Facts at a glance – Smooth sow-thistle


Smooth sow-thistle

(sonchus oleraceus)

Young Plant

The cotyledons are oval with very short stalks. The first leaf ends in prickles and is oval but cut almost square across to the stalk. It is edged with teeth pointing towards the centre of the plant.

Mature Plant

The stem is thick, smooth, hollow and much branched. The green shiny leaves are oblong in outline and lobed, with a triangular lobe at the end. They are slightly prickly and the lower leaves have a short stalk while the upper clasp the stem.


The lemon yellow flowerheads are 20-25mm across and grow in loose clusters. The outer ray florets are purplish underneath. The buds are cylindrical but the base of the flower is flask shaped, becoming more pronounced as the flowerhead dies.

Height 120cm

Smooth sow-thistle Profile

Key feature
  • Leaves are glossy and softly prickly with a wide triangular lobe at the base clasping the stem
  • A single taproot
  • Flowers are lemon yellow
Number of seeds produced per plant Up to 100,000
Seed shed May-October
Germination period March-July and September to November
Germination depth 2cm
Primary dormancy Low
Does it have a secondary dormancy? Yes
Seed longevity >5 years
Factor promoting germination Light
Rate of seed decline with cultivations High
Lifecycle Plants which germinate in autumn overwinter as rosettes, producing flowers in May/june; plants germinating in spring flower in June. The latter can set seed in 10 weeks. Prickly sow-thistle only reproduces by seed and is distributed by wind
Geographical Location Widespread in England and Wales
Soil Type Nitrogen rich loams or nutrient rich sandy and stoney soils which are not too dry
Impact Increasing found in arable rotations particularly in winter crops. Autumn germinating plants can overwinter as rosettes and flower in May, spring germinating plants flower in June
5% yield loss from 50 plants per m2
Resistance risk No resistance in the UK but resistance to a wide range of herbicides in Austrailia.

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