Perennial Sow-thistle (Sonchus arvensis)

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A perennial plant that reproduces both by seed and rhizomes. Stems are upright, leafy at the base, branched in the tops and grow to 2m tall. Cut stems will exude a milky juice. Leaves are alternate and waxy, with weakly prickled edges and the shape is variable. Lower leaves are stalked, but will clasp the stem higher up. Leaf color varies from light to dark green and they can grow up to 20cm long. Flowers are small, yellow and dandelion-like. They are grouped in loose clusters at the ends of stems. The bracts of the flower heads are often covered with sticky hairs. One plant may have up to 20 flower heads, but with only few in bloom at a time. Flowers usually open 2-3 hours after sunrise and close about noon. Flowers have both male & female organs but are generally self-incompatible, and are pollinated by insects. Seeds are tufted to aid in wind dispersal. Sonchus arvensis ssp. uliginosus lacks glandular hairs on the involucral bracts and flower stalks. Its involucral bracts are green with white margins.

Richard Old, XID Services, Inc.,

Key Identifiers
  • Rhizomatous, creeping roots
  • Stems exude milky juice
  • Lobed leaves with prickly margin
  • Flower heads yellow, greater than 2.5 cm across
  • Upper stems and bracts usually have orange hairs
Location in Canada
Perennial Sow-thistle has been reported in all Canadian provinces with the exception of Nunavut.

Alberta Invasive Plants Council - Perennial Sow-thistle

Similar species
Spiny Sow-thistle (Sonchus asper) taproot, very prickly margins, auricles are prominent and rounded and flower is less than 2.5 cm across.
Annual sow-thistle (Sonchus oleraceus) taproot, weakly prickly margins, auricles are sharply pointed and flower is less than 2.5 cm across.
Native lettuces (Lactuca spp)they do not have the extensive root system or their flowers are a blue or purple in color.

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For more information about the content of this document, contact Chris Neeser.
This document is maintained by Shelley Barkley.
This information published to the web on March 29, 2012.
Last Reviewed/Revised on August 21, 2018.