Description: This adventive perennial plant
is 1-3' tall, branching occasionally in the upper half. The light green stems
are slightly ridged. The alternate leaves are up to 8" long and 1" across, but
usually about 3-4" long and 2/3" across. They are lanceolate or narrowly ovate,
pinnately lobed and/or dentate, and are sessile or clasp the stem. Some of the
lower leaves may taper gradually to a petiole-like base. The margins of the
leaves have numerous pale yellow thorns, and tend to be rather crinkly. The
stems and undersides of the leaves can be hairless or somewhat hairy, depending
on the variety. The upper stems develop from one to several flowerheads. Each
flowerhead is about ½–¾" across, consisting of numerous disk
florets and no ray florets. The disk florets are pale pink or lavender, and
subsumed by green bracts that are appressed together. These bracts have pointed
tips, but lack spines. The blooming period usually occurs from early to
mid-summer, although plants may bloom later in the year if their growth is
disrupted by disturbance. The flowerheads have a sweet fragrance. Canada
Thistle is often dioecious, with male and female flowers occurring on separate
plants. The brown achenes of female plants are slender and slightly ridged at
the top. They have tufts of light brown hair, and are carried aloft by the
wind. The root system consists of a slender taproot and horizontal rhizomes.
Canada Thistle often forms vegetative colonies as a result of these rhizomes,
which can extend several feet in all directions.
Cultivation: This plant typically grows in full sun,
slightly moist to mesic soil, and a fertile clay-loam. It can grow in drier
sites with less fertile soil, but the resulting plants will be stunted. The
upper leaves often turn yellow or pale green in response to severe summer heat
and drought, and growth will stop. Partial shade is tolerated. Eradication of
this plant is difficult because it can regenerate from small pieces of the
root. The most effective control method involves the application of broadleaf
herbicides.
Range & Habitat:
Canada Thistle is a common plant that occurs primarily in central and northern
Illinois (see
Distribution
Map). It is apparently less common or absent from many areas of southern
Illinois,
although it could be spreading southward.
Contrary to the common name, this plant is originally from Eurasia. Typical
habitats include cropland, abandoned fields, areas along roads and railroads,
vacant lots, weedy meadows, and degraded prairies. This plant can invade lawns
that are not mowed regularly, and it is aggressive enough to invade many
natural habitats.
Faunal
Associations: Primarily butterflies, skippers, long-tongued bees,
and tiny black beetles visit the flowers for nectar. The caterpillars of the
butterfly Vanessa cardui (Painted Lady) feed on the foliage, as do the
caterpillars of many moth species (see Moth Table). Canada Thistle supplies an early
source of food for Goldfinches as it blooms earlier than most thistle species.
Because of the thorns, mammalian herbivores usually avoid eating this plant.
However, livestock will eat this plant when little else is available, and can
have problems with irritation of mouthparts and the digestive tract as a
result.
Comments: For many
gardeners, farmers, and ecologists in the Northeast and Midwest, this is one of
the most feared thistle plants. This is because Canada Thistle forms dense
colonies that can be hard to eradicate. It is a perennial plant that produces
rhizomes, which is unusual for a Cirsium spp. Canada Thistle can be
distinguished from other Cirsium spp. by considering the following: 1)
It tends to bloom earlier in the year than the biennial thistles, 2) It is less
tall and more slender than most species, and 3) the flowerheads are smaller and
more pale-colored. Many varieties of Canada Thistle have been described.
Walking through a colony of Canada Thistle is an unpleasant and scratchy
experience, but it is less formidably armed than other species, such Cirsium
vulgaris (Bull Thistle) and many Rubus spp.
(Blackberries).