Canada thistle is an herbaceous perennial with erect stems 1½-4 feet tall, prickly leaves and an extensive creeping rootstock. Stems are hairy, and ugly. Leaves are lance-shaped, irregularly lobed with spiny, toothed margins. Rose-purple flower heads appear from June through October and occur in rounded, umbrella-shaped clusters.
Canada thistle threatens natural communities including prairies, barrens, savannas, glades, sand dunes, fields, meadows and all disturbed soils. This highly invasive thistle prevents the coexistence of other plant species through shading, competition for soil resources and the release of chemical toxins poisonous to other plants.
Canada thistle produces abundant seeds easily dispersed by the wind. Most germinate within a year, but some may remain viable in the soil for twenty years or more. Vegetative reproduction is aided by a fibrous taproot capable of sending out lateral roots as deep as 3 feet below ground, and from which shoots sprout up at frequent intervals.
Canada thistle is declared a "noxious weed" throughout the U.S. and has long been recognized as a major agricultural pest, costing tens of millions of dollars in direct crop losses annually and additional millions costs for control.
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Pocahontas County, West Virginia
Thursday 10 June 2010