Musk thistles continue to infest pastures and fields in Missouri. The good news is that fall is a perfect time to start controlling thistles while they are in the young and susceptible rosette state.

"Most rosettes that are seen now will become large plants by spring next year, leading to a release of as many as 10,000 seeds per plant if left uncontrolled," says Tim Schnakenberg, regional agronomy specialist, University of Missouri Extension.

Missouri law says all landowners have the responsibility to keep musk thistle plants from going to seed. The most effective ways of controlling this pest are by maintaining good fertility and grazing management techniques, chemical treatment when appropriate, mowing at proper stages of growth, biological control and digging.

"There are two good times to treat thistles. When it is in the rosette stage the plant is most susceptible to herbicides like 2,4-D and Milestone. To catch the plant in that stage, spray in September and October or in the spring during late March and early April on a warm day," Schnakenberg advises.

Products such as Grazon and Tordon work well on stubborn thistle plants because they can leave a residual in the soil to kill late-emerging rosettes.

Most fields have had a few musk thistle flower head weevils this year that destroyed seeds in the head before they became viable. They won't kill all thistles, but they do have an impact.

"It will take all landowners working together to get a handle on this invasive weed. We will never completely eradicate them, but with a persistent effort, the effect of thistles on property can be lessened," Schnakenberg says.

For more information, contact your local University of Missouri Extension Center for an MU Guide on the control of thistles.