Virginia Tech Weed Identification Guide

Tall Thistle: Cirsium altissimum

Weed Description: A biennial thistle with distinctive pink flower heads, hairy stems, and densely white pubescent (tomentose) leaf undersides.  These plants may reach as much as 9 1/2 feet in some areas and typically flower from July to October.  Tall thistle is found as a weed of pastures, hay fields, roadsides, railroads, and other non-crop areas.

Leaves: Arranged alternately along the flowering stems.  Leaves are mostly without hairs above (glabrous) and are densely white pubescent (tomentose) beneath.  Leaf margins are usually not lobed (entire) but spines occur along the leaf margins.  Leaves typically occur without petioles and are attached directly to the stem (sessile), or some may occur on short petioles.  Leaves may reach as much as 12 inches in length and 4 inches in width.

Stems: Stems are erect, branching, with many hairs.  Stems may reach heights of 9 1/2 feet or more in certain areas.

Roots: A taproot.

Fruit: An achene 6 mm long and 2 mm wide.

Flowers: Single flower heads occur at the ends of the flowering stems.  Each flower may reach as much as 1 1/3 inches in height and width.  Subtending bracts occur below the flower head.  Flowers are pink to purplish in color.

Identifying Characteristics:  The distinctive bulbous-like base below the pink to purplish flowers helps to identify this plant as one of the Cirsium species.  The mostly entire leaves with spiny margins and tomentose leaf undersides are also characteristics that help to distinguish this plant from Canada Thistle (Cirsium arvense), Bull Thistle (Cirsium vulgare), or Musk Thistle (Carduus nutans).