Field Notes: Musk thistle

Valerie Tate

Musk thistle is a noxious weed found throughout Missouri. The musk thistle is sometimes referred to as a nodding thistle because the large single flower may bend over the stem. It can be spread many miles by wind-blown seeds or by moving hay containing musk thistle seed heads. Left uncontrolled in grasslands it can become a problem of epic proportions. Approximately 70 percent of the 10,000 seeds produced by one musk thistle plant will drop to the ground and germinate the first year, 20 percent the second year and the remaining germinate over the next seven to ten years or are blown away.

Generally, musk thistle is a biennial, meaning it takes two years to complete its life cycle. The first year the seed germinates in the late fall or early spring and the plant forms a basal rosette. The plant remains low to the ground and does not flower. The second year, the rosette elongates, reaching heights over six feet. Sharp spines are present on the leaf margins and down the stem. Leaves are grayish-green with light green or white midribs. Pink or purple flowers, 1 ¼ to 2 inches wide, appear on the stems between mid-May and August.

Chemical control is most effective during the early spring or late fall when the plant is in the rosette stage. Broadcast spraying is generally not effective once the plant has bolted, but high-volume spot spraying can be. Musk thistles can be controlled by mowing them within two days of when the flower at the tip of the plant (terminal flower) emerges. This will prevent seed production and rebolting. The problem with this method is that not all the plants will produce flowers at the same time.

Musk thistle rosette weevils and musk thistle head weevils have been released in Missouri to help control the musk thistle. The rosette weevils feed on the crowns, leaves and stems and can kill the plants directly or weaken them resulting in less vigorous plants. The head weevils feed on the seed head and reduce the number of viable seeds produced by the plant. This method will not control 100 percent of the musk thistle plants. It establishes a balance between the number of thistles present and the number of weevils present. As the number of plants declines, the number of weevils also declines. The thistle population will then have an opportunity to increase, which in turn will lead to an increase in the number of weevils present.

For more information contact Valerie Tate, University of Missouri Extension Field Specialist in Agronomy by email at tatev@missouri.edu or by phone at 660-895-5123.