Suppression of Spring Flowering of Poa annua L.   February 25, 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 ball roll is adversely affected by seedheads (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 plants.   Seed production in Poa probably diverts photosynthate away from vegetative tissues (leaves + roots) to the flowers, resulting in reduced root depth and shoot growth after seeding in Poa annua.   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. Of the products listed in Table 1, the best success historically has been found using Embark (mefluidide), but timing and phytotoxicity problems have limited its use, especially on greens.   Several superintendents have also used the wetting agent Aqua-Gro, which is less phytotoxic but provides more variable results.   Also, many superintendents have tried early spring applications of gibberellin inhibitor PGRs (Cutless, Enhancer, Trimmit), and reported some seedhead suppression following early season treatments.   However, seedhead suppression was not the primary goal of these programs. 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, which is no longer manufactured (and supplies are dwindling!).   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 and Primo (trinexapac-ethyl) have shown good results with less turf discoloration. A general overview of field test data from Chicago area trials can be found in Table 2.   Note that the percent seedhead inhibition is an average of all the tests, and results can vary greatly with weather conditions, application timing, and Poa biotypes present in treated areas.   Embark treatments were with the newer "lite" formulation, which is now called Embark T & O.   Phytotoxicity of Embark treatments was observed on green-height creeping bentgrass, and 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 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.   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 (paclobutrazole) 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. In summary, Embark and Proxy appear to have the most activity against Poa seedhead formation, but are not without risks - especially at green height.   If you want to try Embark or Proxy for seedhead suppression, be aware of possible phytotoxic effects.   Treat small test areas or the chipping green/practice tee first, and include some chelated Fe in Embark treatments to reduce discoloration and thinning (note: non-chelated Fe forms interfere with the activity of Embark). Table 1.   Chemicals that have been used for Poa annua seedhead suppression:   Common name Trade name Common name Trade name   maleic hydrazide 'MH' paclobutrazole 'Enancer', 'Trimmet   chlorfluorenol 'Maintain' ethephon 'Proxy'   endothall 'Endothal' flurprimidol 'Cutless'   ethofumasate 'Prograss' trinexapac-ethyl 'Primo'   mefluidide 'Embark' (wetting agent) 'Aqua-Gro L' Table 2.   General overview of percent Poa annua seedhead suppression by PGRs and wetting agents for tests conducted in the Chicago suburbs (2 years, 3 locations). Product App. rates # of apps % seed reduction Comments Embark T&O 1.3 fl oz/ 1000 1 app 80-90% some phyto on bentgrass (add chelated Fe) Proxy + Primo 5 fl oz + 1/8 fl oz 1-2 apps 75-85% less thinning with 1 app (follow-up w/ Primo apps) Proxy alone 5 fl oz/ 1000 1-2 apps 70-80% yellowing with 2 apps (add chelated Fe) Aqua-Gro L 8 + 8 fl oz 8 + 4 + 4 fl oz 2 apps 3 apps 50-70% 50-70% less consistent results less phyto this method Cascade 4 + 4 fl oz 2 apps 10-25% little effect, no phyto Primo .15 to .25 oz 2 apps 0-10% nil effect, no phyto Enhancer 6-10 oz/acre 1-2 apps 0-10% seedheads stayed on stunted plants longer   For more information, e-mail newsletter@interactiveturf.com.