Ontario Weeds: Spiny annual sow 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
  5. Stems and Roots
  6. Flowers and Fruits
  7. Habitat
  8. Similar Species
  9. Related Links

Name: Spiny annual sow-thistle, Sonchus asper (L.) Hill,

Other Names: laiteron rude, Prickly annual sow-thistle, laiteron âpre, laiteron épineux

Family: Composite or Aster Family (Compositae)

General Description: Annual, reproducing only by seed.


Photos and Pictures

Spiny annual sow-thistle. Spiny annual sow-thistle.

Spiny annual sow-thistle.  Flowers.

Spiny annual sow-thistle. A. Base of plant.
Spiny annual sow-thistle. A. Base of plant.

B. Leaf from lower or mid-stem. C. Top of flowering stem.
B. Leaf from lower or mid-stem. C. Top of flowering stem.


Stems & Roots: Resembling Perennial sow-thistle in general appearance but distinguished from it by its annual root (tapering downwards from ground surface and not connected to a strong horizontal root that has other buds), lower and middle stem leaves rather firm, clasping the stem with prominent rounded lobes; all margins with small, moderately spiny teeth.

Flowers & Fruit: Upper stems and involucral bracts of the flower heads smooth or sparsely glandular-hairy and the flower heads smaller. Before opening, the flower heads are short and cylindrical. When fully open they are about 2cm (4/5in.) across and, after the yellow ray florets shrivel and fall, the heads become top-shaped with a broad base and a narrow tip. Seeds brown, 3mm (1/8in.) long, somewhat flat with several lengthwise ribs on each surface but without cross-wrinkles, each with a fluffy white pappus; whole plant with sticky white juice. Flowers from June to September.

Habitat: Spiny annual sow-thistle occurs throughout Ontario in waste places, along fence lines, roadsides, around buildings and occasionally in gardens.

Similar Species: It is distinguished by the rounded, clasping basal lobes on its rather firm leaves, the leaves lobed and generally tapering towards the tip, its tapering annual taproot, its small flower heads, and its seeds with lengthwise ribs but not cross-wrinkled.


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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For more information:
Toll Free: 1-877-424-1300
E-mail: ag.info.omafra@ontario.ca
Author: OMAFRA Staff
Creation Date: 01 June 2000
Last Reviewed: 01 November 2003