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Canada Thistle in Your Field

By Paige Straf, Agriculture Summer Extension Student, Prince Albert Regional Office

June 2017

Canada thistle is known to be an invasive species in many places in North America, including Saskatchewan. As an invasive species, this weed has an advantage that allows it to out-compete native plants or agricultural crops for space, moisture and nutrients. It will often shade other plants, reducing their growth. Canada thistle sometimes can cause greater reduction in crop yields than any other broadleaf weed in Western Canada. It often has a negative impact on the biodiversity of rangelands as well.

Canada thistle can be identified by:

  1. Leaves: Long, shiny dark green, irregularly lobed with sharp spines on the leaf edges, and they alternate on the stem. You will definitely want gloves on if you try to pick one.
  2. Stems: are only slightly prickly. The height of the stems range from approximately 30 cm to 122 cm.
  3. Flowers: Rose purple to light pink with multiple flower heads on each stem. They will bloom from June to the beginning of September.
  4. Roots: Thick, fleshy tap roots that once established create an extensive and deep root system that can spread quickly and be challenging to control. New “clone” plants grow easily from any root segments that may be left in the ground.
  5. Seeds: White feathery tufts, similar to dandelions, that disperse via the air but often don’t travel far from the mother plant. Seeds remain viable usually from three to six years but have been known to last as long as 21 years. Seeds contribute to the vigorous spread of this hardy weed.
  6. Habitat: Croplands, but also tame or native pastures, gardens, roadsides, shelterbelts and gravel pits.

Implementing integrated pest management (IPM) is a recommended strategy to help reduce pests such as weeds. IPM involves cultural controls, but also chemical, mechanical and biological controls, as well. While it has been challenging to find an easy to use biological control specific to Canada thistle, there are still several effective strategies that are helpful to lessen the amount of the weed. It is important to remember using only one strategy is not the best approach. For example, while chemical use is a successful way to eliminate Canada thistle in your field, the overuse of it, especially as a sole means of control, can contribute to herbicide resistance of weeds. For the positive impact to be the greatest, use combinations of as many strategies as possible, along with the following steps:

  1. Prevention – primarily through cultural practices, such as crop rotations or seed spacing, that help the crop to out complete weeds.
  2. Monitoring and forecasting – it is important to be able to identify your weed and understand its lifecycle in order to predict the problems it will cause.
  3. Intervention – scouting your field helps to know where and when to implement a strategy.
  4. Evaluation and record keeping – knowing what worked, where it worked and how it worked will aid in future plans when it comes to weed reduction.

Crop rotations that are often used to minimize Canada thistle include winter wheat or competitive forages (ex. alfalfa or sweet clover). These crop rotations should be implemented for three to four years. Tillage is mechanical strategy that is actually discouraged when it comes to thistle because root pieces will be chopped and only moved around in the field. Little or no tillage is ideal because it also helps to minimize erosion. Trimming the flowers of the plant before it sets to seed is a mechanical strategy that is highly recommended.

For more information, or for questions on identification, reduction strategies, or chemical control of Canada thistle please contact your local Regional Crop Specialist or the Agriculture Knowledge Centre at 1-866-457-2377.

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