Management of Poa annua on Overseeded Fairways

March 16, 2005

Summary: Product labels for several preemergent herbicides currently contain warnings that applications made less than 4 months before overseeding may result in damage to turf. In other words, applications must be made no later than June—too early to obtain season long control in Southern California. Data from the southeastern U.S. suggests that both Ronstar and Barricade can be applied as late as 6 - 8 weeks before overseeding without damaging emerging ryegrass. This replicated trial was set up to confirm these results under southwestern weather and turf management conditions and to investigate the following questions:

Which herbicides can be applied closest to the overseeding date, without damaging turf?

Which herbicides provide the best control of Poa annua?

Key results include the following:

If the correct rates of pre-emergence herbicides are used (50-100 lbs/A Ronstar G, less than 2 lb/A Barricade 65 G), these products can be applied much closer to overseeding (4 - 8 weeks before overseeding) than their labels currently indicate. This strategy will result in little or no turf damage, and good weed control through April or May, or as much as 9 months post-application.
Of the thirty-six different treatments that were evaluated, the best poa control was observed with both pre-emergence herbicides (Ronstar and Barricade) and the post-emergent herbicide Prograss.
Turf quality (damage to ryegrass) was significantly influenced by the rate of Ronstar G used, with a significant negative correlation between rate and turf quality. Interestingly, the timing of application of Ronstar G had little negative impact on turfgrass quality, even when applications were made 10, 8, 6, 4 and 2 weeks before overseeding.
Higher rates of pre-emergence herbicides (Ronstar G at >100 lb/A and Barricade 65 G at 2 lb/A) caused severe damage to ryegrass when applied 2 - 10 weeks before overseeding.
Split applications of Ronstar (50 lb/A, 4-8 weeks before overseeding and 100 lb/A 6-12 weeks after overseeding) provided excellent poa control, with consistent lack of phytotoxicity. This strategy appears to be the most likely to ensure good weed control, without any concomitant damage to turf.

Printable version of full report

Principal Investigators: Wendy Gelernter, Ph.D. and Larry J. Stowell, Ph.D., CPPP, CPAg

Cooperator: Bill Kostes, Desert Dunes Golf Club

Sponsors: AgrEvo, Hi-Lo GCSA, Novartis, Rhone-Poulenc