Vail Weed of the Week: Thistles 101 |

Vail Weed of the Week: Thistles 101

Gregg Barrie
Weeds of the Week
Vail, CO Colorado
Special to the Daily/Gregg Barrie

VAIL, Colorado – If there was a poster child for noxious weeds, thistles could do the job. They are easily noticed with big, pink flowers and sharp spines. They form dense colonies and can be found just about anywhere, from roadsides to golf courses, stream banks to back yards. Simply put, they just look mean. At least five species of non-native, invasive thistles can be found in Eagle County. This article is intended to help residents identify, manage and remove these plants.

It is important to note that not all thistles are bad. In fact, Colorado is home to more than 20 native thistle species. They fill an important niche in biodiversity as pioneer plants in disturbed areas. In Eagle County, they are most often seen along trails and other minimally disturbed areas. Most have a small white flower and a purplish stem.

In Eagle County, the invasive thistles fall into two categories: biennial and perennial. Treatment of these plants is distinctly different. Biennial thistle die after they flower, therefore, eliminating the flower or the plant prevents reproduction. By contrast, perennial thistles flower every year and also spread by aggressive root systems. Removal of flowers in these plants stimulates the root system.

So, how do you tell the difference?

Biennials include Plumeless Thistle (Carduus acanthoides), Musk Thistle (Carduus nutans), Scotch Thistle (Onopordum acanthium) and Bull Thistle (Cirsium vulgare).

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These plants characteristics are as follows: Plumeless and Musk Thistle are the most common in our area.

Plants show a leafy rosette the first year, and then bolt to two feet to six feet tall the second year to flower. All 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. Scotch Thistles are distinctly grayish-green with large, fleshy leaves.

Flowers are pink, red, or purple. Musk Thistle flowers are the biggest with 1 1/2 to 3 inch wide flowers with pinecone-like bracts below. Plumeless Thistles are about one half the size and have solitary flowers at the end of branches. The other fall in between. One thistle plant is capable of producing approximately 9,000 seeds that can remain viable for up to 10 years.

The key to controlling biennial thistles is to prevent seed production. Flower heads and plant can be cut and removed. First year rosettes may also be removed below the crown of the plant, however these methods may need to be repeated throughout the growing season to be effective. Cut plants and flowers should be bagged and taken to the landfill since seeds remain viable for such a long period of time. Over time, the soil seed bank will be eliminated. Biennials can also be managed through the use of herbicides.

Perennial: Canada Thistle (Cirsium arvense)

Characteristics include an upright perennial that can grow to four feet but usually smaller. Plant can form dense colonies in wet areas and lawns. Leaves are light to dark green, irregularly lobed with deep-toothed, spiny margins. Clusters of pink to purple flowers in the high country from late June to September. Spreads with an extensive creeping root system. Seeds are less viable than other invasive thistles but can remain viable in the soil for up to 20 years.

Unlike the big biennial thistles, Canada Thistle cannot be controlled by removing the seed heads. Do not dig, pull, or cut Canada Thistle – this will likely stimulate root growth. Approved herbicide treatments are recommended – once in July (just before the plant goes to bloom) and again after the first frost (when the plant is dieing back). The July treatment will stress the plant because it uses its root reserves to grow more foliage. The fall treatment will allow herbicide to be taken into the root system since the plant is bringing sugars down into the roots to get ready for winter. This will be a multi-year process but with persistence you will gain the upper hand on this tough plant. If the “organic” approach is more your style, cover the infested area with black plastic and leave it on for several growing seasons.

Maintaining healthy native plant communities is the best way to prevent the establishment of any weed. Thistles can quickly invade disturbed areas, making revegetation critical to controlling this plant as well as other noxious weed species.

For more information on these plants and the Weed Management Programs in Eagle County visit or

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