SWEET VERNALGRASS: A NEW WEED PROBLEM FOR NORTHEASTERN TURFGRASS. A.N. Smith* and S. Askew, Virginia Tech, Blacksburg, VA (20)
Sweet vernalgrass is a perennial grass weed found in cool-season turfgrass that has recently become a concern in Virginia. It is highly competitive in the spring due to its rapid growth, early flowering, and potential allelopathic suppression. Research has shown that sweet vernalgrass has high phenotypic plasticity, allowing it to easily adapt to new environmental conditions. Experiments were conducted near Richmond, Virginia in 2010 and 2011 to determine herbicide options for sweet vernalgrass control in cool-season turf. Seven herbicide treatments were evaluated. In 2010, treatments were initially applied on June 15th, with a subsequent mesotrione application applied on July 6th. At 34 DAT, MSMA at 2.1 kg a.i. ha-1, mesotrione applied once at 0.28 kg a.i. ha-1, and mesotrione applied twice at 0.14 kg a.i. ha-1 controlled sweet vernalgrass 73, 63, and 57%, respectively. At 71 DAT, control by MSMA declined to 40%, whereas mesotrione applied once and mesotrione applied twice controlled sweet vernalgrass 100 and 67%, respectively. In 2011, initial treatments were applied April 20th, with a subsequent application of mesotrione applied on May 11th. Mesotrione applied once and mesotrione applied twice were the only herbicides that controlled sweet vernalgrass. However, mesotrione applied once did not maintain control and at 71 DAT, control was 0%. For mesotrione applied twice, control was 100% at 71 DAT. Fenoxaprop, quinclorac, amicarbazone, methiozolin, and sulfentrazone did not control sweet vernalgrass. In 2010, the decline of MSMA control can likely be explained by MSMA’s contact activity and the perennial nature of sweet vernalgrass. The differences seen with mesotrione applications between 2010 and 2011 may be explained by application timing. In 2010, rapid growth of sweet vernalgrass in the spring might explain why mesotrione applied once provided the best control as this herbicide tends to be more effective during rapid growth phases of susceptible plants and applying more active ingredient during sweet vernalgrass peak growth may play an important role in its control. In 2011, however, mesotrione was applied almost two months earlier. A single application may have been too early in the growing season, allowing the weed to recover. Additional research will be conducted on application timing of mesotrione to sweet vernalgrass. Delaying application may result in sufficient control without the need for an additional application.