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Field Testing Plant Growth Regulators and Wetting Agents for Annual Bluegrass Seedhead Suppression
By Randy Kane and Lee Miller
Many of the annual bluegrass biotypes inhabiting the golf courses of Illinois have a winter annual life cycle. That is, these biotypes germinate from seed in autumn, overwinter in a vegetative state, flower and set seed in the spring, and then decline or completely die out during the heat of summer.
Where annual bluegrass is a significant component in a turf, profuse seeding may occur in late April through May and can become objectionable for several reasons. First, profuse seeding can turn an annual bluegrass-contaminated green or fairway almost white in color, prompting questions about grass health. Second, putting greens with significant annual bluegrass populations provide very poor putting surfaces during spring flowering. Seedheads adversely affect ball roll, causing greens to become slower and more bumpy. Third, heavy seeding of annual bluegrass contributes to the seedbank in surface soil and thatch layers, thus promoting the long-term survival and spread of the species.
There is a growing body of evidence that suggests heavy seeding may not be beneficial for the near-term survival of flowering annual bluegrass. Seed production may divert photosynthate away from vegetative tissues (leaves and roots) to the flowers, resulting in reduced root depth and shoot growth after seeding. Annual bluegrass that hasn't set seed (e.g., in treated plots) is usually better able to survive summer stresses than plants that have flowered and set seed (2). It is interesting to note that most of the plants identified as perennial biotypes of annual bluegrass produce less seed than annual biotypes, which may contribute to their longer-term, perennial habit.
Chemical seedhead suppression can help maintain the color and playability of fairways, as well as the speed and trueness of putting greens. Also, many superintendents feel that by reducing seed set and the annual contributions to the seedbank, other chemical and management programs used to reduce or eliminate annual bluegrass from cool-season turf may become more effective. There also is a great deal of interest in trying to preserve the purity of newly renovated turf by keeping nearby annual bluegrass from contaminating the renovated site (e.g., a resurfaced putting green).
How do you reduce or suppress annual bluegrass seed set in the spring? Several herbicides and plant growth regulators are known to inhibit seeding of Poa species and other grasses, including older products like maleic hydrazide, mefluidide, and endothal . However, most products used in annual bluegrass programs have problems with consistency of seedhead suppression, length of time seedheads are suppressed, or phytotoxicity. Also, application timing and proper stage of plant growth are critical for best seed inhibition, and calendar dates for application may vary widely from year to year. Note that there is a "base-50" growing-degree-day prediction model for timing of the first spray for seedhead suppression (3), but this model seems to be as unpredictable as the annual bluegrass itself .
Embark Turf & Ornamental can cause discoloration and thinning of creeping bentgrass mowed at greens height-of-cut (below pen).
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Historically, the best results for seedhead suppression on annual bluegrass fairways have been found using mefluidide ("Embark") (1, 5). However, timing and phytotoxicity problems have limited its use, especially on greens-height turf. Many superintendents have tried early spring applications of gibberellin inhibitor plant growth regulators (PGRs) such as paclobutrazole ("Trimmit") or flurprimidol ("Cutless") to try to slow the encroachment of annual bluegrass into bentgrass turf. They reported some seedhead suppression following early season treatments, but seedhead suppression usually is not the primary goal of these applications.
A few adventurous superintendents have also used the wetting agent Aqua-Gro L (5) to limit spring flowering of annual bluegrass, and they have found that Aqua-Gro is less phytotoxic than Embark, but it provides more variable results. (Aqua-Gro L is no longer manufactured.)
Preliminary field tests have suggested the ethephon ("Proxy") has good activity for annual bluegrass seedhead suppression (4). Proxy is a new PGR for the turf market, but it has been available in agricultural applications for years. Proxy may be safer and have more timing flexibility than Embark, and it could be a potential substitute for Aqua-Gro L. Proxy reportedly has the tendency to make treated turf lighter green to yellow-green, but this can be counteracted to some extent with iron applications. Also, tank-mixes of Proxy plus trinexepac-ethyl (Primo) have shown good results with less turf discoloration.
Three golf course sites were treated with PGRs and wetting agents in April and May of 2000-02, including both greens- and fairway-height turf. Initial treatments were timed to coincide with flowering of the earliest annual bluegrass biotypes. Individual plots were 40-50 sq. ft. in size and were replicated two or three times, depending on space available. Treatments were applied with a CO2-powered backpack sprayer (35 psi, flat fan nozzles).
Proxy was tested alone and in tank mixes with Primo and Trimmit. Single and multiple applications of Proxy were made at 5-7.5 fl. oz. per 1,000 sq. ft. rates. Primo was applied alone and in tank mixes at 5-10 fl. oz. per acre. Trimmit was applied at rates ranging from 6-8 fl. oz. per acre. Aqua-Gro L has been tested for a number of years on putting greens at 8 fl. oz. per 1,000 sq. ft., usually with follow-up applications at 4-8 fl. oz. per 1,000 sq. ft. one week later.
The wetting agent Cascade was also included in the study to see if a different type of wetting agent chemistry could inhibit seedheads (note that the manufacturer makes no claims of seedhead control). Embark (Turf & Ornamental Growth Regulator formulation) at 1.3 fl. oz. per 1,000 sq. ft. was included as a standard, and to test for phytosafety on greens-height turf.
A general overview of field test data from Chicago area trials in 2000-01 on greens-height turf can be found in Table 3. Note that the percent seedhead inhibition is an average of several tests, and results can vary greatly with weather conditions, application timing, and annual bluegrass biotypes present in treated areas. Embark is consistently the best flower suppressor, but phytotoxicity (primarily on creeping bentgrass) remains a major concern in northern Illinois. Phytotoxicity of Embark treatments was expressed as a dark blue-green to brown color, with some thinning of the stand. Once warmer weather arrived, turf color and density recovered.
Proxy and Proxy + Primo treatments provided seedhead suppression approaching that of Embark in our trials in 2000 and 2001. In some cases, suppression with split applications of Proxy lasted longer than single Embark applications. However, higher rates or repeat applications of Proxy caused yellowing and thinning of treated turf, especially at greens height. Note that repeat Proxy applications were made only 7 to 10 days apart; less discoloration has been observed in other tests if the interval between applications is 28-35 days (4). Tank mixing Proxy with Primo appeared to reduce the discoloration and thinning of turf, although further testing will be required to confirm the effect.
Of the other products/rates tested, only Aqua-Gro L exhibited significant seedhead suppression, and the effect was short-lived and inconsistent from site to site and season to season. The anti-gibberellin growth regulators Primo and Trimmit did not appear to inhibit seedhead formation, and in some situations, these treatments appeared to have more seedheads than check plots. This effect could be due to stunting of the seed stalk to the point where the seedheads remained below the cutting height and were not removed by mowing.
For 2002 greens-height trials, we concentrated on Proxy alone or in tank mixes with Primo or Trimmit . We also began a second set of treatments a week later to see if a later application was as effective as a well-targeted first application. The Proxy and Proxy+ tank mixes did not perform as well as in the previous two years. On certain rating dates, the level of seedhead suppression was hovering around 50 percent, with the best levels around 70% suppression. Previous tests provided about 90% suppression. Variability in seedhead suppression with PGRs is common (3, 5) and may be due to differing weather and application timing parameters, as well as to the inherent variability of annual bluegrass biotypes. Proxy treatments applied a week later than the supposed target date still performed well once the time lag was taken into account.
Finally, we took a look at some Proxy tank mixes sprayed on a mixed annual bluegrass/creeping bentgrass fairway (Table 5 and Figure 1). Taking the Proxy rate up to 7.5 fl. oz. per 1,000 sq. ft. improved the seedhead suppression, and no noticeable phytotoxicity was observed at this rate when tank mixed with Primo at 10 fl. oz. per acre. Proxy does not have a separate label rate for fairway treatments or a recommended rate for putting greens on the 2002 pesticide label. It is likely that some broader uses and application rates will appear on future labels.
After three years of testing products for annual bluegrass seedhead suppression, some conclusions can be reached.
. Seedhead production in annual bluegrass is detrimental for various reasons, including poor playability, aesthetics, and reduced plant vigor.
. The most consistent seedhead suppression follows treatments with mefluidide or ethephon, although both chemicals have limitations regarding application timing or possible phytotoxicity.
. Embark can cause discoloration and thinning of bentgrass following cold weather, but it remains the best product for seedhead suppression, especially on fairways, where some phytotoxicity is tolerable.
. Proxy can be nearly as effective as Embark for seedhead suppression, but results are variable from year to year and from site to site.
. Proxy can cause some objectionable color and growth effects, but tank mixes with Primo or other PGRs may alleviate some of these problems.
. If Proxy (+Primo) applications are made early in spring, a follow-up application 4-5 weeks after the first may be beneficial to maintain the seedhead suppression into June.
. Wetting agents gave inconsistent results and were approximately 50% as effective as mefluidide or ethephon, at best.
. Anti-gibberellin PGRs such as paclobutrazole and trinexepac-ethyl did not significantly reduce seedheads in our studies.
. Seedhead suppression can be highly variable from year to year or site to site because of weather fluctuations, application timing, and annual bluegrass variability.
Thanks to our on-site cooperators and supporters for these studies: John Gurke, Superintendent, Aurora C.C.; Dan Anderson, former Superintendent, Fox Valley C.C.; Jim McNair, Superintendent, Orchard Valley G.C.; Jon Jennings, Superintendent, Chicago G.C.; Bruce Branham, Ph.D., Associate Professor, University of Illinois, Department of Natural Resources and Environmental Sciences; Paul Vermeulen, Director, Mid-Continent Region, USGA Green Section.
1. Cooper, R. J., P. R. Henderlong, J. R. Street, and K. J. Karnok. 1987. Root growth, seedhead production, and quality of annual bluegrass as affected by mefluidide and a wetting agent. Agron. J., 79(5):929-934.
2. Cooper, R. J., P. R. Henderlong, and J. R. Street. 1984. Annual bluegrass management: getting to the root of the problem. Golf Course Management, 52(3):39-43.
3. Danneberger, T. K., B. E. Branham, and J. M. Vargas, Jr. 1987. Mefluidide applications for annual bluegrass seedhead suppression based on degree-day accumulation. Agron. J., 79(1):69-71.
4. Gelertner, W., and L. J. Stowell. 2001. Advances in Poa seedhead management. Golf Course Management, 69(10):49-53.
5. Petrovic, A. M., R. H. White, and M. Kligerman. 1985. Annual bluegrass growth and quality as influenced by treatments of growth retardants and wetting agents. Agron. J., 77(5):670-674.
Dr. Randy Kane is director of Turfgrass Programs and Lee Miller is manager of Turfgrass Research for the Chicago District Golf Association.
Left: Research supported by the Chicago District Golf Association tested the ability of plant growth regulators and wetting agents to suppress annual bluegrass seedhead formation. Above: Heavy Poa annua seedhead formation on Midwest putting greens is typical in mid to late May in most years.
Embark Turf & Ornamental can cause discoloration and thinning of creeping bentgrass mowed at greens height-of-cut (below pen).
Shade patterns influence Poa annua growth and its competition with bentgrass.
Higher rates of Proxy (without Primo or chelated iron) caused some discoloration of treated turf mowed at putting green height. Patchy seedhead development is evident in the check plot at left.
Visible seedheads were evident in untreated plots when compared to surrounding treatments that demonstrated varying abilities to suppress seedheads.
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