As the tall green grass fades to brown, the purple flowers and white airborne seeds of thistles can now easily be observed in dense patches throughout Routt County.

Based on its abundance in our area, it may come as no surprise that Canada thistle is one of the most widely distributed noxious weeds in the United States. It arrived here in the 1600s from Europe and has become a problematic weed for recreation, wilderness, crops, rangelands and pasture lands.

Canada thistle is a deep-rooted perennial that spreads by seeds and aggressive creeping roots. The leaves are spiny and bright green. Flowers are varying shades of purple to pink and have numerous flowers occurring in small clusters at the end of stems.

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This species emerges in spring and flowers throughout the summer. Seeds are viable in the soil for 20 years. Mowing every two weeks throughout the growing season can be a good method to suppress Canada thistle.

Mowing in conjunction with applying a herbicide to regrowth is the most effective method of management. Digging is not recommended and will stimulate additional stems and growth. In fact, a root fragment only 0.25 inches is capable of forming a new plant. Thus, moving topsoil can be a good vector for spreading this weed. Know where your topsoil is coming from and always be prepared to manage weeds whenever soil is disturbed.

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Unfortunately, Canada thistle isn’t the only noxious thistle to be on the lookout for. We also have the noxious biennial thistles of Musk thistle, Bull thistle and Scotch thistle. The good news is that these biennial thistles are not perennials, meaning our only goal is to stop them from going to seed, not battling a vast root system underground like with Canada thistle. Biennials can be effectively controlled mechanically. Dig up plants and leave them to decompose on the ground.

However, if they have any flowers or seed, they should be very carefully secured and bagged or burned. If a population becomes too large to mechanically control, then herbicides can be utilized to cover larger areas. We still encourage an integrated approach of collecting as many flowers and seed heads as possible before spraying remaining stems, leaves and rosettes.

The most effective herbicides on thistles include Milestone and Telar XP. A surfactant is always encouraged with herbicide applications. The herbicide you choose to utilize should be dependent on the type of site it is being applied (i.e. roadside, lawn, pasture, etc.) and label restrictions should always be carefully followed. 

It is also important to note, we do have a number of thistles that are native to Colorado and that are valuable forage for our native wildlife species. Please take the time to properly identify specific thistle species before managing them.

For more information on thistles and other noxious weeds, please visit the Routt County noxious weed website at Co.Routt.co.us/216/Weed-Program or email us at weeds@co.routt.co.us.

Tiffany Carlson is the Routt County Noxious Weed Supervisor.