Researchers studying the effects of glyphosate (Rodeo) on Canada thistle found that a fall application of the herbicide decreased the weed's density more consistently than a spring or summer application. Rodeo is often used for weed control near riparian areas because other herbicides such as picloram and clopyralid are not labeled for use near open or shallow water.
The research team also found that the fall application had less effect on desirable vegetation, because the treatment targeted Canada thistle while avoiding the dormant native plants. Finally, the researchers recommend a wick application (rather than broadcast) of Rodeo at 2 pints per acre, which provided optimum control of the Canada thistle while maintaining the native grasses and forbs that are essential for waterfowl.
Montana's riparian areas are important sources of food and cover for wildlife, because the areas contain great diversity and density of plant life. However, riparian areas are especially vulnerable to weed invasion because they are easily degraded by natural and human-caused events such as flooding and agricultural development.
The MSU research team consisted of Roger Sheley, MSU Extension weed specialist; Jane Krueger-Mangold, graduate research assistant; and Bobbi D. Roos, MSU Extension agent in Daniels County. The study was funded by the Montana Department of Agriculture's Noxious Weed Trust Fund in cooperation with northeastern Montana Weed Districts and U.S. Fish and Wildlife Service (Medicine Lake Refuge).
Contact: Roger Sheley, (406) 994-5686