Published online by Cambridge University Press: 12 June 2017
A poisonous pasture weed, western whorled milkweed [Asclepias subverticillata (Gray) Vail] is capable of spreading in a few years from scattered clumps, highly variable in number of stems, to an almost solid, uniform stand. The most effective herbicide evaluated for its control was 4-amino-3,5,6-trichloropicolinic acid (picloram). Use of 2-methoxy-3,6-dichlorobenzoic acid (dicamba) gave only fair control. The herbicides 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4–5-trichlorophenoxyacetic acid (2,4,5-T), 2-(2,4,5-trichlorophenoxy)propionic acid (silvex), and 3-amino-l, 2,4-triazole (amitrole) were ineffective. The addition of amitrole did not enhance the effectiveness of dicamba or picloram for control of this species.