Verdure: Winter applications for spring Poa annua seedhead suppression?

A turf scientist tested whether earlier applications of PGRs could deliver better control of annual bluegrass seedheads.


The fuzzy, white and copious seedheads of annual bluegrass (Poa annua) are a nuisance in golf course putting greens. They look bad, affect turfgrass color, and disturb trueness of golf ball roll. Various chemical treatments are used to reduce Poa annua seedhead populations, with the applications often made in spring.

But what if plants were already initiating seedheads in a warmer winter period? Would winter applications be helpful?

That’s what Shawn Askew, Ph.D., of Virginia Tech set out to investigate — to study the earlier application of plant growth regulators for possible effectiveness on Poa annua seedhead suppression.

To do this, studies were conducted on four golf greens and one fairway (all bentgrass cultivars) throughout Virginia. Treatments were ethephon (Proxy, Bayer) application timings of: 1) spring only (March and April); 2) January and spring; 3) February and spring; and 4) March and spring. These treatments also included applications of trinexapac-ethyl (Primo Maxx, Syngenta) in the spring treatments. Two comparison treatments were also included: one a standard mefluidide (Embark, PBI-Gordon) + chelated iron treatment, spring application only, and the other a non-treated control.

Ethephon was applied at 3.4 pounds a.i./acre (3,812 grams a.i./hectare), trinexapac-ethyl at 0.04 pound a.i./acre (48 grams a.i./hectare), mefluidide at 0.06 pound a.i./acre (70 grams a.i./hectare) and chelated iron at 0.8 pound iron a.i./acre (920 grams a.i./hectare).

Collected data included creeping bentgrass and annual bluegrass injury and density, and annual bluegrass seedhead cover at zero, three and six weeks after the first spring treatment.

At three weeks after the spring treatments were applied, creeping bentgrass was not injured by the application of ethephon in any of the programs. Application of mefluidide injured both the creeping bentgrass (about 5% injury at three weeks after the spring treatment) and the annual bluegrass (about 19% injury). Turfgrass quality was improved when ethephon was applied in winter, with differences most evident at six weeks after spring treatments were first applied.

This improvement in quality was related to annual bluegrass seedhead suppression. At three weeks after the spring treatments were applied, 69% of the annual bluegrass in the control plots (no plant growth regulators applied, of any type) had seedheads. In comparison, the spring-only ethephon treatment produced plots in which 31% of the annual bluegrass had seedheads. Adding the winter application of ethephon reduced seedhead populations to around 5% (January or February application of ethephon). The March application of ethephon was not as effective, with around 13% seedhead cover. For the January and February applications of ethephon, this was a 95% reduction in seedheads when compared with the untreated control plots. The other “standard control” plot of mefluidide (spring-only applications) had 19% seedhead cover in those plots.

So, adding a January or February application of ethephon was a good tool for significantly reducing seedhead populations of Poa annua below those observed when only spring applications were made. The author emphasized that the winter treatment must be paired with a spring treatment based on a conventional growing degree day- (GDD)-based application program. Applying the ethephon in January or February (followed by a spring program) reduced seedhead cover 22% to 55%, and neither the creeping bentgrass nor the Poa annua suffered additional long-term injury.

As an update, since this paper was published, newer work from this researcher indicates that the application window for best spring seedhead control is from Nov. 15 to Feb. 15. And, of course, always follow all label recommendations and instructions for these products.

Source: Askew, S.D. 2017. Plant growth regulators applied in winter improve annual bluegrass (Poa annua) seedhead suppression on golf greens. Weed Technology 31:701-713.

Editor’s note: Read all of Beth Guertal’s recent Verdure columns.

Beth Guertal, Ph.D., is the Rowe Professor of Soil Fertility in the Department of Crop, Soil and Environmental Sciences at Auburn University in Auburn, Ala., and past president of the Crop Science Society of America. She is a 21-year member of GCSAA.