Musk thistle: A persistent pest

July is when you really start to notice them, those large, bright purple, prickly flowers on a tall stalk. Musk thistle can grow up to 8-feet tall, with individual brightly colored flowers at the end of each stalk. One or two along the roadside really catch your eye, a splash of color as you drive by. Hundreds on a hillside in open lands is another story, and while the bright flower may be attractive, a huge patch of musk thistle is virtually impenetrable, making any use of the land impossible. Once those flowers mature they lose their bright color and become a distribution mechanism for thousands of seeds and future musk thistle infestations.

By Ellen Nelson
Larimer County Weed District

Landowners in disturbed areas may see carpets of musk thistle rosettes and thick stands of this large thistle. This plant is more than willing to move in and establish itself in disturbed areas. It is happy to get some roots in the ground and start the process of reestablishing vegetative cover after a fire or flood. And while the vegetative cover is appreciated, if left untended this plant can take over and ultimately prevent other plants for reestablishing.

Fortunately, musk thistle can be controlled if you understand its lifecycle and time your management activities accordingly. Musk thistle is a biennial, it reproduces only from seed, and takes two years to complete its lifecycle. To eliminate musk thistle you need to deplete the seed bank in the soil. To accomplish this, you will need to monitor areas that have had musk thistle and prevent additional seed production for several consecutive years.

A first year musk thistle is a low growing, sometimes flat rosette. The leaves are somewhat wavy and lobed, with white outlined margins and a light or white mid-vein. Some rosettes can be up to a foot in diameter. At this stage, treating with herbicide or pulling the plant is highly effective. The second year of its life, a musk thistle produces stalks with the characteristic prickly purple flowers. Up until the plant produces a flower, treating with herbicide or pulling the plant, making sure to remove the taproot, is very effective.

Once musk thistle produces a flower, treating with herbicide could be a waste of time and money. After producing a flower, the plant has completed its life-cycle and is going to die anyway. Herbicide treatment at this time may not prevent the seeds from maturing. The same is true for pulling these plants, there may be enough energy left in the root and stalk for the seeds in a pulled flower to mature and become viable. Pulling and leaving the flowers on the ground at this time may be aiding their reproduction. If you pull musk thistle with flowers, remove and bag the flowers and take them to the landfill.

For more information about musk thistle or other weeds, for site visits and herbicide recommendations contact the Larimer County Weed District at 970-498-5768 or online at larimer.org/weeds