Abstract: Spot treatments with hexazinone (0.03, 0.046, and 0.06% ai [v/v]) were evaluated for their effects on thistle density, seed production, and crown vetch vigor from 1991 to 1994 to determine if long-term usage of hexazinone caused damage to the vetch and exacerbated the thistle problem. Thistle densities were reduced within a year at any concentration of hexazinone and seed production was reduced or eliminated. Environment had a large impact because initial thistle densities varied widely among years. The amount of precipitation during July of the previous year accounted for 89% of the variation in initial thistle densities in the subsequent year. Vetch injury from hexazinone was also best explained by the amount of precipitation during July. Vigorously growing crown vetch in wet years may have suppressed the thistles enough to reduce density the following year while drought years reduced the competitiveness of the crown vetch, resulting in higher initial densities of thistles in the following year.
Nomenclature: Hexazinone; Canada thistle, Cirsium arvense (L.) Scop. #3 CIRAR; crown vetch, Coronilla varia L.
Additional index words: Herbicide selectivity, interspecific plant competition, weather effects.
Abbreviation: PPFD, photosynthetic photon flux density.