Effects of Herbicide Application Timing and Overseeding On Dallisgrass (Paspalum dilatatum) Control In Tall Fescue (Festuca arundinacea).

Dallisgrass (Paspalum dilatatum) is a perennial warm-season grassy weed found throughout much of the southern United States. Selective control of dallisgrass in tall fescue (Festuca arundinacea) is difficult. Previous reports indicate efficacy of the ACCase-inhibiting herbicide fluazifop-p-butyl (fluazifop) varies with seasonal application timing, but more investigation is warranted. The HPPD-inhibiting herbicides mesotrione, topramezone and tembotrione are being researched for use in turfgrass and may have dallisgrass activity. In 2010 and 2011 field experiments evaluated fluazifop (105 g ha^-1) alone or in combination with mesotrione (280 g ha^-1), topramezone (28 g ha^-1) and tembotrione (92 g ha^-1) applied at different growing degree day- (GDD) or cooling degree day- (CDD) based application timings. GDD’s were calculated using a 10 °C base beginning January 1. CDD’s were calculated subtracting the average daily temperature from a 21 °C base beginning July 1. The influence of fall or spring tall fescue interseeding was also evaluated. Plots were arranged in a split-split plot randomized complete block design and dallisgrass control and tall fescue injury were evaluated 2, 4, 8, 18, and 52 weeks after treatment (WAT). Fluazifop-containing treatments applied at 175 GDD and 5 CDD provided the greatest control 52 WAT. Combining HPPD-inhibitors with fluazifop did not improve control compared to fluazifop alone. Fluazifop-containing treatments caused > 20% injury to tall fescue when applied at 375 and 775 GDD. Data suggest fluazifop applications at 175 GDD or 5 CDD and fall interseeding will provide the greatest dallisgrass control with minimal tall fescue injury. Future research should investigate additional CDD-based applications to determine the most optimal timing to apply herbicides for dallisgrass control in fall.