cool-season turf in the North or Central (transi-

The grass sandbur. Photo courtesy: Max Badgley.

tion zone) regions. In the South, the warm-season grasses enter dormancy beginningin October and do not become aggressiveagain until April or May.

For these reasons, warm-season grassesin the South are frequently overseeded inthe fall. Sports turf managers in the transi-tion zone and parts of the upper South areutilizing cold-tolerant bermudagrasses, suchas Midiron, or turf-type tall fescues to reduceweed invasion during the summer. In allparts of the country, irrigation systems area vital part of summer weed control sinceeven a two-week dry spell can favor theencroachment of weeds.

Perennial ryegrass has become a majorfactor in maintaining cover and turfdominance over weeds in much of the coun-try. It is the primary grass for winter over-seeding in the South. Because it germinatesso rapidly, it is also used heavily in otherparts of the country to fill in thin or bare spotson golf tees, football fields and soccer fields.Wear tests in England and at various univer-sities across the United States have shownthat certain perennial ryegrasses stand upto sports extremely well once established,

Sports turf managers and golf coursesuperintendents who use perennial ryegrassto overseed warm-season grasses shouldselect varieties with low heat tolerance.Ryegrass can be considered a weed inwarm-season grasses if it competes toostrongly with bermudagrass or other warm-season grasses as they come out of dor-mancy in the spring. However, late springgolf tournaments, baseball and soccer sea-sons have forced some sports turf managersto maintain the ryegrass through May.

Since overseeding and reseeding are soimportant to maintaining aggressive turf,sports turf managers must either time appli-cations of preemergence herbicides verycarefully or depend completely uponpostemergence herbicides for weed con-trol. See the story in the February issue on"Scheduling Preemergence Weed Control."

The primary times to apply preemergenceapplications are in the spring and late sum-mer. Unfortunately, these are also the besttimes for sports turf managers to fit over-seeding and reseeding into their work sched-ules. All preemergence herbicides, exceptsiduron (Tupersan), will affect turf seed ger-mination.

If Poa annua, annual bluegrass, is not amajor problem, weed control specialists sug-gest sports turf managers forgoe fall appli-cations of preemergence herbicides and todo as much spring renovation and reseedingin the fall instead of the spring. Then theycan utilize preemergence herbicides in thespring, when weed seed germination is mostsevere.

Renovation and overseeding commonbermudagrass fields and fairways in the latespring should also be scheduled after pree-memergence herbicides in the soil havebeen exhausted. In most cases, herbicideresidues in the soil are below control levelsafter 30 to 60 days. Always check the label;the length of weed control varies accord-ing to rates and products used.

Even with preemergence applications,surface damage caused by sports activitycan create openings in the herbicide bar-rier in the soil. Furthermore, some majorweeds continue to germinate through thespring and summer, long after their peakgermination period in late winter or spring.As a result, postemergence herbicidesremain very important. Fortunately, the typesof weeds that commonly invade sports turfcan be narrowed down to a few by exten-sion turf specialists. They can recommendeffective control measu res for nearly all ofthe primary weeds.

A convenient way to look at weeds alsoexists. All plants can be divided into twogroups, annual or perennial. Annual weedslive for one year and reproduce by seed.Examples of annual weeds are annualbluegrass, crabgrass, knotweed, henbit, fox-

continued on page 26

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April, 1988 25

Postemergence Weed Controlcontinued from page 25

tail, prostrate spurge, hen bit and lamb-squarters. Each year these plants flower andproduce seed for the next generation. Thisseed falls to the ground, where it may liedormant for months or even years.

Perennial weeds live for more than oneyear (biennials live for two years) andreproduce both by seed and vegetatively.Vegetative reproduction is growth of newplants from parts of old plants, such as roots,stolons, rhizomes, bulbs and tubers. Exam-ples of perennial weeds are goosegrass,nutsedge, dandelion, dallisgrass, clover,chickweed, wild garlic, oxalis, Virginia but-

tonweed, yellow wood sorrel, andquackgrass.

While preemergence weed control canprevent seed in the soil from germinating,postemergence weed control can kill theweed before it produces more seed. Bysterilizing soil prior to planting, weed seedand reproductive plant parts in the soil canbe destroyed. Nature eventually restoresthe bank of weed seed in the soil with seedblown onto sports turf from adjacent sites,deposited by birds, or tracked in on shoes.

Perennial weeds can invade an area asseed first, then get a foothold by spread-ing vegetatively. Only recently have new her-bicides been developed that can control the

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vegetative structures of some major peren-nial weeds.

For control purposes, weeds are furtherdistinguished as either broadleaf or grassy.Herbicide manufacturers have createdproducts to selectively remove broadleafweeds and grassy weeds from desirablegrasses. The phenoxies, including 2,4-D,MCPP (mecoprop), and 2,4-DP (dichlorprop),and dicamba (Banvel) have long been usedeffectively for broad leaf weed control.Triclopyr (Turflon) is relatively recent additionto the line of broadleaf herbicides for turf.

Two-, three- and four-way combinationsof these products are available commerciallytoday for broad-spectrum broadleaf weedcontrol. A few examples are Trimec, Trex-San, Weedone DPC, Triamine, Lesco Three-Way and Turflon-D. Generally, these her-bicides should not be used soon before orsoon after seeding. Bromoxynil (Buctril) isa selective broad leaf herbicide that can beused shortly after seeding.

The arsenates, including MSMA, DSMAand CMA, have been the backbone of grassyweed control for many years. While they con-trol most annual grassy weeds, they are noteffective long-term on perennial grassyweeds.

Sports turf managers have been able touse systemic grassy herbicides such as ben-tazon (Basagran) and imazaquin (Image)to control the vegetative structures of majorperennial grassy weeds. Other productswhich have helped control of grassy weedsare metribuzin (Sencor), fenoxaprop-ethyl(Acclaim), ethofumesate (Prograss) andpronamide (Kerb).

In dormant turf, postemergence non-selective herbicides are being used to controlwinter weeds which stand out as greenpatches in otherwise tan turf. TAG and caco-dylic acid kill the exposed green foliage ofweeds active in the winter. Glyphosate(Roundup) moves inside the weed to kill thefoliage and the roots. A second advantageof glyphosate is that it lasts only a few daysand is deactivated upon contact with soil.After a week, seed can be planted withoutany harm from residue. Heavily weed-infested turf areas can be chemically reno-vated in this manner.

Each region of the country has certainweeds that frequently invade sports turf. Therest of this article is devoted to advice fromextention turf specialists and other expertsfrom across the country. They focus onweeds that are found in sports turf in theirarea during the summer and those that arecommon in the winter.

For 30 years, Dr. Richard Skogley hasdispensed turf advice at the University ofRhode Island in Kingston. The weed thathas been causing the most problems latelyin ryegrass and Kentucky bluegrass sportsfields and fairways is knotweed. The broad-leaf weed excels in compacted, worn turfnear cart paths, on soccer goal boxes andin the center of football fields.

"What makes knotweed so hard to con-trol," explains Skogley, "is it germinatesbefore crabgrass. Sometimes it's already

up and growing before all the snow melts.Preemergence applications timed for crab-grass will miss germinating knotweed. For-tunately, dicamba is very effective on knot-weed. The fact that fields have knotweedindicates that more fields are needed to carrythe load."

Timing is not the only problem with pree-mergence weed control for knotweed andcrabgrass. Long rainy periods in the springextend the germination of crabgrass beyondthe residual life of the herbicides. Skogleystates that applications of three-way her-bicides containing dicamba effectively knockout knotweed, crabgrass, dandelion andplantain in sports turf.

However, most schools renovate theirfields in the spring instead of the fall. Forthem, Skogley recommends applying sid-uron for preemergence weed control anddicamba for the knotweed a few weeks priorto renovation. Once the turf is reseeded,applications of bromoxynil (Buctril) will con-trol young broad leaf weeds without harm-ing the ryegrass or bluegrass.

Yellow nutsedge. Photo courtesy: LarryLeuthold.

According to Skogley, golf course superin-tendents are using fenoxaprop (Acclaim) incombination with 2,4-0 to control annualbluegrass and a variety of other weeds inryegrass in the late spring. "Poa is a bigproblem here, largely in areas that are over-irrigated, compacted, poorly drained, andhave grasses that aren't adapted to sportsin Rhode Island, such as fine fescue andtall fescue," adds Skogley. "More and moresuperintendents are realizing that poainfestation is related to management. Theycan see that too much water, too much fer-tilizer and mowing too close are encouragingthe weed." Since poa germinates in the fall,Skogley urges superintendents to fit appli-cations of preemergents around fall reno-vation.

Prostrate spurge is an annual broadleafweed that invades some heavily used turfareas in July and August. Skogley says thatthe preemergence herbicide pendamethalinand the postemergence herbicide dicambamake control of prostrate spurge effectivein Rhode Island.

Knotweed is also a problem in Kansas,says Larry Leuthold, at Kansas State Univer-sity in Manhattan. Phenoxies can kill youngknotweed early in the spring, but dicambais needed later in the season. "Knotweed

is a good indicator of compaction," statesLeuthold. "Part of control should be moremanagement to correct compaction." Oryza-lin (Surflan) and OCPA (Oacthal) do a goodjob as preemergents on knotweed if you getthem down early enough, he adds.

In addition to crabgrass, foxtail and com-mon broadleaf weeds, Leuthold is especiallyconcerned about grass sandbur and punc-ture vine. Both form spiny burs in late springthat are painful to players when they fall onthe weeds. "You have to control these weedsbefore the burs form," he says. "Once theyform, you can kill the plant, but the bursdry out, get hard and really hurt when youfall on them."

Sports turf managers can control grasssandbur, foxtail and crabgrass together withrepeat applications of MSMA or OSMA. Incool-season turf, fenoxaprop (Acclaim) canbe combined with preemergence herbicidesto control all three weeds. Goosegrass isanother grassy weed that is effectively con-trolled with fenoxaprop. However, fenox-aprop can't be applied to bermudagrass.Trimec-type herbicides will take care of thepuncture vine.

Yellow nutsedge is a hard-to-control weedin Kansas sports turf. Applications of MSMAwill knock back the foliage but won't kill thereproductive nutlets in the soil. Professional

continued on page 28

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Postemergence Weed Controlcontinued from page 27

applicators can use the systemic herbicidebentazon (Basagran) to kill the nutlets.

Kansas, being in the transition zone, hasa variety of turfgrasses on its fields and golfcourses. Leuthold says that while most golfcourses have bluegrass/ryegrass fairways,a growing number of courses areexperimenting with zoysiagrass. Golfers likethe zoysia because it holds the ball uprightmore than bluegrass. However, the zoysiais slow to establish, fills in divots slowly, andhas a long dormant season. Because zoy-sia is so dense, overseeding with ryegrassis difficult. Glyphosate is used to controlwinter weeds in both dormant zoysia andbermudagrass in the state.

"Don't assume the turf is dormant," saysLeuthold. "Check the undergrowth to makesure the stems and tillers aren't still green."Midiron bermudagrass is a winter hardy ber-mudagrass developed at Kansas StateUniversity with a relatively short winter dor-mancy. The majority of athletic fields in thestate are tall fescue with a trend towardperennial ryegrass. A growing problem isberrnudaqrass invading cool-season turf,especlallytert in full sun. At the present time,the only way to correct the situation is tochemically renovate the invaded area withglyphosate and to reseed. Leuthold pointsout that water used to apply glyphosate mustnot be muddy. Even dusty turf will reduce

Prostrate spurge. Photo courtesy: Larry Leuthold.

the effectiveness of the material.As in Rhode Island, prostrate spurge

invades heat- and drought-stressed cool-season turf in Kansas during the summer.Leuthold recommends Trimec-type herbi-cides for postemergence control of spurgeand applications of either pendamethalinor oxadiazon (Ronstar) for preemergencecontrol. An occasional weed in sports turfis wild violet. Leuthold suggests triclopyrplus 2,4-0 (Turflon-D) to control it.

Leuthold's neighbor, Dr. David Minner at

the University of Missouri in Columbia, saysmany weed problems can be avoided dur-ing spring renovation. Nutsedge, carpetweedand goosegrass take advantage of fieldsopened up for renovation or resodding. "Ifyou try to seed in May, you'll get lots ofgoosegrass," says Minner. If late springseeding is unavoidable, he recommendsapplying siduron (Tupersan) before seed-ing followed by applications of bromoxyniland/or fenoxaprop.

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including knotweed, that emerge, and fenox-aprop will take care of goosegrass, crab-grass, foxtail and some broadleaf weedsin ryegrass or tall fescue turf. Yellow nut-sedge control is best accomplished by twoapplications of bentazon, one in early Juneand a repeat application later in the summer.

There is no effective preemergence con-trol of dallisgrass, says Minner. For the timebeing, multiple applications of MSMA mustbe made where the weed is a problem. Pros-trate spurge is a common problem but iscontrolled effectively with late spring appli-cations of Trimec-type herbicides and pree-mergence herbicides earlier in the year.

"The number one technical problem inthe South Central U,S,," reports Dr. EuelCoates at Mississippi State University, isa perennial, subtropical flower called theVirginia buttonweed. The summer weedspreads vegetatively and by seed. So far,ester formulations of combination postemer-gence herbicides, such as Weedone DPCEster, Super Trimec and Tri-Ester, will knockthe buttonweed back for four weeks to twomonths. The problem often crops up on teesor when a superintendent widens a greeninto a collar area. Buttonweed survives shortcutting height and thrives on high moisture.Coates says a more effective product isneeded to get full control of the weed.

Control of prostrate spurge is improving,says Coates, as sports turf managers startusing surfactants with postemergence com-

Knotweed. Photo courtesy: Larry Leuthold.

bination herbicides containing MCPP. Weakturf encourages spurge and yellow woodsorrel, another summer broadleaf weed con-trolled fairly easily with three-way or two-way herbicides containing both 2,4-0 and2,4-DP.

Summer grassy weeds on Coates' "hit list"are goosegrass, crabgrass, and purple nut-sedge. Goosegrass control in warm-seasonturf is improved by combining MSMA withmetribuzin (Sencor) and a surfactant. Theapplication should be repeated five to sevendays later.

Purple nutsedge was very difficult to con-trol with MSMA, but a new herbicide, imaza-quin (Image), has brought it under control.A combination of MSMA and imazaquin willkill both the foliage and the nutlets, as wellas some broad leaf weeds, such as sand-bur and wild garlic. Imazaquin is not for useon cool-season grasses or bahiagrass.

Coates points out that economical con-trol of some fall and winter germinatingweeds in non-overseeded turf IS possiblewith the preemergence herbicide simazine

continued on page 30

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an advantage over the turf."Yellow nutsedge and purple nutsedge

control have become more effective sincethe introduction of bentazon (Basagran) andimazaquin (Image), says Barnes. He usesa combination of MSMA and bentazon forthe yellow nutsedge, and adds imazaquinfor purple nutsedge. The MSMA kills thefoliage while the bentazon and imazaquintake care of the nutlets.

Goosegrass in the fields not overseededwith ryegrass is controlled with applicationsof metribuzin (Sencor). There is a problemcontrolling goosegrass on overseeded fieldsbecause metribuzin can harm the ryegrassand fenoxaprop can harm the ber-mudagrass. For those fields he must relyon preemergence herbicides and repeatedapplications of MSMA to control goosegrass.However, in late winter, the metribuzin cancome in handy to both control thegoosegrass and phase out the ryegrass.

"This far south, bermudagrass doesn't gocompletely dormant, it just slows down," saysBarnes. For this reason, he stays away fromusing glyphosate (Roundup) to control winterweeds. He is experimenting with a light rateof glyphosate to take out rye between theend of soccer season in February and thebeginning of spring football in late April.

He treats overseeded fields in the winterlike a northern sports turf manager would,depending mainly on 2,4-D and dicambafor broadleaf weed control and MSMA for

Foxtail. Photo courtesy: Larry Leuthold.

Postemergence Weed Controlcontinued from page 29

(Princep). Split applications of simazine canprovide control of poa, hen bit, chickweedand some clovers from October throughApril. He prefers controlling winter weedsin dormant turf this way as opposed toglyphosate.

Dallisgrass control is just as tough in Mis-sissippi as it is in Missouri and Kansas.Sometimes six applications of MSMA areneeded during the summer to control thegrassy weed.

Sports turf managers in Florida face both

yellow and purple nutsedge, explains JimBarnes, pesticide specialist for BrowardCounty School District. Barnes and his pes-ticide applicator, Lanny Dixon, keep all 22fields on a tight schedule of both preemer-gence and postemergence herbicide appli-cations and overseeding to keep weedsunder control.

"We frequently work weekends to fit weedcontrol into a heavy sports program at theschool district," Barnes adds. "We don'tspray whole fields all the time. We concen-trate on the compacted areas like benchareas and around tracks where weeds have

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grassy weed control. Prickly sida is a hard-to-control broad leaf weed in compactedpractice fields. It takes up to four applica-tions of dicamba to control, says Barnes.Fields which aren't overseeded are aerified,topdressed and treated with preemergenceherbicides in the fall.

Summer weed control at Broward CountySchool District consists of spot treatmentswith postemergence herbicides with MSMA,metribuzin, 2,4-0 and dicamba. "Many ofour fields are common bermudagrass,"Barnes states. "We drill seed in commonbermudagrass seed to keep a dense standof turf and to keep weeds from getting estab-lished."

Glyphosate is a major herbicide for con-trol of winter weeds in Texas, says Dr. BillKnoop, with Texas A&M University's exten-sion center in Dallas. Many schools don'thave the budgets to overseed, and eventhough glyphosate is a relatively expensiveherbicide, it is cheaper in both cost and man-power requirements than overseeding.

The state even has a label for glyphosateat very low rates for summer weed controlin bermudagrass. The winter rate is onequart per acre in 50 gallons of water, whilethe summer rate is one pint per acre. "Youmay hurt the bermudagrass a little or delayspring transition with glyphosate, but at theserates you won't kill it. If you can live throughsome temporary discoloration, you can con-trol many tough weeds fairly effectively. You

Crabgrass. Photo courtesy: Max Badgley.

can also use glyphosate in cooler temper-atures and get better control than withMSMA," Knoop explains.

TAG is another nonselective postemer-gence herbicide schools use to knock outwinter grasses such as annual bluegrassand rescuegrass. Ryegrass and six weeksfescue are other grassy weeds in non-overseeded field, says Knoop. "In over-seeded turf, you wait until the bermudagrasscomes back and then take out the grassyweeds with postemergents." Trimec-typeherbicides take care of henbit and otherwinter broad leaf weeds.

Knoop's list of summer weeds includesgoosegrass, dallisgrass, crabgrass, spurge,johnsongrass and knotweed. Grassy peren-nial weeds in bermudagrass can be con-trolled during the summer with a light rateof glyphosate, one application of metribuzin,or several applications of MSMA. Trimec-type herbicides are effective on summerbroaoleat weeds. Some sports turf managersare mixing glyphosate with Trimec or Wee-done DPC to make one herbicide applica-tion during the summer, says Knoop.

Both cool- and warm-season turfgrassesare used year-round in California,makingweed control a mixture of different methods.Dr. David Cudney, weed specialist for theUniversity of California at Riverside, statesthat kikuyugrass is the worst weed of alltypes of turfgrass in the state. The aggres-sive, low-maintenance grass was importedto Southern California from Africa for ero-sion control in 1918. It spreads both vegeta-tivelyand by seed produced in small seed-heads which form below the cutting heightof mowers. Kikuyugrass is best adapted toMediterranean coastal climates. Once estab-lished, it forms a thick mat which makesthe turf feel spongy but resists other weedsfrom invading.

Many sports turf managers and golfcourse superintendents find it easier to letthe kikuyu take over since it resists weeds,requires less water and uses less fertilizer,"

continued on page 32

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32 sportsTURF

Plaintain. Photo courtesy: Max Badgley.

Postemergence Weed Controlcontinued from page 31

says Cudney. "But they will have to dethatchit heavily to keep it from getting spongy."The only selective herbicide that will beginto control kikuyugrass is MSMA. Failing that,the only way to get rid of it is with nonselec-tive postemergence herbicides. "You alsoneed a barrier of turf without kikuyugrassaround tees, fairways and fields to keep itfrom moving back in."

Common bermudagrass sports turf canbe as much as 90 percent Poa annua in thewinter, Cudney points out. Ethofumesate(Prograss) could be used to control it, butwith poa being such a high percentage ofthe turf, it's better to manage it until the turfcan be renovated, he says.

"Too many sports turf managers wait toolate in the fall to overseed with perennialryegrass," Cudney adds. They try to holdonto the bermudagrass until the last minute.As a result, the less agressive bermuda doesnotresist invasion by winter broad leaf weedssuch as oxalis, clover, dandelion, plantain,dock and southern brass buttons. By over-seeding earlier, the ryegrass can becomecompetitive sooner. This is also one rea-son for some poa infestations, he adds.

Oxalis can be a problem on overseededturfgrasses in Southern California in thewinter. Cudney has had success with appli-cations oftriclopyr (Turflon) on oxalT~

Cool-season turf lacks competitivenessduring the summer in Cudney's area. Pros-trate spurge, oxalis, dallisgrass, goosegrassand other weeds take advantage of the weakturf to get established. Spring seedings ofcool-season grasses and late spring seed-ings with common bermudagrass need tobe treated with bromoxynil to keep out sum-mer broad leaf weeds, he says.

Overseeded perennial ryegrasses canhang on into late spring if the weather ismild and hybrid bermudagrasses don'tnecessarily go completely dormant. Sportsturf managers should select ryegrasses with

low heat tolerance for overseeding for thisreason.

Steve Cockerham, director of the turf trialsat the university, has seen some ryegrassesdo exceptionally well throughout the yearat Riverside. However, summer waterrequirements for ryegrass as compared tobermudagrass discourage their use. Turf-type tall fescues are growing rapidly forresidential and commercial uses in the areaand have a lower water requirement thanryegrasses. But their use for sports turf .remains limited.

A growing number of superintendents aremaintaining creeping bentgrass greens inSouthern California, especially resortcourses. These superintendents have towatch the bent carefully for both summerweeds and diseases. They rely heavily onpreemergence weed control and preven-tative disease control.

No matter in what part of the countrysports turf managers strive to control weeds,the consistent factor in weed prevention ishealthy, aggressive turf. Weed control can-not make up for poor maintenance duringthe year. Proper irrigation, fertilization, anddrainage are the first steps in weed con-trol. Periodic renovation may still be requiredunder high use conditions to keep turf denseand weeds under control.

The good news is that the number ofweeds in sports turf that have no effectivecontrol are decreasing as new herbicidesreach the market, More golf course superin-tendents and sports turf managers are tryingthese products out with excellent results.Some may cost more than others 011 asingle-application basis, but they can alsoreduce the total number of applicationsnecessary.

If sports turf is truly important to the user,then funds will be found to accomplish weedcontrol and basic maintenance procedureswhich assure the quality of the turf. Theimportant thing is to know what can be donewhen adequate funding is provided .•