Canada thistle

Identification & Biology
Alias' : Canadian thistle, creeping thistle, lettuce from hell thistle, California thistle, corn thistle, cursed thistle, field thistle, green thistle, hard thistle, perennial thistle, prickly thistle, small-flowered thistle, way thistle, and stinger-needles
Latin Name : Cirsium arvense
Category : Terrestrial Plants
Description :
  • Root creeping perennial 
  • Stalkless, spiny, prickly, dark green leaves 15-10 cm long 
  • Purple or pink or white flowers that have a sweet vanilla scent 
  • Grows 0.3-2 meters in height at maturity 


Canada thistle is a perennial plant that reproduces by seed and vegetatively through creeping, horizontal roots. Root fragments are capable of forming new plants. Seed viability can be low; mature seeds germinate most readily in mid-spring. Non-germinated seeds may remain dormant for up to 3 years. Plants are male or female (dioecious) and grow in circular patches that are often one clone and sex. Canada thistle may produce 1,000 to 1,500 seeds per flowering shoot. Flowers are primarily insect-pollinated and bloom between July and August. Seeds can be dispersed by wind, water, animals, clothing, equipment and vehicles. 

Canada thistle is sometimes confused with bull thistle (Cirsium vulgare), Scotch thistle (Onopordum acanthium), plumeless thistle (Carduus acanthoides), musk thistle (Carduus nutans), and marsh thistle (Cirsium palustre). Canada thistle can be differentiated from all similar species by the lack of spines on the main stem, small flowers, and a height of less than 2 m. 

Look Alikes

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. 

Impact & Risks
  • 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. 
Prevention & Mitigation

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.  
Treatment & Disposal
  • 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 
  • As a biocontrol option, the seed weevil, Larinus planus, has shown promise in suppressing Canada thistle.  
Okanagan Distribution

Priority Level Definitions

Watch For - Poses a significant threat (very high risk) and does NOT presently occur in the region OR is relatively new to the region and is very limited in extent.
High - High risk/impact; limited population with significant potential to spread in the region.
Medium - Medium risk/impact; limited distribution – broader population distribution with potential to spread further in a region.
Low - Low risk/impact; may be widespread or not, may be of concern in specific situations with certain high values – e.g. specific agriculture crops. Some species may be treated primarily with biological control agents.