Question:  At the 2017 MATE show this year, one of the student’s research presentation dealt with treating cool season weedy grasses and broadleaf weeds in dormant warm season grasses. I am looking for an overview of the results. I have a customer worried about Roundup right now as warm as it has been here lately. I do remember there was more damage further into the GDD calendar, just not sure if we’re that far along in 2017 yet. I also remember there were some other options for treatment.

Dr. Shawn Askew (@VTTURFweeds) responds: I would not go too far out on a limb based on one year of experiments, TJ, but we are getting a second year’s trial on dormant zoysiagrass initiated today (2/17/17).   This information is only based on 1 year of data so we can draw no firm conclusions yet, but I will share my thoughts so that your client can make an informed decision about weed control in dormant turf. We are completely dormant in our area now… can't find any green leaves in the canopy.  In our work last year, when the zoysiagrass had about 5% cover of green leaves (put your hand down on the turf and you are covering about 5 to 10 leaves that are down in an otherwise brown canopy) we had great results from Roundup Pro Concentrate at 12 to 16 oz/A, Sureguard at 12 oz/A, Ronstar Flo at 3 lb ai/A, and Revolver at 17 oz/A.  Based on last year’s results, I would not recommend Finale at 6 qt/A as it was too injurious.

When the zoysiagrass was closer to 15% green cover and having many leaves on the canopy surface, Revolver and Ronstar were the only "safe" products.  Of course, Ronstar does not offer postemergence weed control.  At this worse case timing, Roundup caused about a 1 month delay in green-up.  Finale was worse.  Sureguard caused about a 2-3 week delay.  In all cases except Finale, the zoysiagrass made a complete recovery and there was not any long term evidence of turf thinning later in the season, even at this late timing.

Based on all of our investigations to this point, we feel that application volume and possibly nozzle type and pressure could impact the degree of injury to zoysiagrass when spraying Roundup.  Lower spray volumes are best.  We recommend 25 to 30 gallons of spray solution per acre or about 0.5 to 0.75 gallons per 1000 sq ft.  We are testing these types of things in this year's research.  Stay tuned!