Perennial by rhizomes, 2 to 6 feet in height. Often a persistent spreading weed of
are club shaped, dull green in color and relatively thick. Young leaves covered with
short hairs. Leaf margins are wavy with spines. Shoots that emerge from
rhizomes lack cotyledons.
Roots: An extensive
rhizome system that can extend up to 3 1/2 feet into the soil in a creeping horizontal
branching at top, glabrous early but becoming pubescent with maturity.
Leaves: Alternate, sessile,
simple, oblong to lanceolate. Leaves are irregularly lobed, developing into
triangular indentations with age, with spiny margins. Upper surface of mature leaves
is dark green and hairless, while the lower surface is light green in color and may be
with or without hairs.
Flowers: Heads are numerous,
3/4 to 1 1/4 inches in diameter, and are composed of pink, purple, or rarely white disk
flowers surrounded by spineless bracts. Flowers are present from June through
Fruit: A flattened, brownish
achene 2 1/2 to 4 mm long which encloses the seed.
Plants in patches due to horizontal rhizome growth. Young leaves are covered with
short hairs (illustrated above). The flowers of Canada thistle do not have spines or
prickles unlike bull or musk thistle. Stems are also spineless unlike Bull Thistle (Cirsium vulgare) or Musk Thistle (Carduus nutans).