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A Thorny Problem

Contributed by:Progressive Farmer

Author: Victoria G. Meyers

Eventually everyone with land has to deal with musk thistle. This spiny weed with the large, attractive reddish-purple flowers has been in the United States for nearly 100 years. It came from Europe where it was grown as an ornamental. But Europe had something the United States didn't”a musk thistle weevil that kept seed production low in the plants.

Now the United States has the weevil and several other good control options to keep the weed in check.

Prevention. Good grazing management is one key to keeping musk thistle at bay. Stocking rates should be such that they prevent overgrazing, allowing a competitive cover to exist and avoiding bare ground. In cool-season pastures, keep in mind the need for vigorous competition against the weed. Maintain good soil fertility and avoid stocking rates that are too high.

Burning. A prescribed burn in the late spring as grasses start to grow will help in a musk thistle control program. The burn doesn't kill the musk thistle; it stimulates warm-season grasses that help prevent the musk thistle from becoming established.

Mowing. Seed production can be reduced by mowing musk thistle when the plants are in their late-bloom phase. But because all the weeds won't be at the same stage of growth, the area will have to be mowed again as needed to keep the weed cut back.

Hand removal. If you just have a few musk thistle plants, hand removal is a good option. It will be worth your time”just one of these beauties can provide more than 10,0000 seeds for the next crop. The seeds are spread by the wind, so they can move into a lot of areas quickly. To remove by hand, cut through the taproot at least 2 inches below the crown. Removal of the individual flowering heads will help, but new heads will grow unless the entire plant is dug up. Be sure to place any plants or flowering heads in a tight container and bury them or allow them to rot to prevent seed germination.

Herbicides. Sometimes the sheer quantity of a weed will make chemical applications the most efficient means of getting the upper hand. There are several herbicides labeled for use on musk thistle. These include 2,4-D, dicamba, picloram, metsulfuron methyl, clopyralid and triasulfuron. Check label directions for rates and best times of application. For the most part, chemical controls begin to lose their effectiveness after the plant bolts and begins to flower. The best time to treat the weed and avoid seed production is at the rosette or bolting stage. Waiting until the bud or early flower shows can mean a dramatic increase in how much seed the plant will be able to produce.

Biological. The musk thistle weevil was introduced in the United States in 1969. Many states have had successful releases of the weevil, and they have made an impact on musk thistle populations. The weevils lay eggs in the musk thistle plants, and the larvae feed at the base of the flower, interfering with seed production and viability. Weevil release sites should not be grazed, mowed or sprayed for two years. It is estimated it takes from six to 10 years for weevil populations in a new release area to build up enough to significantly reduce musk thistle seed production.