Annual bluegrass control: Implications for eliminating a historic weed in golf course turfgrass

Patrick E. McCullough

Golf courses play a major role in the recreational development of most areas and represent a major business commodity to the United States economy. Since golf course turfgrass is grown for recreation, superintendents are scrutinized by the public, media, and environmentalists over management practices required for successful long-term culture. Modern golf course management emphasizes reduced water, fertilizer and pesticide use for environmental conservation. However, competition among golf courses to produce idealistic turfgrasses has superintendents employing management strategies that push turfgrasses to physiological extremes. Unconventional fertility, mowing, and watering regimes reduce turfgrass growth, vigor, and recuperative potential from environmental stresses which enable problematic weeds to become established.

Annual bluegrass, or Poa annua, is a historically problematic weed in golf course management that superintendents have unsuccessfully controlled (20). Annual bluegrass populations reduce turfgrass aesthetics and functionality due to its light green color, unsightly seedheads and shallow root system (7, 17, 31). Annual bluegrass tolerates close mowing, germinates rapidly, and has undesirable qualities, including poor disease, drought and wear tolerances that create unsightly patches in golf course turf (6, 8, 17). Consequently, golf courses infested with annual bluegrass require greater water, fungicides and intensive management to maintain acceptable turfgrass swards.

Contrary to its name, both annual (live for one season) and perennial (live for many seasons) biotypes of annual bluegrass are found in golf course turfgrass. Perennial biotypes have lateral stems and are more prevalent on putting greens that are routinely irrigated and fertilized (32). While the two biotypes may not be easily distinguished from each other, annual types exhibit bunch-type growth habits and produce greater seedheads than lower-growing perennial types.

Annual bluegrass is a prolific seedhead producer as single plants may produce greater than 2,000 viable seeds during spring and early summer, thus increasing the weediness of the plant in subsequent growing seasons (6, 17). Annual bluegrass grows well under short day lengths and cool conditions, and may out-compete other turf species for light, water and nutrients during late fall and early spring. Annual bluegrass often dies from summer stresses but may survive if irrigated, and pests are adequately controlled.

Annual Bluegrass Control: Inconsistency of Past Herbicides
Selectively controlling annual bluegrass in golf course turf has been traditionally difficult since previous chemistries have inconsistent herbicide activity and cause excessive turfgrass injury. Thus, superintendents have traditionally been forced to manage annual bluegrass as part of the desired turf. Consequently, greens, tees and fairways with significant annual bluegrass populations produce inferior playing surfaces and are economically more expensive to maintain high quality turf.

Prograss (ethofumesate) is a postemergence herbicide that for years was the only selective material available for annual bluegrass control in cool-season turfgrasses. However, Prograss has a narrow window for application dates (late fall/early winter), which often provide erratic annual bluegrass control and unacceptable turfgrass injury (1, 11). For most cool-season turfgrasses, injury concerns force superintendents to reduce Prograss application rates from those most effective for annual bluegrass control (3).
Golf course superintendents have also resorted to spot treating non-selective herbicides, such as Roundup (glyphosate), for annual bluegrass control. This method is effective for controlling annual bluegrass but may not be applicable when heavy populations are present or when an entire golf course is infested. Additionally, drift from non-selective herbicide applications may kill surrounding turfgrass species, which reduces turf recuperation over voids left from annual bluegrass plants eliminated by the herbicide. Unsightly voids following Roundup applications often reduce turfgrass aesthetics and may increase the potential for crabgrass or other annual weeds to establish.

Rather than applying postemergence herbicides, golf course superintendents have resorted to suppressing annual bluegrass populations with plant growth regulators (PGRs). Early gibberellin synthesis inhibitors, such as Trimmit (paclobutrazol) or Cutless (flurprimidol), are applied throughout the fall and spring for annual bluegrass management. Suppression from these PGRs result from more significant annual bluegrass growth inhibition than most turfgrass species, thus creating a competitive growth advantage for other grasses. Long-term PGR use (two or more years) has shown to effectively reduce annual bluegrass present in golf course greens, tees and fairways (14, 21). However, PGRs do not completely control annual bluegrass.

Another effective but costly method to remove annual bluegrass is to sod over heavily infested areas. Planting new sod significantly reduces germination potential of annual bluegrass seeds since the soil surface is covered by established turf. However, this procedure requires hours of labor, quality sod and equipment that is costly for golf course management. Furthermore, introducing new sod is not guaranteed to be free of dormant seed and annual bluegrass may germinate in subsequent years.

Annual Bluegrass Control: New Herbicide Chemistries for Selective Control
Until 2005, golf course superintendents have had no postemergence herbicides registered for selective annual bluegrass control. Velocity (bispyribac-sodium) is a herbicide recently introduced to the turfgrass industry that was discovered for annual bluegrass control in field experiments at Michigan State University (9). Initially, Velocity was investigated as a PGR for annual bluegrass suppression but researchers noted annual bluegrass was completely desiccated by applications of the herbicide. Velocity was then tested throughout the United States and was federally registered for use in golf course turfgrass in November 2004.

Velocity belongs to the pyrimidinyl carboxy herbicide family and controls susceptible weeds by inhibiting acetolactate synthase (ALS) enzyme involved in branch chain amino acid synthesis (30). This mode of action is similar to sulfonylurea herbicides like Manage (halosulfuron) used for sedge and grassy weed control (29, 30). The active ingredient of Velocity, bispyribac-sodium, has traditionally been used for selective postemergence control of barnyardgrass and other weeds in rice production (29, 30, 33, 34). Field research with Velocity indicates applications at 30 to 60 g a.i. per acre control annual bluegrass populations in creeping bentgrass and perennial ryegrass turf (5, 18, 19, 26).

Tolerant turfgrasses to Velocity metabolically inactivate the herbicide through translocation away from meristematic regions. Conversely, the toxicity of Velocity is more substantial on annual bluegrass because the plant cannot metabollicaly inactivate the herbicide before lethal injury. Susceptible weeds decline from arrested photosynthesis and toxic accumulation of the ALS enzyme as a result of branch chain amino acid synthesis inhibition.

Velocity Herbicide: Successful Integration into Management Regimes
Velocity herbicide is an innovative material that is unparalleled by other herbicide chemistries. However, successful integration of Velocity into golf course management programs has several fundamental precautionary elements. Velocity is currently labeled at 30 to 45 g a.i. per acre for annual bluegrass control in creeping bentgrass and perennial ryegrass fairways. Sequential applications are required two to three weeks after initial applications for best results (4). Leaf chlorosis is common following applications of Velocity but turf generally recovers within 7 to 14 days when temperatures are moderate (70° to 85° F) during the treatment period. Turfgrass chlorosis may persist longer when temperatures are not within the 70° to 85° F range (19).

Most herbicides, especially Velocity, must be applied under appropriate environmental conditions for turfgrass safety and effective annual bluegrass control (25). Research in New Jersey shows Velocity most effectively controls annual bluegrass when applied in early summer compared to fall or spring applications (19). Conversely, creeping bentgrass tolerance to Velocity herbicide is greatest in early summer rather than fall or spring applications. Growth chamber experiments confirm seasonal variations in Velocity efficacy are a result of temperature on herbicide activity and turfgrass growth (13, 22).

Prior to applications of Velocity herbicide, superintendents must survey targeted areas for applications and estimate the annual bluegrass population of the turfgrass area. If annual bluegrass is the predominant species in the turfgrass stand, severe voids in turf will result from herbicidal activity. Superintendents are recommended to reduce annual bluegrass populations before Velocity applications with plant growth regulators, such as Trimmit, when annual bluegrass populations exceed 30% to promote quicker turfgrass recovery over Velocity treated areas.

Research shows creeping bentgrass and perennial ryegrass can be seeded into areas treated with Velocity 10 to 14 days after sequential applications. In New Jersey, applications of Velocity at labeled rates, 30 to 45 g a.i. per acre, to a 100% annual bluegrass turf resulted in 90% to 95% control by early July (12). Slit-seeding ‘L-93’ creeping bentgrass at 1 lb per 1000 sq ft on July 1 resulted in 90% to 95% creeping bentgrass cover by Aug. 29, which increased up to 96% and 100% by September. In field experiments at Echo Lake Golf Course in New Jersey, Velocity was applied to a creeping bentgrass fairway with approximately 25% annual bluegrass cover. Applications of Velocity provided 90% to 100% annual bluegrass control with sequential applications. Since the annual bluegrass populations were under 30%, slit-seeding creeping bentgrass had no influence on turf coverage since the fairway naturally recovered over areas once covered by annual bluegrass. Thus, turf managers have the capacity to control moderate (25%) annual bluegrass populations with exclusive Velocity use but slit-seeding over treated areas is necessary when annual bluegrass is the dominant species.

Velocity: Cultural Practices Influence Herbicide Efficacy
Several cultural aspects must be considered prior to Velocity use for turfgrass safety and annual bluegrass control. Nitrogen is the nutrient required in greatest quantities by turfgrasses and is a key constituent of chlorophyll, proteins and amino acids. Nitrogen applications generally enhance turfgrass color and quality but nitrogen rates should not be increased prior to Velocity applications. Field and greenhouse experiments have noted increased weekly nitrogen applications from 0.125 lbs/1000 sq. ft may exacerbate creeping bentgrass leaf chlorosis while enhancing annual bluegrass tolerance to the herbicide (12, 23). This likely resulted from increased translocation of the herbicide to leaf tissue as a result of stimulated shoot growth from increased nitrogen rates.

Since leaf chlorosis is a concern following Velocity applications, superintendents may wish to tank-mix chelated iron with the herbicide. Field experiments in Connecticut have noted applying Velocity with iron effectively masks creeping, colonial and velvet bentgrass chlorosis following Velocity use without compromising annual bluegrass control (24). In New Jersey, tank mixing the plant growth regulator Primo effectively mitigated creeping bentgrass chlorosis from the herbicide without reducing efficacy for annual bluegrass control (23).

Velocity herbicide has potential to stunt turfgrass growth thereby reducing recuperative capacities relative to untreated turf. Cultivation practices such as aerification, verticutting or grooming should be withheld until turfgrasses are actively growing following Velocity treatments. Turfgrass tolerance to Velocity herbicide is also a function of mowing height. Creeping bentgrass and perennial ryegrass fairways should not be cut lower than 0.25 inches when treated with Velocity while raising the height of cut prior to applications is recommended.

Annual Bluegrass Control: Future Implications of Selective Herbicides
Recent advances in herbicide chemistry have promising implications for long-term successful annual bluegrass control in golf course turfgrass. After one year on the market, Velocity is only labeled for use on creeping bentgrass and perennial ryegrass fairways. As with most herbicides used in turf, the Velocity label will likely be amended to include other turfgrass species including tall fescue. Velocity should not be used on putting greens, collars or approaches but researchers are currently investigating potential applications of Velocity on creeping and velvet bentgrass golf greens. Once comprehensive investigations have been conducted with conclusive results, Velocity will likely be labeled for golf greens within the next few years.

The market for annual bluegrass control in Kentucky bluegrass has much greater potential than any other turfgrass species. Kentucky bluegrass is the most widely used species in the world for its desirable color, quality and cold tolerance in roughs, lawns and sports fields. However, Kentucky bluegrass is the most sensitive turfgrass to Velocity applications because of the genetic similarity to annual bluegrass. Researchers are currently investigating physiological mechanisms of varieties that have greater tolerance to Velocity. Because Kentucky bluegrass varieties have wide genetic diversity, DNA sequencing of tolerant varieties is allowing breeders to produce germplasm for breeding Velocity tolerant Kentucky bluegrass cultivars.

Currently, a similar herbicide to Velocity called Beacon (primisulfuron) has a 24-C special local needs label for annual bluegrass control in Kentucky bluegrass grown for seed production. Beacon is a sulfonylurea herbicide previously used for grassy weed control in grass crops (2, 10, 16, 27, 28). Although this herbicide is not labeled for use on golf courses yet, Kentucky bluegrass has shown exceptional tolerance to Beacon in field and greenhouse experiments with substantial activity for annual bluegrass control (12). Beacon herbicide has promising implications for future registrations in golf course turfgrass and may be available within the next few years.

Finally, the history of golf course weed control took a dramatic turn in 2005. Superintendents now have the potential to eliminate one of the most problematic maladies in golf course management. Superintendents who apply Velocity according to recommendations can now selectively control annual bluegrass in early summer to establish a dense, uniform turf prior to annual bluegrass germination in fall. Preemergence herbicide programs in conjunction with Velocity use will ultimately be the most effective regime for successful long-term annual bluegrass control. Betasan (bensulide) and Dimension (dithiopyr) are preemergence herbicides most suitable for northern golf course turfgrass while southern managers may also apply Ronstar (oxadiazon) or Rubigan (fenarimol). Overall, Velocity herbicide has revolutionized the potential for turfgrass weed control and golf course superintendents may no longer have to compromise aesthetics or functionality of their turf from annual bluegrass infestations.

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Patrick E. McCullough is a Ph.D. graduate student at Rutgers University.