Winter Glyphosate Applications to Bermudagrass
Glyphosate applications to dormant bermudagrass in winter are excellent for general weed control. Many weeds, including Poa annua, can be effectively controlled at the labeled rate of 0.5 lbs ai/a of glyphosate (1 pt/acre for the 4 lb/gallon product). Furthermore, glyphosate use is an important tool for resistance management of weeds in bermudagrass.
There are two common questions turfgrass managers frequently ask: 1) How important is ambient temperature for efficacy, and 2) Can damage to bermudagrass occur if bermudagrass is not totally dormant.
Importance of ambient temperature. The short answer is, temperature is the most important factor for good control. It is even more important than application rate of glyphosate. The general rule is to apply glyphosate on a day when temperatures exceed about 60 degrees. Figure 1 (top) shows the importance of air temperature on glyphosate activity. The test was done on tall fescue at an intentionally low rate in order to tease out the effects of air temperature. The target application temperatures ranged from 50-60 to 90-100. Note that when glyphosate was applied in the 50s, tall fescue control was 0. However, when applied in the 60s, control jumped to 59%. In the 70s, control increased to 92%. Therefore, my recommendation is to apply on a warm day. At least 60 degrees but 70 is even better.
What if bermudagrass is not completely dormant? The best scenario is a 70 degree day in winter when bermudagrass is totally dormant. If there is any green bermudagrass, glyphosate applications will likely result in a slight delay in greenup. However, as long as no more than 0.5 lbs ai/a is applied, my research has shown no permanent damage to bermudagrass. In fact, Figure 2 (bottom) shows the results of a trial on 15% greened up ‘Tifway’ bermudagrass. Note the 16 oz/acre rate at 9 days after treatment (9 DAT) reduced greenup from 50% in nontreated plots to 18%. However, by 24 days after treatment (DAT), the 16 oz/acre rate was 65% greened up compared to 80% for the nontreated. This delay, assuming broadcast sprayed, will likely not be noticeable. However, if spot spraying, then the delay in greenup will be much more noticeable. In summary, dormant bermudagrass is always recommended but slightly green bermudagrass will only be delayed if the 0.5 lbs ai/a rate is used