Stemming seed production key to noxious-weed control |

Stemming seed production key to noxious-weed control

Daily Staff Report
Amadej Trnkoczy | Special to the DailyMusk thistle and plumeless thistles (below) are both categorized as noxious weeds in Colorado.

In an effort to raise awareness about this growing problem, Gov. Bill Ritter declared last week Colorado Weed Awareness Week. The Vail Daily, in conjunction with Eagle County, Colorado State University Extension and the town of Vail, will be featuring a “Weed of the Week” each Monday throughout the summer, highlighting a noxious weed, its characteristics and various control method.

Have you ever noticed the tall, pink, prickly flowers that appear in midsummer in our open space areas? Or, maybe the white daisies gracing our roadsides? How about the yellow blooms beginning to creep up the Forest Service trails around the county?

What do these plants have in common? They’re all included on the Colorado Noxious Weed List. The multiple species of thistles, oxeye daisies and yellow toadflax described above are among as many as 30 different noxious weed species found throughout Eagle County.

Noxious weeds are highly aggressive, non-native plants that move into our natural areas and displace our native plants, thereby reducing wildlife forage and eliminating biodiversity. These plants also threaten agriculture, recreational opportunities, and even affect property values in the Colorado high country.

Weeds of the week

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Plumeless Thistle (Carduus acanthoides) and Musk Thistle (Carduus nutans).

These commonly seen plants are beginning to bloom around Eagle County and have similar growth habits and control methods.


– Biennial plants showing a leafy rosette the first year, and then bolting to 1 to 6 feet tall the second year. Once the plant seeds, it dies.

– Both plants are very spiny. Musk yhistles 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 11⁄2 to 3-inches wide with pinecone-like bracts below. Plumeless thistles are about half the size and have solitary flowers at the ends of branches.

– One thistle plant is capable of producing approximately 9,000 seeds that can remain viable for as long as 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 also may 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 since 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 affected areas with an appropriate plant is essential in control.

For more information on these plants and the weed management programs in Eagle County, visit or Or call the Eagle County Weed and Pest Department at 970-328-3540, the town of Vail Department of Public Works at 970-479-2158 or the Eagle County Extension Office at 970-328-8630.

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