Canada thistle

Canada thistle is native to Eurasia, but was accidentally introduced to North America in the 1600s and now is widespread. In British Columbia, Canada thistle is found in almost every plant community over a wide range of elevations. Canada thistle is commonly found on roadsides, cultivated fields and pastures, logged roads, and other disturbed areas. They are also found in margins of forests, meadows, wetlands, and native plant communities. They are best adapted to rich, heavy loam, clay loam, and sandy loam. They grow poorly in shaded conditions. They can tolerate saline, wet, or dry soils, but do not tolerate waterlogged or poorly aerated soils. 

• Plants can form dense patches which crowd out forage grasses in pastures and rangelands, reducing yields and productivity.  
• Canada thistle can degrade wildlife habitat, and can hinder reforestation and landscape restoration efforts.  
• It crowds out native plants and reduces biodiversity. 

The most effective way to ensure that your lands do not become infested with Canada thistle is by prevention. Here are some recommendations to prevent Canada thistle from invading your property: 

• Maintain your crops and natural lands in a healthy, vigorous condition to ensure a competitive plant community. 
• Regularly patrol your property for Canada thistle plants and immediately control or remove infestations before seed set.  
• Cooperate with adjacent landowners and encourage them to prevent Canada thistle spread. 
• Immediately re-vegetate disturbed, bare soils with a suitable seed mixture that provides dense, early colonization to prevent weed invasion.  
• Ensure soil, gravel, and other fill material are not contaminated. Do not move contaminated soils to a new area.  
• Avoid unloading, parking, or storing equipment and vehicles in infested areas.  
• Remove plants, plant parts, and seeds from personal gear, clothing, pets, vehicles, and equipment. Wash vehicles, including tires and undercarriage, and equipment at designated cleaning sites before leaving infested areas.  
• Take special care when controlling Canada thistle near streams, or ditch lines, to prevent the movement of plant parts downstream.  

• Canada thistle can be controlled mechanically, chemically, and with biocontrol.  
• Mowing is most effective when completed at the bud stage.  
• Regular cutting or tillage can help reduce plant growth but is not likely to kill the plant.  
• If plants are cut prior to flowering, the plant material can be left on the site to decompose. If plants are cut post flowering, all plant parts should be bagged and buried deeply at a landfill.  
• Chemical control is also an option. Before applying herbicides, read the label for full use and precautionary instructions.  
• For further information on the selection and application of chemicals to protect your crop, contact AgriService BC at 1-888-221-7141 or email AgriServiceBC@gov.bc.ca.  
• As a biocontrol option, the seed weevil, Larinus planus, has shown promise in suppressing Canada thistle.