Suppression of Spring Flowering of Poa annua L. Updated Version - 2002 Data December 16, 2002 Many of the Poa annua 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 die during the heat of summer. Fortunately, in most years, much of the Poa north of I-80 survives the summer, and therefore behaves more like a true perennial - thanks to better understanding of the plant's biology and careful management by superintendents. Biotypes of Poa annua common to northern Illinois often seed profusely in late April through May, which can become objectionable for several reasons. First, profuse seeding can turn a Poa contaminated green or fairway almost white in color, prompting questions about the health of the grass. Second, putting greens with significant Poa populations provide very poor putting surfaces in spring. As we all know, seedheads adversely affect green-speed and smoothness (just ask Johnny Miller). Third, there is a growing body of evidence to suggest that heavy seeding may not be beneficial for the near-term survival of flowering Poa annua. Seed production in Poa may divert photosynthate away from vegetative tissues (leaves + roots) to the flowers, resulting in reduced root depth and shoot growth after seeding. Also, Poa that doesn't set seed (e.g. in treated plots) is usually better able to survive summer stresses than Poa that has flowered and set seed. So, how do you reduce or suppress Poa annua seeding in spring? Several herbicides and plant growth regulators are known to inhibit Poa seeding, including older products like maleic hydrazide, endothall, and mefluidide (see Table 1). However, most products have problems with consistency of seedhead suppression, length of time seedheads are suppressed, or phytotoxicity. Also, application timing and stage of plant growth is critical for best seed inhibition, and calendar dates for application may vary widely from year to year. (There is a growing-degree-day prediction model for timing of the first spray for seedhead suppression, but this model seems to be as unpredictable as the Poa itself - see Table 2). Historically, the best results for seedhead suppression have been found using mefluidide (Embark T&O). However, timing and phytotoxicity problems have limited its use, especially on greens. Many superintendents have tried early spring applications of gibberellin inhibitor PGRs (paclobutrazole, flurprimidol) to try to slow the encroachment of Poa 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 to limit spring flowering of Poa annua, and have found that Aqua-Gro is less phytotoxic than mefluidide, but provides more variable results. Note that Aqua-Gro L is no longer manufactured. More recent reports have suggested that ethephon (Proxy) has good activity for Poa annua seedhead suppression. Proxy is a "new" PGR for the turf market that may be safer and have more timing flexibility than Embark, and 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. Also, tank-mixes of Proxy plus Primo (trinexepac-ethyl) have shown good results with less turf discoloration. 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 Poa biotypes present in treated areas. Embark is consistently the best Poa flower suppressor, but phytotoxicity 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, the color and turf density recovered. Proxy and Proxy + Primo treatments provided seedhead suppression approaching that of Embark in our trials in 2000 and 2001. In some cases, flowering 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 green 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. Tank mixing Proxy with 1/8 oz of Primo appeared to reduce the discoloration and thinning of turf, although further testing will be required to confirm the effect. Of the other products 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 Enhancer/Trimmit did not appear to inhibit seedhead formation, and in some cases 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. For the 2002 trials, we concentrated on Proxy alone or in tank mixes with Primo or Trimmit in our greens-height trials (see Table 4). We also began a second set of treatments a week later than the first, to see if a later application is as effective as a "well-targeted" first application. You can see that 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 hovered around 50%, with the best levels around 70% suppression. Previous tests were around 90% suppression. Proxy treatments applied a week later 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 Poa annua / creeping bentgrass fairway (see Table 5), also in 2002. Taking the Proxy rate up to 7.5 fl oz per 1000 improved the seedhead suppression, and no noticeable phytotoxicity was observed at this rate when tank mixed with .25 oz of Primo. Proxy does not have a separate 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. Here are some conclusions we have reached after three years of testing products for Poa annua seedhead suppression. Embark can cause discoloration and thinning of bentgrass following cold weather, but 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 seedhead suppression into June. Thanks to our cooperators and supporters for these studies: John Gurke, Supt., Aurora CC Dan Anderson, former Supt., Fox Valley CC Jim McNair, Supt., Orchard Valley GC Jon Jennings, Supt., Chicago GC Bruce Branham, Prof, UIUC NRES USGA Green Section - Regional Grants - Paul Vermeulen, Director, Mid-Continent Region Aquatrols, Bayer(Aventis), Syngenta, Precision Labs Table 1.   Chemicals that have been used for Poa annua seedhead suppression: Trade name Common name PGR Mode of Action       MH or SlowGro maleic hydrazide Type I cell division Endothal endothall Type I cell division Embark mefluidide Type I cell division Prograss ethofumesate Type I (?) Enhancer, Trimmit paclobutrazole Type II GA inhibitor Cutless flurprimidol Type II GA inhibitor Primo trinexepac-ethyl Type II GA inhibitor Proxy ethephon ethylene effects Aqua-Gro L N/A (wetting agent) unknown   Table 2.   Comparison of Growing Degree Daybase 50 Model to first visible flowering in Poa annua over the last four years. Year Date that GDD50 >=50 1st Visible Flowering       2002 April 15 April 24-28 2001 April 12 April 27-29 2000 April 7 or April 24 May 3-7 1999 April 4 April 15   Table 3: General overview of percent Poa annua seedhead suppression by PGRs and wetting agents for tests conducted in the Chicago suburbs (2000 - 01).       Percent Poa annua with seedheads Product # of apps Rate/1000 ft2 May 10 May 24 June 1 Control - - - - 10 20 15 Aqua-Gro L 3 8,4,4 fl oz 4.5 10 15 Cascade 2 4 fl oz 7.5 20 12 Enhancer 2 0.18 fl oz 20 20 20 Primo 2 0.125 - 0.25 fl oz 10 15 15 Proxy 1-2 5 - 7.5 fl oz 2 3 3 Proxy + Primo 1-2 5 + 0.125 fl oz 2 3 3 Embark T & O 1 1.3 fl oz <1 <1 3 * Treatments applied mid to late April at Aurora CC and Fox Valley CC*   Table 4: Percent Poa annua seedhead suppression by Proxy alone and in tank mixes with Type II PGRs (2002 studies).       Percent Poa annua with seedheads Product Rate/1000 ft2 app date May 10 May 24 June 1 Control - - - - 12 27 25 Proxy 5 fl oz April 18 3 12 15 Proxy 5 fl oz April 24 11 15 13 Proxy + Primo 5 + 0.125 fl oz April 18 2 7 12 Proxy + Primo 5 + 0.125 fl oz April 24 8 6 8 Proxy + Trimmit 5 + 0.14 fl oz April 18 7 8 8 Proxy + Trimmit 5 + 0.14 fl oz April 24 10 7 15 * Aurora CC chipping green; seedheads first noted around May 1.*   Table 5: Percent Poa annua seedhead suppression on fairway turf - 2002.     Percent Poa annua with seedheads Product Rate/1000 ft2 May 10 May 16 May 24 May 31 June 7 Control - - 73 80 45 25 13 Proxy 5 fl oz 35 33 5 13 6 Proxy + Primo 5 + 0.25 fl oz 30 20 9 20 9 Proxy + Primo 7.5 + 0.25 fl oz 20 18 5 3 1 Proxy + Trimmit 5 + 0.28 fl oz 50 50 30 15 15 * Treatments applied April 23; Orchard Valley GC #9 fairway.*     For more information, e-mail newsletter@interactiveturf.com.