Perennial Sow-Thistle

Composite or aster (Asteraceae family):

Sonchus arvensis L.

EPPO code:

SONAR

Other names:

Creeping sow-thistle, field sow-thistle, milk thistle, sow-thistle

Species information

Lifecycle:

Perennial.

Propagation:

Reproduces by seed and spreads by horizontal roots.

Emergence:

Shoots emerge in late April to early May. The seed tends to germinate in late May as soil temperatures warm.

Habitat:

Perennial sow-thistle is very widespread in the province and can be found in waste places, road sides, gardens, lawns and in cultivated fields, especially under reduced tillage.

Competitiveness:

Perennial sow-thistle is extremely competitive. Where dense patches of perennial sow-thistle exist there is often little crop to harvest.

Identification clues

Seedling

Cotyledons:

Orbicular.

First leaves:

The first leaves appear as a basal rosette; the leaves are oval to elongated with irregularly toothed margins.

Mature leaves:

The mature leaves of perennial sow-thistle are elongated, wavy toothed to deeply divided and have spiny margins and a prominent white central vein.

Vegetative shoots:

While shoots arise in clusters from root buds, the leaf shape will vary from elongated to deeply-divided.

Mature plant

Stems:

Its stem is erect and hollow. It is hairless at the base, but hairy as you go up the stem.

Flowers:

The flower heads are yellow, 2.5–4 cm in diameter and similar in appearance to those of dandelion as they contain only ray florets. The flowering head’s bracts are densely glandular and hairy.

Seeds:

Its seeds are brown, 2.5–3.5 mm in length and have five or more prominent ribs on each side. The seeds are attached to a white feathery pappus that most refer to as “fluff”.

Roots:

Thick, spreading horizontal roots that help in spreading patches.

Identification tip

If parts of the plant are broken, it will excrete a sticky white juice (latex).

Often mistaken for

I know it's not Prickly lettuce because perennial sow-thistle has spreading horizontal roots and lacks the row of spines on the underside of its leaf midrib.

I know it's not Annual sow-thistle because perennial sow-thistle has spreading horizontal roots and flowering heads with densely hairy bracts. It lacks the pointed basal leaf lobes of annual sow-thistle.

I know it's not Spiny annual sow-thistle because perennial sow-thistle has spreading horizontal roots and flowering heads with densely hairy bracts. It also lacks the rounded basal leaf lobes of spiny annual sow-thistle.