Mid-May is Best time to Spray Pastures for
Plumeless Thistle

Minnesota has nearly one million acres in pastureland providing forage to livestock throughout the growing season. One of the more problematic weeds for NW Minnesota pastures is plumeless thistle (Figure 1). Thistle plants are not consumed by cattle and the thorny stems and leaves quickly train animals to avoid heavily infested areas. The result can be areas of pasture left relatively ungrazed. This lost forage productivity is unnecessary.

Plumeless thistle is a noxious and invasive weed located statewide and commonly found in pastures, CRP, and wastelands. Management of plumeless thistle must focus on reducing seed production in the attempt for long term control. Unlike Canada thistle, plumeless is relatively easy to control with herbicides. Our research has indicated that the timing of herbicide applications can commonly be more significant than the thistle herbicide selected. Table 1. illustrates the effect of well and poorly timed herbicide applications for the control of plumeless thistle. Applying herbicides in May while the 1st and 2nd year plants are still in the vegetative (rosette) stage (Figure 2), has provided greater than 90% control. Waiting until 2nd year plants transition into reproductive growth (bolting), places much more importance on the herbicide selected and rate applied. A factor that often influences control decisions is the desire to preserve legumes in mixed grass/legume pastures. This is a difficult problem, and all of the herbicides/rates we evaluated, that were effective on plumeless thistle, eliminated legumes from the pasture.

Mowing has traditionally been used to remove flowers and seed heads. Much of the mowing done has proven cosmetic as apposed to effective. Plumeless thistle is reproductive from June through September and seeds become viable seven to eight days post flowering. Mowing can work if cutting heights are used to remove the terminal growing points and repeated throughout the season as lateral branches develop in response to mowing.