Integrated Control of Musk Thistle
Musk thistles were accidentally introduced into the United States in the mid-to- late 1800ís. It has since spread throughout North America, with heavy infestations in this region. The musk thistle is noted for its attractive purple flower and spiny foliage. It usually grows in pastures, along roadsides, railroad-rights of way, fence borders, and wastelands. Economic impact is from costs for control and reduced pasture productivity. Moderate musk thistle infestations can reduce pasture yields 23 percent if the thistle is not controlled. Musk thistle is a noxious weed in both Kansas and Missouri.
Musk thistle reproduces entirely by seed. One plant produces an average of 10,000 seeds per plant. An integrated approach to musk thistle control, with the use of chemical, mechanical and biological methods is the most effective strategy for control. Chemical control is effective if properly timed. Tordon and 2,4-D are recommended herbicides for control. Herbicides should be applied from mid-March to late April in the spring, or September and October in the fall when the musk thistle is in the rosette stage. Later applications are too late for good control.
The best time to mow thistles is within two days after the terminal or primary flowerhead blooms, if you do not have thistle weevils present. This inhibits seed production the most. Biological control can be achieved with thistle weevils. Several years ago the musk thistle weevil was introduced into the area and has become established. The rosette weevil is also being introduced. These two natural enemies of the musk thistle can contribute to a substantial reduction in thistle populations. If thistle weevils are present, by mowing in mid-July, and spraying during the previously mentioned times, little harm will be done to the weevils and control will be improved. For more information on the integrated control of musk thistles, contact your local Extension office.