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How to Manage Pests
Identification: Weed Photo Gallery
Canada thistle
Scientific name: Cirsium arvense (Sunflower Family: Asteraceae)
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DESCRIPTION:
Characteristics
Canada thistle, a perennial broadleaf, is found in most of the western states. The extensive root system may be 20 feet (6 m) deep and reach 15 feet (4 .5 m) horizontally.
Seedling
Cotyledons (seed leaves) are oval to oblong, fused at the base, thick, smooth or slightly glandular. First leaf margins are somewhat wavy to unevenly toothed. Surfaces are covered with stiff hairs. Lower leaf surfaces are often sparsely covered with soft weblike hairs. Seedlings have poorly developed rosettes.
Mature plant
Leaves alternate along the stem, are oblong to lance-shaped, mostly 2 to 8 inches long (5 to 20 cm), and prickly. Leaf bases may extend down the stem joints as prickly wings up to almost 1/2 inch long (1 cm). Margins are toothed and range from nearly smooth to shallow-lobed. The upper leaf surface is hairless to nearly hairless and the lower surface sometimes sparsely woolly. Mature plants have either a few rosette leaves or lack them altogether. Stems are slender and hairless or nearly so.
Flower
White to purplish pink flower heads, about 1/2 inch (1.3 cm) in diameter, form on separate male and female plants.
Propagation
Wind dispersed seeds can be viable for many years when buried. Plants will grow from cut roots.
Management
Control plants before they can regenerate food reserves or produce seed. Prevent spread to new areas and eliminate the weed from noncrop locations.
Broadleaf ID illustration.
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