Vail Valley weed of the week: Plumeless & musk thistle

VAIL, Colorado -Plumeless thistle (Carduus acanthoides) and musk thistle (carduus nutans) are commonly seen plants that are currently blooming around Eagle County. They grow similarly and can be controlled similarly. These plants are targeted by Eagle County’s municipal and county Noxious Weed Management Programs. Characteristics• Biennial plants showing a leafy rosette the first year, and then bolting up to a foot to six feet tall the second year. Once the plant seeds, it dies.• Both plants are very spiny. Musk Thistles have thick stems and large, deeply lobed leaves with five points per lobe. Plumeless thistles have spines along the stems and smaller, but highly spiny leaves.• Flowers are pink, red or purple. Musk Thistle flowers are 1 1/2 to three inches wide with pinecone-like bracts below. Plumeless thistles are about half the size and have solitary flowers at the end of branches.• One thistle plant is capable of producing approximately 9,000 seeds that can remain viable for up to 10 years.• Both plants invade disturbed areas, pasture, rangeland, forests and cropland throughout most of the United States.• There are 13 native thistle species in Colorado, many at higher elevations, so careful identification is important. The difference between native thistles and others is that musk, plumeless and Canada thistles are highly aggressive. Canada thistle, a perennial plant, will be featured at a later date.

Flower heads can be cut and removed to eliminate seed production. Rosettes may also be removed below the crown of the plant, however both of these methods must be repeated throughout the growing season as well as annually to be effective. Cut-plants should be bagged and taken to the landfill because seeds remain viable for such a long period of time. The key to controlling musk and plumeless thistles is to prevent seed production. Eliminating the seed bank of these plants, then reseeding effected areas with an appropriate plant is essential in control.For more information on these plants and the weed management programs in Eagle County visit http://www.eaglecounty.us/weed or http://www.vailgov.com/weeds. Or call the Eagle County Weed and Pest Department, 970-328-3540, the town of Vail Department of Public Works 970-479-2158, or the Eagle County Extension Office 970-328-8630.