Canada Thistle (Cirsium arvense)

Gallery:

Canadian thistle leaf

Canadian thistle flower heads

Canadian thistle flowers

Canadian thistle flower head and bracts

Canadian thistle pappus

Common names:

Canada thistle, Canadian thistle, corn thistle, creeping thistle

Scientific Name:

Cirsium arvense (syns. Breea arvensis, Carduus arvensis, Cirsium incanum, Cirsium orchrolepideum, Cirsium setosum, Cnicus arvensis)

Description:

Canada thistle is a tall, prickly, creeping rhizomatous perennial with multiple purple flowers. The erect plant stands around 2 to 4 feet tall on rigid, hairy, branched stems that usually die back during winter. This aggressive noxious plant forms dense unisex patches on disturbed land, and produces allelopathic chemical that inhibit growth of other plants. Canada thistle is native to Europe, not Canada, and can be found throughout the United States.

Life cycle:

Perennial

Height of mature plants

5 feet

Flower color:

Purple

Bloom time:

Flowers appear from June to October.

Look-a-likes:

There are several native and nonnative thistles that resemble Canada thistle. Please see this guide for identification clarification.

Habitat:

Canada thistle can be found in disturbed areas with abundant sun and moist but not wet soils. Crop fields, forest openings, gardens, hillsides, logged areas, pastures, range land, roadsides, stream banks, vacant lots and waste places are the usual infestation locations for Canada thistle. It is found throughout most of the continental United States.

Impacts:

Once established, it spreads quickly replacing native plants. It grows in circular patches, spreading vegetatively through roots which can spread 10 -12′ in one season. It poses an economic threat to the agriculture industry by reducing crop yields.