Flowers: Deep pink-purple, gumdrop-shaped flowers to the head of each stem during flowering times. Flower heads are about 4 – 5 cm wide, surrounded by spines that extend from the base all the way down the stems.
Stem: Stems are stiff and spiny; Bull Thistle can grow to be up to 2 m tall.
Leaves: Deeply lobes leaves alternate on stems and branches, donning distinctive spiny tips. Rather dull in colour, the leaves are silvery green on top, with white hairs underneath.
Roots: With a main short, fleshy taproot in place, Bull Thistle grows many smaller, lateral roots to invade the surrounding area
Wavy Leaf Thistle (Cirsium undulatum), is sometimes mistaken for Bull Thistle, but it is much shorter (0.3 – 1.2 m tall).
Scotch Thistle (Onopordum acanthium): One of the more distinguishing factors between Scotch and Bull Thistle are wooly stems and leaves – both of which are unique to Scotch Thistle. If you have a keen enough eye, you might also notice the flowers on Scotch Thistle are much larger, and more numerous, than Bull Thistle.
Milk Thistle (Silybum marianum) has distinctive marbling patterns on its leaves.
Habitat and Origin
Introduced from Europe and Asia, Bull Thistle is believed to have made its way to North America as a seed contaminant.
Thriving in disturbed soils, this thorny plant can be found along clear-cuts, roadsides, and pastures. Resilient as it may be, Bull Thistle can be picky about its light requirements, and doesn’t fare so well in healthy, densely-canopied forests.
Propagation & Vectors of Spread
Bull Thistle spreads primarily through seeds, which germinate easily and can lay dormant for up to 3 years. Mature Bull Thistle can produce up to 4,000 seeds per plant. Although this plant relies heavily on wind dispersal, and seeds can spread more than 2km per year by wind, they usually only land about 2m away from their parent plant.
Ecological, Economic, & Health Impacts
- Establishes quickly in disturbed areas, rapidly encroaching on native floral populations.
- Reduces biodiversity.
- Proves unpalatable to native wildlife.
- Decreases forage value by contaminating hay bales.
- Limits livestock migration.
What Can I Do?
Bull Thistle is found in communities throughout the Sea to Sky region, so PREVENTION of further spread is key.
Learn to identify Bull Thistle: use the images presented in this profile page to learn how to identify Bull Thistle
What to do if you spot it: You can report any Bull Thistle sighting by clicking here.
- Regularly monitor properties for weed infestations.
- Ensure soil and gravel are uncontaminated before transport.
- Ensure plants (particularly flowering heads and seeds) are bagged or covered to prevent spread during transport to designated disposal sites (e.g., landfill).
- Do not plant Bull Thistle in a garden, no matter how well-contained its enclosure may seem.
- Do not move soil that has been contaminated with Bull Thistle.
- Do not compost!
Hand-pulling small infestations or repeated mowing before seed set has proven to be an effective control method against Bull Thistle.
- Dicamba, glyphosate and 2,4-D have been proven to be effective for Bull Thistle control.
- Picloram has also shown positive results, however it is not suitable for wet, coastal soils.
- Herbicides are most effective when applied after mowing or during the plant’s rosette stage.
- We recommend that any herbicide application is carried out by a person holding a valid BC Pesticide Applicator Certificate. Before selecting and applying herbicides, you must review and follow herbicide labels and application rates; municipal, regional, provincial and federal laws and regulations; species-specific treatment recommendations, and site-specific goals and objectives.
A seed weevil, Larinus planus, has been released in the Sea to Sky to feed exclusively on invasive thistles.
A combination of prevention, cultural, biological, mechanical and/or chemical control holds the highest success rate among Bull Thistle management plans. Reseeding disturbed or reclaimed areas is absolutely critical to preventing new infestations. Chemical control methods aren’t viable with the presence of bio-control agents.
- Government of BC, Invasive Plants with Biocontrol, https://www2.gov.bc.ca/gov/content/environment/plants-animals-ecosystems/invasive-species/management/plants/biological-control/invasive-plants-with-biocontrol
- Invasive Species Council of BC, Canada Thistle TIPS Factsheet, https://bcinvasives.ca/wp-content/uploads/2021/01/Canada-Thistle_Factsheet_26032019.pdf
- Invasive Species Council of BC, Field Guide to Noxious and Other Selected Invasive Plants of British Columbia, https://bcinvasives.ca/documents/Field_Guide_to_Noxious_Weeds_Final_WEB_09-25-2014.pdf
- Invasive Species Council of BC, Scotch Thistle, https://bcinvasives.ca/invasives/scotch-thistle/
- King County, Bull Thistle Identification and Control, https://www.kingcounty.gov/services/environment/animals-and-plants/noxious-weeds/weed-identification/bull-thistle.aspx
- King County Noxious Weed Control Program, Best Management Practices for Bull Thistle, https://your.kingcounty.gov/dnrp/library/water-and-land/weeds/BMPs/bull-thistle-control.pdf
- Northwest Invasive Plant Council, Bull Thistle, http://nwipc.org/plants/bull-thistle
- Pacific Northwest Pest Management Handbooks, Thistles, https://pnwhandbooks.org/weed/problem-weeds/thistle-bull-cirsium-arvense-milk-silybum-marianum-musk-carduus-nutans-scotch-onopordum-acanthium