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Ontario Weeds: Spotted knapweed

Author: OMAFRA Staff
Creation Date: 01 June 2000
Last Reviewed: 01 November 2003

Return to the Ontario Weeds Gallery

Excerpt from Publication 505, Ontario Weeds, Order this publication

Table of Contents

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

Name: Spotted knapweed, Centaurea maculosa Lam.,

Other Names: centaurée maculée, centaurée tachetée

Family: Composite or Aster Family (Compositae)

General Description: Biennial or short-lived perennial, reproducing only by seed.

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Photos and Pictures

Spotted knapweed.

Spotted knapweed.

Spotted knapweed. A. Base of plant. B. Stem with flower heads. C 1 involucral bract.

Spotted knapweed. A. Base of plant. B. Stem with flower heads. C 1 involucral bract.

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Stems & Roots: Stems erect, much-branched, 20-180cm (8-72in.) high, greenish or purplish striped; leaves in a basal rosette the first year, compound (pinnately divided) with several simple (not lobed) or irregularly lobed leaflets widely spaced along each side, the margins of each leaflet smooth or toothed; basal leaves long-stalked, with scattered, tiny, round, shining glands embedded in both upper and lower surfaces and a few translucent dots through the blade; lower and middle stem leaves similar to those of the rosette but smaller with shorter stalks, alternate (1 per node); upper leaves small, narrow, stalkless, simple or with a few small lobes.

Flowers & Fruit: Flower heads small, erect on slender stiff branches, hard; ray florets absent; flower heads consisting only of many rosy, purplish disk florets; the corolla of each floret ending in 5 long slender lobes; the outer florets longer than the central ones; the head surrounded by an involucre of small, somewhat dry and membranous bracts, each bract ovate with a distinct brownish-black tip ending in a fine fringe like the teeth of a comb. Flowers from July to September.

Habitat: Spotted knapweed occurs throughout Ontario but is most common in the southern part of the province, especially in Grey and Hastings Counties.

Similar Species: It is distinguished by its pinnately divided rosette and lower stem leaves, the leaves slightly woolly, with tiny shining glands in both surfaces visible to the naked eye as shiny specks and with a few translucent dots visible as pinpricks through the leaf blade when viewed against the light, and by its showy rosy-purple flower heads surrounded by involucral bracts with dry and membranous brownish-black tips fringed like the teeth of a comb.

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Related Links

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

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