Cirsium arvense - (Canada thistle)

COMMON-NAMES = Canada Thistle, creeping Thistle, Californian Thistle,

DESCRIPTION

Cirsium arvense is a perennial thistle up to 1.5 meters tall. It is distinguished by its creeping horizontal lateral roots, which produce dense patches of aerial shoots. The sessile, pinnatifed and prickly leaves are dark green, sometimes wooly beneath (Fernald 1950, Gleason 1957, Moore and Frankton 1974). Plants are imperfectly dioecious. The numerous heads of purple to white flowers (in forma albiflorum) are 1 to 2.5 cm long and one-third to one-fourth as wide. The corolla of the globose male heads is projecting, 12-14 mm long with anthers to 4 mm long. Male flowers occasionally have functional female parts and set seed (Hodgson 1968, Kay 1985, Hayden 1934). Pistillate heads are oblong, with corollas 23-26 mm long, the conspicuous pappus about 14 mm long in fresh flowers.

Thistles of the genus Cirsium are distinguished by their plumose pappus from members of the genus Carduus which are known as "plumeless thistles" because of their simple pappus. Within the genus Cirsium, C. arvense is the most often confused with C. vulgare. The two species can be distinguished by the larger head and isolated distribution of C. vulgar (Hansen 1918). Gray's Manual of Botany (Fernald 1950) distinguishes C. arvense from all other thistles by its habit of arising from sprouts of a widely creeping root system and its almost universally dioecious plants.

Within C. arvense, four varieties are commonly recognized: the typical var. horridum, and vars. mite, integrifolium and vestitum (Fernald 1950, Moore and Frankton 1974, Moore 1975).

HABITAT

Despite its common name, Cirsium arvense is native to Eurasia and was apparently introduced to North America during the colonial period. By 1918, it was already on the noxious weed lists of 25 northern states. It is now widespread in all states and Canadian provinces north of 37 degrees N and south of 58-59 degrees N (Moore 1975). Infestations are particularly troublesome in the northwest and northcentral states, and in the eastern provinces of Canada (Moore 1975)

The northern limit of the zone of highest density in Canada corresponds with the 18 degrees mean January isotherm, whereas the southern limit of the species is probably controlled by high summer temperatures and short-day length (Moore 1975).

Cirsium arvense is found in open, mesophytic areas, with optimal growth between 50 and 75 cm annual rainfall (Hodgson 1968). It occurs on all but waterlogged, poorly-aerated soils (Rogers 1928, Bakker 1960, Hodgson 1968, Moore 1975) and in nearly every community within its range where sufficient light is available. Bakker (1960) found an average density of 39 shoots per m with 41 flower heads per shoot in open sites, and a density of 11 shoots per m with 18 flower heads per shoot in shaded areas. Patches of C. arvense are most prevalent in disturbed areas such as agricultural land, roadsides, ditch spoil banks, gopher mounds and overgrazed pastures (Moore 1975).

Cirsium arvense apparently has difficulty becoming established from seed in undisturbed areas. Amor and Harris (1974) report no seedling establishment from seed artificially sown in pastures, whereas 7 to 13% of seeds sown on bare dirt emerged and 78 to 93% of these became established. In spring wheat fields, C. arvense increased in abundance by 192% over a four-year period, whereas over the same interval it declined to 1% of its previous value in alfalfa fields mowed twice yearly for hay (Hodgson 1968).

Thistles are another plant we can't seem to appreciate. Thistles are wonderful, not only as rich nectar sources for butterflies but as host plants. Thistles of all kinds are devoured by larvae of painted ladies (Vanessa cardui), whereas the swamp metalmark (Calephelis mutica), a state threatened species, is a thistle connoisseur, feeding only on swamp thistle (Cirsium muticum), itself a state threatened species.

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