Virginia Tech Weed Identification Guide

Canada Thistle: Cirsium arvense

Weed Description:   Perennial by rhizomes, 2 to 6 feet in height.  Often a persistent spreading weed of many pastures.

Seedling:  Cotyledons 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 growth pattern.

Stems:  Grooved, 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 August. canthistle7-23.jpg (72959 bytes)
Fruit:  A flattened, brownish achene 2 1/2 to 4 mm long which encloses the seed.
Identifying Characteristics:   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).