Weed of the Month: Canada Thistle

Canada thistle can be a real pain in our side—literally and figuratively! Sometimes you’ll feel it before you even see it. Canada thistle is difficult to control and quite unpleasant to the touch. Even if you’re wearing thick jeans, the long, sharp thistles of this plant can penetrate through clothing, leaving a stinging sensation. 

Unfortunately, canada thistle is prevalent in most states because of its early introduction and rapid spread. However, hope is not lost. With the correct timing and a little bit of tenacity, we can control and decrease canada thistle around us. Read more to find out why September is a GREAT time to kill canada thistle.

Canada thistle’s threat to Teton County

Once Canada thistle is established it quickly spreads and outcompetes native thistles and other plants—thus reducing biodiversity. Many of our native animal species like elk, coyotes, and bison steer clear of canada thistle. This lack of foraging allows canada thistle to spread quickly because it is not a desirable plant to eat. 

Plus, canada thistle is an opportunist plant. Meaning, it likes to grow quickly in disturbed areas such as landslide locations, construction areas, and roadsides.

Identification of Canada Thistle

Canada thistle is a perennial plant which means that it comes back every year. Plus, it has a robust root system that can penetrate deep into soil and aggressively spread and propagate. This means that canada thistle is extremely difficult to kill.   

Other identifying features:

  • The leaves alternate and are deeply divided with spines along the edge
  • Stems typically do not have spines
  • Can distribute up to 5,000 seeds
  • Seeds can stay viable in the soil for a decades
  • Coloring can range from a dull to a vibrant green depending on soil and water conditions 
  • It has purple flowers
  • Plants typically bloom in late spring and early summer
  • Seeds have feathery white tufts that allow it to effectively disperse by wind

Although the name may be misleading, canada thistle did not originate in Canada but likely Europe or Asia. It is suspected that it spread through contaminated crop seed in the 1700s.

PlayCleanGo to stop canada thistle

The PlayCleanGo motto is simple for canada thistle. The best way to stop this invasive species in its tracks is to: 

  • REMOVE - plants and mud from your boots, gear, pets, and vehicles
  • CLEAN - your gear before AND after recreational activities
  • STAY - on designated roads and trails
  • USE - certified or local firewood and hay

Like we said before, canada thistle is an opportunist. It will try its best to spread in any way it can. Use these four simple steps to ensure that our backcountry, trails, roadsides, and yards stay canada thistle-free.  

How to control canada thistle

Although the PlayCleanGo message is effective for thwarting invasive species BEFORE they are established, different methods need to be employed if canada thistle patches already exist. 

Few invasive plants are as annoying and frustrating as canada thistle. However, there is a silver-lining. September is a GREAT time to treat canada thistle. 

If you choose to use herbicide to control canada thistle, September in Teton County is the perfect time to do so because we usually see our first hard frost around now. What does a frost have to do with anything? Canada thistle goes dormant in the fall, typically after a late summer frost. When canada thistle goes dormant it takes its energy from its leaves and stem down to its roots—thus giving you a window of opportunity to use herbicide to kill the entire plant. Remember, canada thistle is a perennial with an extensive root system. Therefore, you need to target its extensive root system to permanently destroy the plant. 

Herbicide Control Instructions: Apply herbicide around or a couple of weeks before the first hard frost.

If you choose not to apply herbicide and opt for mechanical means of control, you can do so throughout the summer. Be aware that mechanical control takes a lot of work and potentially many years for an effective outcome. 

Mechanical Control Instructions: Repeatedly pull or cut the plant until its energy reserves are exhausted and the plant does not return in the spring.

We are here to answer your canada thistle questions

As always, the staff here at TCWP is available to answer your questions regarding canada thistle control methods, tricky look-alike plants, and any other questions you may have.