Ontario Weeds: Perennial sow thistle

Table of Contents

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

Name: Perennial sow-thistle, Sonchus arvensis L.,

Other Names: laiteron des champs, Creeping sow-thistle, Field sow-thistle, Glandular-hairy perennial sow-thistle, crève-z-yeux, laiteron vivace

Family: Composite or Aster Family (Compositae)

General Description: Perennial, reproducing by seed and from buds on widely spreading, creamy white, brittle, underground roots.

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Photos and Pictures

Perennial sow thistle (A - group of flowering stem; B - one stem beginning to flower).

Perennial sow-thistle. A. Young plant from seed. B. Base of plant from perennial root. C. Top of flowering stem.

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Stems & Roots: Stems erect, 60-150cm (2-5ft) high, smooth and hairless on the lower part but glandular-hairy towards the top and on branches, hollow; leaves of seedling plants broadly club-shaped with irregularly toothed margins, the teeth ending in weak prickles, remnants of the cotyledons (seed leaves) often visible; leaves of shoots from perennial roots variable in shape, the lower ones shallowly to deeply lobed and irregularly toothed, reminiscent of Dandelion leaves but the teeth ending in small, weak prickles, with winged stalks; alternate (1 per node); middle and upper leaves similar but smaller with shorter stalks and clasping the stem with small, rounded basal lobes, or sometimes with larger basal lobes similar to those of Spiny annual sow-thistle as at; uppermost leaves small, narrow, without lobes and with only a few teeth; upper stems, branches and involucral bracts surrounding the flower heads usually densely covered with dark hair; each hair with a tiny gland at its tip (these hairs, when seen under magnification, resemble a tiny lollipop) (these glandular hairs do not occur in its sister variety, Smooth perennial sow-thistle).

Flowers & Fruit: Flower heads showy, bright yellow, 2.5-4cm (1-1¾in.) across; each head containing only strap-shaped ray florets (like Dandelion flower heads and similar to those of Smooth perennial sow-thistle); seeds brown with lengthwise ridges and finer cross ridges; whole plant with sticky white juice and a rather sour odour. Flowers from June to late autumn.

Habitat: Perennial sow-thistle occurs throughout Ontario in cultivated fields, pastures, meadows, woodland, waste places, roadsides, gardens and occasionally in lawns.

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Similar Species: Non-flowering plants are distinguished by their variously lobed leaves with weakly spiny teeth; by their soft, somewhat fleshy, bright green stems mostly arising directly from horizontal yellowish-white, fleshy, brittle, cord-like roots, these roots bearing many, small, whitish buds able to grow into other new leafy stems; the whitish vertical underground portion of each upright stem resembling a root, but usually not tapering downwards except in first-year plants which started from seed. Flowering plants are distinguished by their showy, large, bright yellow flower heads and the covering of glandular hairs on the involucre and the upper stems and branches.

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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