Thistles continue to be a problem in Barry County as well as other parts of southwest Missouri according to Tim Schnakenberg, agronomy specialist, University of Missouri Extension.

The good news is that fall is a perfect time for landowners to start controlling thistles on their land 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,” said Schnakenberg.

Missouri law says all landowners have the responsibility to keep musk thistle plants from going to seed. Schnakenberg says this region also has plenty of bull and tall thistles that can be just as invasive, though not on the noxious weed list of Missouri.

Landowners should rely on a variety of approaches to control thistle.

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,” said Schnakenberg.

If the plant begins to bolt it is not as susceptible and may require stronger pesticides. Products such as Grazon and Tordon also work well because they can leave a residue in the soil to kill late-emerging rosettes.

Multiple passes with a brush hog earlier in the season can be effective if timed properly. The first pass should occur immediately after the terminal bloom flower head blooms. Since viable seed can be produced within seven to 10 days of the flower turning pink, waiting later can allow some seed to be produced and spread with the brush hog. Mowing earlier can lead to re-growth with the result of additional flower heads produced.

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,” said Schnakenberg.

For more information, including an MU Guide on the control of thistles, contact the University of Missouri Extension Center in Cassville at (417) 847-3161.

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