Bull Thistle

Bull thistle is a biennial forb in the Asteraceae (daisy) family. It grows from a taproot to form a basal rosette in its first year of growth; in the second year of growth it produces one to many stems that may reach between 2 to 6 feet tall and occasionally up to 7 feet tall. Basal leaves may between 3 and 12 inches long and are usually deeply lobed. Occasionally basal leaves are not lobed but are rather lanceolate with toothed margins. Leaves occur alternately on stems and decrease in size up the plant. Leaves are deep green. Spines occur on the margins, tops, and under midrib of leaves, as well as along wings run vertically along stems. Bull thistle flowers from early summer to fall. Flowers are large, reaching up to1.5 to 2 inches across, and terminal. Flowers are usually solitary on stems, but may occasionally occur in clusters. Florets may be pink to deep purple and are enclosed in long, spiny bracts that form a bulb at the base of the flower. Seeds are oblong achenes up to 5mm long with pappi.

Bull thistle is native to Europe, Asia, and Northern Africa and was accidentally introduced to North America in themid-1800s possibly as a contaminant in ship ballasts or as a seed contaminant. It spreads solely by seed. Seeds are dispersed by the wind, animals, water, humans, and equipment. Bull thistle is frequently found in forests, rangeland and pastures, landscapes, lawns, roadsides, and other disturbed sites.

Prevention and cultural control strategies should be utilized as much as possible.

There are no biological control agents currently available for release in Teton County.

Because bull thistle has a taproot, it can be readily managed by hand pulling or digging out the root. Tilling or hoeing is also effective for management when the taproot is severed below ground. Controlled grazing by goats may be effective when plants are in flower. Mowing or chopping will not eradicate stands of bull thistle, but it can be beneficial in preventing seed production. Do not mow while seeds are present.

Larger infestations can be controlled with herbicides. Spring and fall treatments of rosettes are recommended, but treatment of bolting plants can be effective. Flowering plants may be chopped and remaining rosette and stem treated with herbicide to prevent reflowering. Read the label before using any herbicide. Contact TCWP if you have any questions about application rates or how to use an herbicide.

• Weed Report: Bull thistle from Weed Control inNatural Areas in the Western United States (2013) DiTomaso, Kyser et al.University of California.
  
• King County Noxious Weed Alert: Bull Thistle(2008). King County Noxious Weed Control Program, King County, Washington.
• Bull thistle (Cirsium vulgare). Minnesota Department of Natural Resources.