Bull Thistle

Native

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 prefers rich, moist soils and does not tolerate shade. As a result, it doesn’t fare well in healthy, densely-canopied forests.

• Bull Thistle spreads through seeds, which germinate easily. In optimal conditions, mature Bull Thistle plants can produce 100 – 300 seeds per flower head, with one to more than 400 flower heads per plant.
• As a biennial or monocarpic perennial, individual Bull Thistle plants only set seed once before drying.
• If the seeds are buried 15 cm or deeper, they may remain viable in the soil for up to 3 years or more.
• Bull Thistle seeds can spread more than 2 km per year in the wind, but usually land near the parent plant.
• Seeds are also spread by vehicles and equipment, or in contaminated soil.

Bull Thistle is found in the Sea to Sky region and its distribution is beyond landscape-level control. When Bull Thistle is present at high-priority locations and negatively impacting them, their control is considered a high priority.

Otherwise, the goal is to prevent Bull Thistle from spreading to new (uninfested) areas, and to control it where possible to limit its impact on biodiversity.

 

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.

 

DO:

• 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:

• 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!

 

Control

Mechanical

• Hand-pulling (while wearing gloves to protect yourself against the plant’s prickly spines) is effective for small infestations.
• Mowing before seed set is also an effective control method against Bull Thistle, since the plant dies off after setting seed.
• However, note that since plants mature and set seed at varying rates, repeated mowing throughout the season is recommended.
• Since Bull Thistle only reproduces by seed, control measures that sever the root below the soil surface are generally viewed as effective.

Chemical

• Aminopyralid, clopyralid, dicamba, glyphosate, triclopyr, chlorsulfuron, imazapyr, metsulfuron and 2,4-D have proven 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 used during the plant’s rosette stage or after mowing.
• The use of a surfactant is recommended, due to the hairy leaf surface.
• 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.

Biological

• A seed weevil, Larinus planus, has been released in the Sea to Sky to feed exclusively on invasive thistles.
• Most animals do not care for the bitter taste of Bull Thistle and will not consume it.

Integrated Control

• A combination of prevention and various control methods holds the highest success rate for Bull Thistle management and control.
• Reducing disturbances and reseeding are critical to preventing the plant’s return after treatment.
• Chemical control methods must be avoided if biocontrol agents have been released.