Appearance: Biennial herbaceous plant, between 3 – 6′ tall with one erect branched stem. It grows a rosette in its first year and blooms in its second year.
Leaves: Alternate, coarsely lobed, each lobe with a spine at its tip. Spines extend downward from the leaves along prominent ridges of the stem. Upper leaf surface is rough.
Flowers: Disk-shaped flower heads contain hundreds of tiny individual purple flowers which bloom from July through August.
Seeds: Numerous straw-colored seeds with plume-like bristles are dispersed by wind. They remain viable in the soil for over 10 years.
Roots: Each plant has a fleshy taproot.
Reasons Bull Thistle is a Problem:
- It colonizes primarily in disturbed areas such as pastures, roadsides, and ditch banks, but also in hayfields and disturbed prairies.
- The large size and rapid growth of bull thistle means it out-competes and shades many prairie plants.
- Bull thistle is distasteful to most grazing animals, giving it a competitive edge.
- Seeds spread by wind and have high dispersal rates.
- Seeds remain viable in the soil for up to 10 years.
Similar to Canadian thistle, the more robust bull thistle was introduced to the United States in the 1800’s from Europe and Asia, and has now spread to 42 states, being declared a noxious weed in 16. Bull thistle is common in disturbed areas but is not a significant threat to well established native ecosystems. Like Canadian thistle, the most worrying aspect are the seeds, which can disperse within 7 to 10 days of flowering. The bulk of seed is produced on the upper branches, and germination may run as high as 95%. Each plant is capable of producing up to 10,000 seeds and wind dispersal allows for movement over long distances. Seeds can remain viable in the soil for over 10 years. The arboretum experiences its biggest problems with bull thistle in the newer prairie fields, those in their second and third growing season. Bull thistle is uncommon in the older restorations.
Cowling Arboretum Procedures for Removal:
Backpack sprayers are used to directly herbicide spray individual plants in the early summer, along with Canadian thistle. It is important to kill plants before seed can be produced to prevent a persistence seed bank .