Herbicide mixtures applied in the spring or fall for absinth wormwood control. Rodney G. Lym. (Department of Plant Sciences, North Dakota State University, Fargo, ND 58108-6050).
Aminopyralid [Milestone specialty herbicide] and clopyralid [Transline specialty herbicide] are commonly used to control absinth wormwood in a variety of environments. Aminocyclopyrachlor (AMCP) has also been used to control absinth wormwood in non-grazed or hayed areas. Often combinations of herbicides have provided better long-term control of invasive species than a single herbicide used alone. The purpose of this research was to evaluate aminopyralid or AMCP applied at reduced rates with other herbicides for long-term absinth wormwood control. The aminopyralid study was established on an active gravel quarry near Valley City, ND that was heavily infested with absinth wormwood. The treatments were applied on May 26 or September 15, 2011. Absinth wormwood was in the vegetative growth stage and 11 to 18 inches tall when treatments were applied in May. Because absinth wormwood grows 4 to 6 feet tall, the plot area was mowed in late-July 2011. The plants had regrown and were 6 to 8 inches tall when the fall treatments were applied. The AMCP study was established within a fenced area of a horse pasture near Spiritwood, ND. Herbicides were applied on June 3, 2013 when absinth wormwood was 4 to 16 inches tall in the rosette growth stage. Fall treatments were applied on September 13, 2013 to plants that had 12 to 18 inches of regrowth after being mowed in August. Herbicides were applied using a hand-held boom sprayer delivering 17 gpa at 3 5 psi. Experimental plots were 10 by 30 feet at Valley City and 10 by 25 feet at Spiritwood. Treatments were replicated four times in a randomized complete block design. Absinth wormwood control was evaluated visually using percent stand reduction compared to the untreated control. All treatments that contained aminopyralid or clopyralid provided 90% or better absinth wormwood control36 months after treatment (MAT) whether applied in June or September (Table 1 ). The most cost-effective treatment was clopyralid plus aminopyralid at 2.4 + 0.5 ozJA which provided 95% absinth wormwood control36 MAT treatment. The least effective treatment was dicamba applied at 16 ozJ A in the spring which provided 56% absinth wormwood control by the end of the study. All treatments that contained AMCP or aminopyralid applied alone provided 90% or more absinth wormwood control the season after treatment in the pasture location (Table 2). Control was similar whether AMCP at 1.1 or 1.8 ozJA was applied with chlorsulfuron, metsulfuron, or 2,4-D. In summary, absinth wormwood was easily controlled with aminopyralid or clopyralid applied alone or in combination and with any treatment that contained AMCP whether applied in the spring or fall. The choice of treatments should be based on both cost and other target weeds in the same area where absinth wormwood has established.