Common Names: Creeping thistle, Field thistle, Californian thistle
Description: This species is listed as a prohibited noxious weed by the Michigan Department of Agriculture. Canada thistle was introduced to North America from Europe in the early 1600s.
Habit: Perennial, rhizomatous thistle ranging in height from 0.6-1.5 m (2-5 ft), forms large monocultures.
Leaves: Simple, alternate, lance-shaped, crinkly, tapering, irregular lobes and spiny toothed margins.
Stems: Upright; slender and branching towards the top; becoming increasingly hairy with age.
Flowers: Numerous, purple-lavender in color, small flower heads, less than 2.5 cm (1 in) high, clustered at the tops of stems, fragrant; bloom June through September.
Fruit and seeds: Seeds are small, light brown; tufts of hair attached to the tip for wind dispersal; one plant produces between 1500-5000 seeds, which can germinate 8-10 days after flowering begins and persist in the seed bank for up to 20 years.
Habitat: Found in disturbed open areas, roadsides, agricultural fields; invades prairie and riparian areas; salt-tolerant; shade intolerant.
Reproduction: Primarily by creeping, laterally spreading rhizomes, but also by prolific seed production; dioecious, with separate male and female clones; some hermaphroditic forms.
Similar species: Native swamp thistle (
Cirsium muticum) has sticky flower heads; non-native weeds bull thistle (
Cirsium vulgare), European swamp thistle (
C. palustre), and musk thistle (
Carduus nutans) have spiny winged stems.
Monitoring and rapid response: Monitor sunny, disturbed sites including degraded grasslands, open woodlands, edge habitats and restoration sites. Begin control efforts in highest quality areas; pull seedlings within 2.5 weeks after germination or they become perennial; Canada thistle is clonal; resprouts from root fragments. Herbicides are most effective with two applications per season: in spring, just before flowering, and in fall on new growth after mowing, treat all stems. Different strains of Canada thistle respond differently to the same herbicide; may require 5-10 years of ongoing efforts. This species is extremely difficult to eradicate - research control options thoroughly.
Credits: The
Michigan Natural Features Inventory (MNFI) has partnered with MISIN to provide the information in this fact sheet. Species images and/or information were used with permission from "
A Field Identification Guide to Invasive Plants in Michigan's Natural Communities" and "
A Field Guide to Invasive Plants of Aquatic and Wetland Habitats for Michigan.