Canada Thistle
Canada thistle is a very aggressive noxious weed. It has an extensive underground root system, making it extremely hardy and difficult to get rid of. It can grow on all soil types and infests crops, pastures, rangelands, waste areas, roadsides, edges of sloughs and dugouts, shelter belts, and non-crop areas. Infestations typically start on disturbed ground, including ditch banks, overgrazed pastures, tilled fields or abandoned sites.
Canada thistle is a creeping perennial which begins growth in the early spring. Flowering occurs primarily in early spring but may occur throughout the summer and fall. It reproduces primarily by its intricate rooting system (rhizomes), which enable the plant to recover from a variety of control methods. Horizontal roots may extend 15 feet or more and vertical roots may grow six to 15 feet deep. Small root pieces approximately 0.25 inches long have enough stored energy to develop new plants. These small roots can survive approximately 100 days without nutrient replenishment from photosynthesis. Canada thistle reproduces by seed as well. Most seeds do not stray very far from the parent plant unless they are floating downstream. Contrary to popular belief, most of the thistle fluff seen blowing in the wind is just that: fluff. Viable seeds are too heavy to travel more than a few feet.
One thistle plant can colonize an area three to six feet in diameter in one to two years. Seeds can remain viable in the soil for up to 20 years. With its rapid spreading abilities and strongly competitive nature, Canada thistle causes greater crop loss than any other broadleaf weed in Western Canada.
Control Options:
- Combining control methods is the best way to manage this weed.
- Always maintain desirable plant cover to compete with the thistle.
- Mowing can be effective if combined with a herbicide treatment or if repeated at regular intervals (monthly) for several years.
- If Canada thistle is in your crop, be sure to use proper crop rotations.
- Persistence is important. Continued stress will exhaust the root system and hopefully kill the weed. Don’t expect to eradicate the thistle completely with only one herbicide treatment or mowing pass.
- Check your Crop Protection Guide for herbicide recommendations.
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