Spiny Annual Sow-Thistle

Composite or aster (Asteraceae family):

Sonchus asper (L.) Hill

EPPO code:

SONAS

Other names:

Prickly annual sow-thistle

Species information

Lifecycle:

Annual.

Propagation:

Reproduces by seed.

Emergence:

The majority of seed germinates and emerges in mid to late May as soil temperatures warm, but emergence has been observed throughout the season.

Habitat:

Spiny annual sow-thistle is prevalent in agricultural crops and waste areas where soil has been disturbed.

Competitiveness:

Crop yield loss depends on the density of spiny annual sow-thistle, but due to its wind-borne seed, this species can quickly become densely populated.

Identification clues

Seedling

Cotyledons:

Orbicular.

First leaves:

Spiny annual sow-thistle’s first leaves appear as a basal rosette with rounded oval to elongated leaves with wavy toothed margins. It is difficult to differentiate them from annual sow-thistle seedlings.

Mature leaves:

Mature leaves feel waxy and taper to a pointed tip. They have very spiny margins and two rounded lobes at the base that clasp the stem.

Mature plant

Stems:

Spiny annual sow-thistle’s stem is erect, hollow and hairless.

Flowers:

Its flower heads are yellow and look similar to those of dandelion as they contain only ray florets. They grow to 1.5–2.5 cm in diameter. The flowering head’s bracts are sparsely haired to hairless.

Seeds:

The plant’s seeds are brown in colour. They grow to be 2–4 mm long and have 3–5 (or more) prominent ribs on each side. The seeds are attached to a white feathery pappus that most refer to as “fluff”.

Roots:

Fleshy taproot.

Identification Tip

The plant will excrete a sticky white juice when parts of it are broken.

Often mistaken for

I know it's not Prickly lettuce because spiny annual sow-thistle lacks the row of spines on the underside of its leaf midrib.

I know it's not Perennial sow-thistle because spiny annual sow-thistle does not have spreading rhizomes, and its flowering heads have hairless bracts.

I know it's not Annual sow-thistle because the leaf of spiny annual sow-thistle has a waxy surface, is very prickly or spiny when you run your hand along the leaf margins and it clasps the stem with round lobes.