By Mike Proctor
Characteristics: Musk thistle is an introduced forb with abundant spines and large pink flowers. It is a biennial, which means it lives for two years. Mature plants can reach more than six feet in height. The flowers are quite attractive but are covered by spines similar to those on the rest of the plant. The seeds may be the only part of the plant that doesn’t have spines.
Musk thistle germinates in the fall, forming flattened “rosettes” (think dandelions) that over-winter and continue to grow in diameter until the second spring. At this time, they “bolt” rapidly to several feet in height and begin to produce as many as 100 flowers, each of which may contain more than 1,000 seeds.
Area of Importance: Musk thistle is classified as a noxious weed by the Oklahoma legislature, having been introduced from the Mediterranean. Opinions vary as to when and how it arrived in the United States. Some sources indicate it was in the United States as early as 1842, but it wasn’t really noticed until the early 1940s. It has been documented to occur in nearly every Oklahoma county.
Attributes: In Oklahoma, this species has formed colonies with more than 20,000 plants per acre. If you consider that a plant could easily be head high and three or four feet in diameter, there’s not much room left for things like cows, or grass even, in a pasture with a population of thistles like that. Control of musk thistle focuses mainly on minimizing seed production. Since they produce seed abundantly and the seed can remain viable in the soil for several years, it only takes a couple seasons to create a serious problem. Oklahoma’s Noxious Weed Law defines a “severe infestation” as 10 plants per acre.
Musk thistle is most likely to become established in overgrazed pastures and other disturbed areas like roadsides. Arid conditions, dense shade from trees, and competition with native grasses and forbs reduce the likelihood of establishment. Proper grazing management practices can help hinder the spread of musk thistle.
Any sort of treatment requires recognizing the plants while still in the rosette stage. It’s not very photogenic then, so finding photos of rosettes is a little hard. We’ve added some images to the Noble Research Institute’s Plant Image Gallery to address that. Simply having lots of spines and pink or purple flowers doesn’t make it a musk thistle.
Oklahoma has several native thistles that do not cause problems as well as five introduced species that do. Proper identification is necessary to apply the most effective control techniques to the right species and to avoid wasting time, effort and expense on the native species that support pollinators.
If the plant has already flowered, it’s too late to do much about it other than attempt to cut off all the heads by hand.
This can be effective for small infestations as long as the heads are sealed in a bag and disposed of properly. Biological control with a couple of different weevils seems to be an effective method for large infestations, but this can still take several years to have a significant impact. Combining mowing, spraying and biological methods has also shown to be effective but involves more timing and coordination to be successful.
Musk thistle is of little value to livestock or wildlife.