Thistle identification: They all have purple flowers

By Ethan Proud
PREVIEW Columnist
There are five species of thistle on the Colorado Noxious Weed List, and there are many native species that do not require treatment. Four of the five noxious thistles are biennials and can be treated with herbicides or dug up for effective control.
Canada thistle is a perennial and spreads from its roots. Mowing or hand-pulling plants is not effective unless it is done aggressively. Mowing the thistle every two weeks at least will exhaust the root reserves, but each cut will trigger new growth and if the mowing is lax, the plant will quickly take over.
Tips to identify thistle species:
Musk thistle is the most prevalent List B noxious weed in Archuleta County and can be found on nearly every county road. It has white leaf margins and highly dissected leaves and may grow between 3 to 7 feet tall. Musk thistle has one terminal bloom at the end of each stem and may have five or more blooms per plant. The stem bends under the weight of the bloom and gives it the nickname of “nodding thistle.”
Plumeless thistle has not yet been recorded in Archuleta County and has a very similar appearance to musk thistle, but its blooms do not nod and are urn-shaped. The stems are covered in spiny wings. Plumeless thistle and musk thistle may hybridize.
Bull thistle also has an urn-shaped flower receptacle, with spiny bracts. Its leaves are more dissected than either musk or plumeless thistle and are a deep forest green and are very hairy on the surface. It does not produce as many seeds as musk thistle and is less aggressive, but can be found growing in the same environments.
Scotch thistle has been reported in Archuleta County, though it is not prevalent. It can be distinguished easily by its size; plants may grow in excess of 10 feet tall and the leaves may be 20 inches long. The leaves are a blue-grey color (almost like sagebrush) and are hairy. Stands of scotch thistle can be too thick for livestock or wildlife to navigate through.
Canada thistle is the only perennial noxious thistle species in Colorado. The plants may grow from 1 to 4 feet tall and have lance-shaped leaves that may be slightly dissected or entire. The underside of the leaves is often lighter than the top. The flowers are much smaller than other noxious thistles and are often only 1 inch long and are a lighter purple, or lavender, compared to other noxious thistles. A colony of Canada thistle is typically only a few individual plants, connected by the roots. Chemical control is the most effective option.
The color of the flower is not a good identifying feature. All thistle flowers (in the genera carduus, cirsium and onopordum) can range from white to purple, though some species have white flowers more frequently than others. Knapweeds, teasel, dames rocket, loosestrife, bouncingbet and burdock all have purple flowers.
If the thistle you find in your yard lack the characteristics above, then you may not have a noxious weed and treatment may not be required. Native thistles fill a niche in the ecosystem and are part of the native environment. If you need help identifying a plant, take a picture or, preferably, a sample to the county weed and pest or Extension office.
Archuleta County Weed and Pest is your local resource for managing noxious weed populations and controlling other pests. For more information, please contact Ethan Proud at 264-6773 or email at eproud@archuletacounty.org.
Seed potato orders
Orders are now being taken for Colorado-certified seed potatoes from the San Luis Valley. There are red, white, blue and pink varieties for $1 per pound.
They will be ready to pick up May 9-11. Please call the CSU Extension office to place your order.
4-H paper clovers
Through April 22, buy a paper clover for $1 at Tractor Supply and help our 4-H youth attend leadership events through the 4-H Paper Clover Campaign.
CPR and first aid classes
CPR and first aid certification classes are now being offered monthly by the CSU Extension office on the second Monday and Wednesday of each month from 6 to 10 pm. Anyone needing to receive or renew certification can register by calling the Extension office at 264-5931.
We will also attempt to schedule classes on additional dates with five or more registrations. Cost for the classes is $80 for combined CPR/first aid and $55 for CPR, first aid or recertification. The type of first aid information provided will vary by the needs of the audience.
More about
CSU Extension
CSU Extension is your local university community connection for research-based information about natural resource management; living well through raising kids, eating right and spending smart; gardening and commercial horticulture; the latest agricultural production technologies; and community development.
Extension 4-H and youth development programs reach more than 100,000 young people annually.
CSU Extension programs are available to all without discrimination.

This story was posted on April 21, 2018.