Canada thistle  (Cirsium arvense (Sunflower Family: Asteraceae) )

Canada thistle, a perennial broadleaf plant, is found in most of the western states except for southern Sierra Nevada, the Sonoran and Mojave deserts, and the Channel Islands. It is scattered throughout California to about 5900 feet (1800 m). Canada thistle inhabits agricultural land and other disturbed locations. According to some taxonomists, four varieties or biotypes exist that differ in growth habit, leaf characteristics, seed germination, and development.

Canada thistle is a state-listed noxious weed in California and many other states. It is important to control plants before they regenerate food reserves in their roots or produce seed. Do not let this weed move to new areas and eliminate it from noncrop locations.

Habitat

Stream banks, forest openings, rangeland, hillsides, moist depressions, gardens, crop fields, roadsides, and other open, disturbed sites.

Seedling

Cotyledons (seed leaves) are oval to oblong, usually from 1/5 to 1/2 of an inch (5–14 mm) long, thick, fused at the base, and smooth or slightly glandular. The midvein on their lower surface is shiny. The first leaves are alternate to one another on the stem and their edges are somewhat wavy to unevenly toothed. The leaf surface is covered with stiff hairs, but lower surfaces are often covered sparsely with soft, webby, hairs. Seedlings have poorly developed rosettes.

Mature plant

Canada thistle stands erect to 3-1/3 feet (1 m) tall and grows in clumps or patches. Its stems are slender and hairless or nearly hairless. Leaves are oblong to lance shaped, mostly 2 to 8 inches long (5–20 cm), prickly, and are alternate to one another along the stem. Sometimes leaf bases extend down the stem joints as prickly wings that are 1/2 of an inch (1 cm) long. Leaf edges range from nearly smooth to shallow lobed and toothed. The upper leaf surface is hairless to nearly hairless and the lower surface is sometimes sparsely woolly. Rosette leaves are either few or lacking altogether.

The extensive root system consists of a network of vertical and creeping horizontal roots. Although most roots occur in the top 1-1/2 feet (45 cm) of the soil, vertical roots from 6-1/2 to almost 10 feet (2–3 m) deep are common.

Flowers

Flowers bloom from June through October. The flower head is cylindrical or narrowly egg to bell shaped, about 1/5 to 4/5 of an inch (0.5–2 cm) in diameter, and consists of several overlapping rows of scalelike structures (bracts) tipped with spines. Narrow, white, purple, or pink flowers (disk flowers) cluster above.

Fruits

Fruits are single seeded (achene), egg to football shaped, tan, about 1/12 to 1/6 of an inch (2–4 mm) long, and end in a long tuft of feathery bristles that are about 1/2 to 4/5 of an inch (12–20 mm) long.

Reproduction

Reproduces by seed and from cut roots. Seeds are wind dispersed and can be viable for many years.

More information

• Broadleaf ID illustration
• Calflora's distribution map
• For agriculture: UC IPM Pest Management Guidelines