Ontario Weeds: Scotch thistle

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Excerpt from Publication 505, Ontario Weeds, Order this publication

Table of Contents
  1. Name
  2. Other Names
  3. Family
  4. General Description
    1. Photos and Pictures
  5. Stems and Roots
  6. Flowers and Fruit
  7. Habitat
  8. Similar Species
  9. Related Links

Name: Scotch thistle, Onopordum acanthium L.,

Other Names: acanthe sauvage, Cotton thistle, White thistle

Family:Composite or Aster Family (Compositae

General Description:Biennial or sometimes annual, reproducing only by seed.

Photos and Pictures:

Scotch thistle.

Scotch thistle.

Scotch thistle. A. Rosette of basal leaves. B. Branch with flower head.

Scotch thistle. A. Rosette of basal leaves. B. Branch with flower head.

Stems & Roots: Stems erect, tall, 1-2.4m (3-8ft) high, often much-branched, densely white woolly with broad, spiny, leaf-like wings; leaves of first-year rosette large (to 60cm, 24in. long and 30cm, 12in. wide), lobed or coarsely toothed, densely white woolly (like a felt insole), margins wavy and spiny; stem leaves similar but smaller, alternate (1 per node), the margins of the leaf blades usually continuing down the stem as wings.

Flowers & Fruit: Flower heads (at ends of branches and from leaf axils, large, 2.5-5cm (1-2in.) across, nearly spherical, surrounded by numerous, over-lapping, narrow, spine-tipped involucral bracts; ray florets absent; disk florets numerous, purple, showy; seeds about 5mm (1/5in.) long, light brownish-gray to dark gray and wrinkled crosswise. Flowers from late June to September.

Habitat: Scotch thistle occurs in scattered localities throughout southern Ontario in waste places, fence lines, and around old buildings, usually in gravelly soils where it has escaped from cultivation.

Similar Species: It is distinguished from all other thistles in Ontario by the very dense, white woolly covering on stems and leaves.

Related Links

... on general Weed topics
... on OMAFRA Noxious Weeds Profiles
... on weed identification, order OMAFRA Publication 505: Ontario Weeds
... on weed control, order OMAFRA Publication 75: Guide To Weed Control

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