Musk thistle (Carduus nutans) is a biennial plant commonly considered an invasive weed. It can easily grow to 6 feet. The plant only reproduces via seed production. One plant will produce more than 20,000 seeds each year, according to Colorado State University. Seeds are dispersed by the wind. The plant produces spiny, lobed foliage in a rosette fashion. Flowers range in color from pink to purple.
Musk Thistle Flowering
Each spring, the musk thistle produces an abundance of flowers. Flower-heads range in size from 1 to 3 inches in diameter. Seeds begin to form seven to 10 days after the flowers open up. A fully grown musk thistle plant will easily produce more than 100 flower heads, according to the Colorado State University. Flowers are produced non-stop over seven to nine weeks.
The seeds have parachute-like hairs that allow the wind to catch the seeds and lift them from the seed-heads. The hairs give the seeds the ability to fly on the wind with ease so dispersal is easier. Once the wind blows the seed heads, only 5 percent will remain attached to the plant and unable to be dispersed. It is believed that only one third of the seeds are viable from each flower-head.
The habitat of the musk thistle is wide ranging. It will easily grow aggressively from sea level up to 8,000 feet, according to the National Forest Service. It enjoys open areas such as pastures, meadows, roadsides and fields. The plant does not like excessively wet soil or areas of shade. It enjoys well-draining soil and full sunlight. The weed poses a serious threat to native plants due to its aggressive nature.
Herbicides that work well to control musk thistle include picloram, Milestone, metsulfuron, Banvel/Vanquish/Clarity (dicamba), Transline, glyphosate and 2,4-D, according to the Kansas Department of Agriculture. Herbicides must be applied in the early spring and fall to interrupt the plant's seed production and end its lifespan. Mowing the flower heads prior to seed production also offers moderate control.
The Colorado Department of Agriculture has distributed the weevil, Trichosirocalus horridus, across the state to offer biological control against musk thistle. The larvae of the weevil enjoys feeding on the seeds of the musk thistle and is effect at ending the plant's life cycle by decimating its seed production. Unfortunately, distribution has been placed on hold of the weevil for control because it has been found to also enjoy consuming native thistle seeds.