Noxious Weed of the Month: Canada thistle


By Ethan Proud

PREVIEW Columnist

Canada thistle is, in fact, not from Canada. Like many of our noxious invaders, it hails from Eurasia. It is spread throughout Archuleta County and is usually found in riparian areas, despite being able to grow in drier soils.

Unlike musk, bull and scotch thistle, it is a perennial and the same plant will return year after year. Mowing is a much better option than pulling as root fragments will yield a new plant. However, both can be done diligently to manage the population.

The root system of Canada thistle makes even herbicide applications difficult, as the roots can spread 30 feet laterally and 15 or more feet deep. Repeated applications are necessary as spring treatments primarily affect seed set and fall treatments target the root.

Canada thistle can produce 5,000 seeds which are viable in the soil for 20 years. Herbicides with residual can control germinating seedlings for up to two years. A fall application will take the herbicide deep into the root system as photosynthate is being translocated and stored as starch.

Knowing the life cycle of a perennial like Canada is essential to effective control. In early spring, seedlings are susceptible to herbicide, though a mature plant will be moving nutrients to the shoots and buds, which will prevent much of the herbicide from reaching the roots. An application in late spring to early summer before seed production will see more herbicide translocated into the roots as the plant is in a vegetative state and nutrients are flowing to both the roots and new growth.

A new biological control, puccinia punctiformis, is showing potential; it is a naturally occurring fungus that is capable of eradicating Canada thistle if it has a little human help.

For more information on the rust fungus, contact the Archuleta County Weed and Pest or keep an eye out for the next article.

Upcoming events

July 13: Archuleta County Annual Weed Tour.

Aug. 1-4: Archuleta County Fair. Do you quilt or sew, can vegetables or fruit, grow hay crops, veggies or flowers? Maybe you do leather or wood work? Possibly brew beer or make wine? Or, maybe you have a hidden crafting talent that you would like share with us? If so, then you can enter the Archuleta County Fair Open Classes. Go to to find out how to enter. It’s homegrown and county pride.

CPR and first aid classes

CPR and first aid certification classes are offered monthly by the CSU Extension office on the second Monday and Wednesday of each month from 6 to 10 p.m. 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.