Cirsium arvense

Name and classification

Cirsium arvense (L.) Scop
Family Asteraceae, Tribe Cynareae

NOTE: This species is actionable only when found in field or vegetable seed for planting; tolerance applies.

Common names

Canada thistle

Disseminule

fruit

Description

Fruit an achene, oblong to obovate, 2.5–4 mm long, 0.8–1.5 mm wide, 0.4–0.8 mm thick, sometimes curved, truncate at base and apex, obscurely angled towards base, oval or 4-sided in cross section. Surface smooth, dull to lustrous, straw-colored to golden-brown, sometimes with light colored longitudinal stripes. Scar basal. Apical collar often light colored, oblique, thin, prominently rimmed above a constricted band. Style base cylindrical or conical, sometimes with a small knob on top. Pappus of white, plumose hairs, 12–20 mm long, deciduous as a ring. Embryo linear-spatulate; endosperm absent.

Identification considerations

Achenes are usually easily identifiable, primarily because they are smaller than most commonly found Cirsium achenes.

Similar species

Cirsium ochrocentrum A. Gray (non-FNW)

Cirsium vulgare (savi) Ten. (non-FNW)

Distribution

widespread in cool temperate regions of Africa, Europe, western Asia, North and South America; also Australia and New Zealand

native to western temperate Asia, Europe

Habitat

subhumid to humid cool-temperate regions, open areas up to subalpine levels; grows on a wide range of soil types and moisture conditions; a weed of pastures, ditch banks, forest edges, open, neglected land

General information

Cirsium arvense is a spiny-leaved perennial herb up to 1 m tall, dioecious; it is the only thistle with unisexual flower heads. Cirsium arvense is the worst thistle problem in much of its range. It infests both cultivated lands and pastures. Contamination of agricultural seed is responsible for much of its dispersal worldwide. Vegetative reproduction, however, is its major means of spread. New shoot buds and roots can arise anywhere on the plant’s extensive, branched system of horizontal and vertical roots. A small root fragment can produce plants covering 25 square meters. Entire patches of the plant may not produce seed either because the flower heads are solely staminate or because male and female plants are too far apart. Achenes are often devoid of seeds. Cirsium arvense is one of ten species covered under the Federal Seed Act for which tolerance is applicable to its introduction (7 CFR 361.6(a)(2)).

achenes

achenes

apex of achene

apex of achene

basal scar of achene

basal scar of achene

longitudinal section of achene showing embryo (left) and naked embryo (right)

longitudinal section of achene showing embryo (left) and naked embryo (right)