Wildflowers, Plants and Ferns, Pacific Northwest
Bull Thistle grows all over the south coast including all of Vancouver Island and I am now seeing it in the remotest spots on the coast. Bull thistle is a widespread thistle originally from Europe and Asia, but now introduced throughout North America. Although it is intimidating in appearance and can sometimes form large infestations, this thistle is not as challenging to control as many others and is mainly a problem in hay fields and pastures. Bull thistle is also commonly found along trails, roads and vacant fields.
Bull thistle prefers sunny, open areas and can tolerate a wide range of conditions, from moist to dry soils, and is typically found in disturbed areas such as roadsides, trails, logged areas, vacant land, pastures and cultivated land. Overgrazed pastures are susceptible to bull thistle encroachment, and it can sometimes form dense stands that reduce productivity and stocking levels. Bull thistle also dominates forest clear cuts and reduces growth of tree seedlings.
Bull thistle has a two year life cycle, flowering and setting seed in the second year. Seeds are short lived on the soil surface but can persist for many years when they are buried, such as from cultivation. Seed germination generally occurs in the fall and spring. Basal rosettes form and continue to grow until winter and can grow quite large, up to 1 meter in diameter.
Rosettes that are not large enough by spring may not flower until the following year. Flowering usually starts in mid-June and continues into early fall. Plants can be self-pollinated or insect pollinated by bees. Bull thistle does not reproduce vegetative and does not have rhizomes.
Bull thistle only reproduces by seed so prevention of seeding and taking care not to spread seeds are key to preventing new infestations. Do not leave cut stems of flowering bull thistle on the ground because they are likely to form viable seed even after they are cut.
Bull thistle can be dug up with a shovel. Usually removing the top couple of inches of root is sufficient to kill the plant. A shovel or other tool can used to chop off leaves from one side of the plant to gain easier access to the roots, which can then be dug up. Burn or other wise destroy the plants to prevent reseeding.