J.T. Brosnan, Ph.D.; G.K. Breeden, M.S.; and A.J. Patton, Ph.D.
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Summer stress from heat, drought, and disease as well as mechanical injury from equipment and golfer traffic can compromise putting green quality and provide opportunity for weed invasion.
Photos by A. Patton
The two most common turfgrass species planted on golf course putting greens are creeping bentgrass (Agrostis stolonifera) and hybrid bermudagrass (Cynodon dactylon × C. transvaalensis). When managed appropriately, both species provide high-quality playing surfaces. However, turfgrasses managed for putting green use are subjected to a considerable amount of stress. These grasses are often mowed daily at heights less than 0.15 inch (3.81 millimeters) and are subjected to heavy traffic from both golfers and maintenance equipment.
Broadleaf and monocot (grasses, sedges, kyllinga) weeds can invade putting greens lacking density and vigor. Reduction in density and vigor may come from mechanical injury following cultivation and traffic, voids left from ball marks, damage from insects and disease, and environmental stresses, which can all lead to weed invasion. Weed control on golf course putting greens can be difficult as few herbicides are labeled for use on putting greens because stress renders them more susceptible to herbicide injury that can compromise both aesthetic and functional turf quality. Additionally, putting green turf is the most valuable acreage on the golf course and is expensive to repair if injured. Many companies do not register herbicides for use on greens because they want to avoid being liable for potential injury and because putting greens make up an infinitesimally small percentage of the total turf acreage in the world. In many instances, herbicide labels neither allow nor restrict applications to putting greens, which places all liability on the end users.
Smooth crabgrass growing in a creeping bentgrass putting green maintained at less than 0.150 inch.
Crabgrass (Digitaria species), goosegrass (Eleusine indica) and annual bluegrass (Poa annua) are three of the most common annual grassy weeds of creeping bentgrass and hybrid bermudagrass putting greens.
Crabgrass and goosegrass
Summer annual species such as crabgrass and goosegrass germinate in spring, and seedlings mature throughout the summer. Preemergence control of these weed species is the easiest means of control. A list of pre-emergence herbicides labeled for creeping bentgrass and hybrid bermudagrass greens is presented in Table 1. For each herbicide, check the label to ensure the product is safe to use on the species and cultivar grown. For example, older creeping bentgrass varieties such as Cohansey, Carmen, Seaside and Washington are more susceptible to dithiopyr and siduron (Tupersan) injury (6). It is also important to use only labeled herbicides at labeled rates as off-label applications can injure root systems and compromise putting green quality.
Goosegrass is a problematic summer annual in creeping bentgrass putting greens maintained in the transition zone and in bermudagrass putting greens maintained throughout the southern U.S.
Currently, no herbicides are labeled for selective post-emergence control of crabgrass or goosegrass on creeping bentgrass putting greens (Table 2). However, research trials often indicate good control of one-tiller or smaller crabgrass and goosegrass with fenoxaprop (Acclaim Extra, Bayer) at 3.5 fluid ounces/acre (256 milliliters/hectare) on creeping bentgrass putting greens with no appreciable injury. Furthermore, newly published research shows that a pre-mixture of 2,4-D + dicamba + MCPP + carfentrazone (Speed- Zone) can control goosegrass without injuring creeping bentgrass putting greens (12). Despite these reports, these tools are not available to superintendents since neither product is registered for use on creeping bentgrass putting greens.
On hybrid bermudagrass putting greens, diclofop (Illoxan, Bayer) and foramsulfuron (for example, Revolver, Bayer) can be used for post-emergence goosegrass control. However, diclofop will no longer be available after December 2014 (Jeff Michel, Bayer Environmental Science, personal communication). For post-emergence crabgrass control on bermudagrass greens, no effective options are available, as herbicides such as trifloxysulfuron (Monument, Syngenta) exhibit only marginal activity against crabgrass species
Annual bluegrass (Poa annua) is a winter annual grassy weed common in creeping bentgrass and hybrid bermudagrass putting greens. Pre-emergence control of annual bluegrass can be erratic because this weed is able to germinate in a wide range of environments (9,10). Post-emergence control is difficult not only because of the limited number of labeled herbicides, but also because of the possibility of multiple annual bluegrass biotypes persisting in putting greens, including both an annual- type (P. annua cv. annua) and perennialtype (P. annua cv. reptans).
Currently, no herbicides are labeled for selective, post-emergence annual bluegrass control in creeping bentgrass putting green turf. However, sequential applications of plant growth regulators such as paclobutrazol (for example, Trimmit, Syngenta) and flurprimidol (for example, Cutless, SePRO) often reduce annual bluegrass populations in creeping bentgrass putting greens and/or control seedhead production (13). Although the products are not labeled for use on putting greens, some turfgrass managers have successfully reduced annual bluegrass populations with the herbicides Velocity SG (bispyribac-sodium, Valent) and Xonerate (amicarbazone, Arysta Life- Science) (8,13). Additionally, a great deal of interest exists among golf course superintendents in the experimental herbicide PoaCure (methiozolin, Moghu), which can control annual bluegrass in creeping bentgrass putting greens. The future of this experimental herbicide is still largely unknown at this time.
On hybrid bermudagrass greens, trifloxysulfuron can be used for post-emergence annual bluegrass control, along with foramsulfuron (Revolver), pronamide (Kerb, Dow AgroSciences), and rimsulfuron (for example, TranXit, DuPont). Caution should be heeded when attempting to control annual bluegrass with sulfonylurea herbicides labeled for putting greens (trifloxysulfuron, rimsulfuron) as annual bluegrass populations resistant to these herbicides were recently found in Alabama, Georgia and South Carolina (5,11) and these herbicides can move laterally and damage adjacent cool-season turfgrasses.
Sedge and kyllinga species
Creeping bentgrass roots were severely injured in this putting green following an off-label application of dithiopyr (Dimension 2EW) to a putting green. Roots were severely stunted and clubbed, and rooting depths were extremely shallow causing summer management problems/challenges on this putting green.
Sedge (Cyperus species) and kyllinga (Kyl- linga species) species can invade both creeping bentgrass and hybrid bermudagrass putting greens. These species often invade poor-draining or over-irrigated soils. Kyllinga species tend to tolerate low putting green mowing heights and greater mowing frequencies better than sedges (4); therefore, kyllinga infestations tend to be more prevalent in putting greens. No herbicides are labeled for selective control of either weed species on creeping bentgrass putting greens although some have successfully used spot treatments of halosulfuron (for example, SedgeHammer, Gowan), which is labeled for use on all areas of the golf course except putting greens. On hybrid bermudagrass putting greens, applications of trifloxysulfuron provide kyllinga suppression (Table 2).
Annual bluegrass is a problematic weed because of its ability to produce viable seed under close mowing.
Although most broadleaf weeds cannot survive at mowing heights used to maintain putting greens, species such as white clover (Trifolium repens), mouse-ear chickweed (Cerastium vulgatum), dandelion (Taraxacum officinale), cudweed (Gnaphalium or Gamochaeta species) and prostrate spurge (Euphorbia humistrata) can persist even with the use of sound management practices. Broadleaf herbicides like 2,4-D have been used on putting greens since the 1940s for weed control (1,2), but many superintendents today are hesitant to use broadleaf herbicides on their putting greens for fear that turfgrass injury might occur — especially from 2,4-D applications in summer. Labels for many of the herbicides listed in Table 2 neither allow nor restrict applications to creeping bentgrass or hybrid bermudagrass greens, leaving liability on the end user in the event undesirable turfgrass injury occurs after application.
Mixtures of synthetic auxin herbicides can be used at reduced rates to control broadleaf weeds on putting greens. For example, 2,4-D + MCPP + dicamba (for example, Trimec Classic, PBI/Gordon) can be applied to creeping bentgrass putting greens at 1 fluid ounce of formulated product per 1,500 square feet. The label does caution against applications when creeping bentgrass putting green turf is under heat or drought stress and highlights that injury after application will be shortlived. A different formulation (for example, Trimec Bentgrass Formula, PBI/Gordon) is labeled for use on creeping bentgrass greens at rates less than or equal to 1 fluid ounce of formulated product per 1,000 square feet (3.18 milliliters/hectare). Trimec Bentgrass Formula applies less 2,4-D (0.15 pound ae/acre vs. 0.45 pound ae/acre (168 grams ae/hectare vs. 504 grams ae/hectare) than Trimec Classic when applied per label directions.
White clover in a creeping bentgrass putting green.
Carfentrazone (for example, QuickSilver T&O, FMC Corp.) is a post-emergence broadleaf weed control herbicide labeled for use on creeping bentgrass and hybrid bermudagrass putting greens. Rates range from 1.0 to 2.0 fluid ounces/acre (73 to 146 milliliters/ hectare); however, the product can be used at 6.7 fluid ounces/acre (490 milliliters/hectare) for managing silvery thread moss (Bryum argenteum). Research in Tennessee indicates Quicksilver applications combined with appropriate cultural practices (for example, increased nitrogen fertility and sand topdressing) control silvery thread moss better than simply spraying the herbicide alone (3).
Recent broadleaf research
There is very little data on the safety of broadleaf herbicides on putting greens despite pesticide labels that suggest that they can be used without injuring turf. Research was conducted at Purdue University and the University of Tennessee during 2011-2012 to determine the safety of post-emergence broadleaf herbicides on putting green-height creeping bentgrass turf.
An experiment was conducted twice at the W.H. Daniel Turfgrass Research and Diagnostic Center at Purdue University in West Lafayette, Ind., and the East Tennessee Research and Education Center in Knoxville, Tenn., at the University of Tennessee. The Indiana location was Pennlinks creeping bentgrass grown on a sand root zone built to USGA recommendations and, in Tennessee, the site was L-93 creeping bentgrass grown on a silt loam soil frequently topdressed with a USGA-recommended sand. The locations were maintained as putting greens. Plots were treated with herbicide on Oct. 24, 2011, and an adjacent location on May 22, 2012, in Indiana and on Oct. 17, 2011, and an adjacent location on May 1, 2012, in Tennessee. At both locations, herbicides were applied in 2 gallons water/1,000 square feet at 30 psi (81.5 milliliters/ square meter at 207 kilopascals) with a CO2-pressurized boom sprayer equipped with an XR8002VS flat-fan nozzle.
Table 3. Injury ratings following applications of broadleaf herbicides to creeping bentgrass putting greens in the fall or the
spring in Indiana and Tennessee at label rates and twice (2×) the label application rates.
Herbicides included in this study (Table 3) were all labeled for use on creeping bentgrass putting greens and applied at the putting green label rate and at a rate twice (2×) this labeled rate. One exception to this was the October 2011 application timing in Tennessee where only the label rate of each herbicide was applied. An untreated check was included for comparison at each location. Injury to creeping bentgrass and turf quality data were collected. All data were analyzed using statistical software.
Mouse-ear chickweed is a common weed in close-mowed bentgrasses.
Fall applications. Minor and transient injury was observed from fall treatments on creeping bentgrass putting greens in Indiana, but injury levels were acceptable (≥ 7, on a scale of 9-1, where 9 = no injury) for all treatments including herbicides applied at a 2× rate (Table 3). Minor but acceptable injury occurred from applications of 4-Speed, 4-Speed XT, Banvel, Trimec Bentgrass, Trimec Classic, Trimec Encore and Trimec Southern. In Tennessee, injury was minimal (<4%, on a scale of 0%-100%, where 0% = no injury) and transient from labeled application rates following 4-Speed, 4-Speed XT, Trimec Classic, and Trimec Southern applications. Differences in turf quality were not seen among treatments in Indiana or Tennessee (data not shown).
Silvery-thread moss is a common putting green weed, especially in close-mowed putting greens that receive frequent irrigation and low fertility.
Spring applications. The experiments were repeated in May 2012 to determine if more injury might be expected from late spring and summer applications during warmer temperatures. At both locations, more injury was observed from May 2012 applications than October 2011 applications. Applications at label rates did not cause unacceptable injury when applied in Indiana in May, but 2× rates of 4-Speed XT, Banvel and Trimec Southern did cause unacceptable injury two weeks after application (Table 3). That injury was acceptable by three weeks after application (data not shown). Results were similar in Tennessee with 4-Speed XT, Banvel and Trimec Southern applied at the 2× rate also causing the most injury (11%-19%) (Table 2), but with injury decreasing to <4% three weeks after application (data not shown). While a labeled application rate of Banvel caused 10% injury two weeks after application in Tennessee, other products such as Mecomec 2.5, QuickSilver T&O, Trimec Bentgrass, Trimec Classic and Trimec Encore had <2% injury when applied at the labeled rate in May, similar to responses observed in Indiana. These data are supported by reports on the safe use of carfentrazone on creeping bentgrass for silvery thread moss (Bryum argenteum) control (15) and the use of Trimec Bentgrass for lesser swinecress (Croronopus didymus) control (14).
Findings of this research were that broadleaf herbicides labeled for putting green use can be safely applied at labeled rates in the spring and fall; injury is more likely to occur from May herbicide applications than from October applications; some herbicides are safer than others with high rates of dicamba, triclopyr and 2,4-D causing injury; and unacceptable injury can occur from higherthan- labeled herbicide rates such as from spot applications.
Although most golf course superintendents have few weed problems other than annual bluegrass in their putting greens, weeds do occasionally invade. If only a handful of weeds are present within a single green, mechanically removing these weeds is the most efficient method of control. Herbicide applications are only warranted when weed pressure is significant. In these cases, research has confirmed that several herbicides labeled for use on putting greens can be used safely and successfully to control weeds when label directions are followed.
More herbicides are registered for use on fairways and tees than on putting greens. Considering the scope of efficacy and tolerance testing required across a wide geographic region before a herbicide receives labeling, there is likely good reason a particular product is not labeled for putting green use — it could result in undesirable injury. Herbicide labels that neither restrict nor allow putting green use place all liability on the end user, so use caution with these products. For these reasons, it is best to use only products with specific label instructions for putting greens.
The authors would like to thank Drs. Lambert McCarty and Travis Gannon for their assistance with this manuscript.
This article is a compilation of previously published University of Tennessee and Purdue University Extension publications and a research article authored by the same authors in Applied Turfgrass Science.
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- Patton, A.J., and D.V. Weisenberger. 2014. Turfgrass Weed Control for Professionals. AY-336. Purdue University Extension Publication, West Lafayette, Ind.
- Patton, A.J., D.V. Weisenberger, J.T. Brosnan and G.K. Breeden. 2013. Safety of labeled herbicides for broadleaf weed control in creeping bentgrass putting greens. Applied Turfgrass Science doi:10.1094/ATS- 2013-0523-01-BR.
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- Reicher, Z. 2014. Controlling moss or goosegrass in new creeping bentgrass seedlings. Online. http://turf. unl.edu/pdfctarticles/June9_CBG_Seedlings_moss_ goosegrass.pdf
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- Straw, C., G. Henry, T. Williams et al. 2012. Postemergence control of lesser swinecress in creeping bentgrass putting greens. ASA, CSSA, and SSSA Annual Meetings, Cincinnati, Ohio. Oct. 21-24, 2012. Paper 105-7.
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Jim Brosnan (Twitter: @UTTurf weeds) is an associate professor of turfgrass weed science and Greg Breeden is an Extension specialist at the University of Tennessee, Knoxville, Tenn. Aaron Patton is an associate professor at Purdue University, West Lafayette, Ind.