Weed of the Month: Canada Thistle


Can YOU outsmart the smartest weed in the West?

Canada thistle may be the cleverest weed in the West

Canada Thistle, Cirsium arvense, may be the cleverest weed West of the Mississippi. It boasts a long list of weedy characteristics, including:

  • Perennial growth habit
  • Ability to reproduce from creeping rhizomes (underground stems that creep out laterally in all directions)
  • An abundance of wind-dispersed seeds that are also long-lived (up to 22 years!)
  • Spiny leaves that are avoided by wildlife and most livestock   
  • Tolerant of a wide range of soil conditions
  • And worst of all, roots that penetrate anywhere from 6 – 15 feet deep, sequestering nutrients that allow Canada Thistle to recover from any attempts to control it at the surface.

To outsmart a weed like this, one must get to know it intimately and discover and exploit its weakness.

So, let’s get to know Canada thistle!

This first thing to know is that it is NOT from Canada. This thistle is native to the Mediterranean regions of Europe and North Africa. It was accidentally introduced to Canada in a shipment of contaminated grain in the 1600s. It spread down the east coast of the US and was first discovered in the Rocky Mountain west in the 1800s. It is currently the most widespread of all thistle species. 

Canada thistle is native to the Mediterranean region, but is now widespread in North America

The next thing to know is that Canada thistle is not like any other thistle you have encountered. Unlike Musk, Bull, Scotch, and Plumeless thistles, which all emerge from a stout taproot as an individual plant, Canada thistle produces extensive patches with hundreds or thousands of tiny stems that are genetically identical and arise from a shared root system. Therefore, pulling one “plant” in a patch of Canada thistle is as effective as picking a single leaf from a tree hoping to kill the tree – NOT effective at all!

Canada thistle has a creeping perennial growth habit which means this entire patch is probably one genetic individual, much like a clone of Aspen trees

Canada thistle also differs from the other thistles in being dioecious. This means that each plant (or patch in this case) is either male or female and bears flowers that will produce either pollen or seeds. A Canada thistle patch that is female must be pollinated by a patch that is male in order to produce viable seed and having male and female plants separated in space may reduce the probability of viable seed production. However, when male and female plants do find each other, one flowering shoot can produce 1,000-1,500 seeds that can lay dormant in the soil for up to 22 years. So, it is still worthwhile to cut and bag any seed heads you encounter. 

Canada thistle seeds can lay dormant in the soil for up to 22 years awaiting the right conditions for germination

A large patch of Canada thistle is essentially one clonal plant, either male or female, and to kill it, one must kill ALL of it. The challenge is finding a way to kill the extensive underground root system that penetrates to a depth of up to 15 feet and extends to a breadth of at least that. The only way to succeed in eliminating Canada thistle is to injure and exhaust this root system, and do so repeatedly, treating for multiple seasons and multiple times within a season. 

To eliminate Canada thistle, you must be able to kill its extensive root system. This is a challenge.

If you want to outsmart this weed, follow these steps:

  • Spring treatment: All winter, Canada thistle lays dormant under the ground, subsisting on the nutrients stored in its roots and rhizomes. When the snow melts in spring, it will use that precious stored energy to push a flush of leaves out of the ground, which will start out as a patch of spiny rosettes.
A spring flush of Canada thistle rosettes

The rosettes will leaf out and grow taller and eventually produce buds and flowers for reproduction. Once the buds emerge, the energy stored in the root system is at a seasonal low. Now is the time to severely weaken the root system by abruptly removing all the surface vegetation, either by mowing it close to the ground or by applying an herbicide like Glyphosate, Speedzone, or Milestone to the leaves. This will force the thistle to further deplete its nutrient reserves in order to replace the vegetation it lost. 

The Spring treatment of Canada thistle should be applied at the bud stage of growth
  • Fall Treatment:   After the spring treatment, the weakened plant will take some time to recover. But recover, it will. In the late summer/early fall, in preparation for winter dormancy, it will muster the remaining energy reserves from its roots and push up another flush of leaves. The purpose of these leaves is to vigorously photosynthesize and transport all the sugars produced down into the root system so it can survive the winter and have energy for a flush of leaves in the spring. Now is the BEST time to treat with an herbicide because the chemical will inadvertently be transported down to the root system, along with the photosynthates. The herbicide recommended by TCWP for fall treatment in non-lawn areas is Milestone. If you are treating the thistle in a turf grass lawn you will need to use a lawn-approved herbicide like Speedzone, Tri-Mec, or Mecamine-D. You will want to treat before a killing frost, but after the first frost to have the best chance of getting the herbicide down to the evasive root system.
  • NOTE: If the plant was able to produce healthy regrowth soon after the spring treatment, plan to cut it back 3-5 weeks before applying the fall Milestone treatment. This will encourage active growth in the plant and produce clean new leaves that lack the waxy cuticle present on mature leaves, allowing the herbicide to penetrate the leaves and circulate throughout the plant more easily, increasing its effectiveness. Whenever using herbicide, refrain from cutting or mowing treated vegetation until after the herbicide has had a chance to work. This could be as long as 2-3 weeks.
  • Repeat: This procedure should be repeated for at least 3 years.
  • Replant: Plant new vegetation to compete with the Canada thistle and any other weeds that may try to get established as soon as possible after herbicide treatments begin to take effect. Check the label of the herbicide you used to see how long you must wait before replanting an area that was treated. For Milestone it can be up to 18 months.

These four steps are the most effective means of eliminating Canada thistle from your property. This requires patience, vigilance and use of chemical herbicide.

If herbicide must be avoided, it may be possible to exhaust the root system of Canada thistle using a combination of mechanical methods such as mowing, digging, and grazing. However, this process will require A LOT of patience, diligence, and time. 

For more information on Canada thistle treatment see these links: