Annual Sow-Thistle

Composite or aster (Asteraceae family):

Sonchus oleraceus L.

EPPO code:

SONOL

Other names:

Common annual sow-thistle, hare’s lettuce, colewort, milk thistle

Species information

Lifecycle:

Annual

Propagation:

Reproduces by seed.

Emergence:

The majority of seeds germinate and emerge in mid to late May as soil temperatures warm, but emergence has been observed throughout the season.

Habitat:

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 annual sow-thistle, but due to its windborne seed it can quickly become densely populated.

Identification clues

Seedling

Cotyledons:

Orbicular

First leaves:

The first leaves of annual sow-thistle appear in a basal rosette with rounded, oval to elongated leaves with wavy-toothed margins. The leaf tip starts out rounded, but later has the characteristic triangular or arrowhead-shaped tip.

Mature leaves:

Mature leaves are elongated and deeply divided with a triangular or arrowhead-shaped tip. The margin is soft-spiny. There are two angular or pointed lobes at the base that clasp the stem. Leaves appear in an alternate arrangement on the stem.

Mature plant

Stems:

Erect and hollow, annual sow-thistle’s stem is hairless at the base. As you go up the stem, though, it becomes hairy at the top.

Flowers:

The flower heads of annual sow-thistle are yellow, 1.5–2.5 cm in diameter and similar in appearance to those of dandelion as they are composed only of ray florets. The flowering head’s bracts are sparsely hairy to hairless.

Seeds:

The plant’s seeds are brown and 2–4 mm long with 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 (latex) when parts of it are broken.

Often mistaken for

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

I know it's not Perennial sow-thistle because annual sow-thistle does not have spreading rhizomes, its leaf tips are triangular or arrowhead shaped and its flowering heads have hairless bracts.

I know it's not Spiny annual sow-thistle because annual sow-thistle lacks the rounded basal leaf lobes of spiny annual sow-thistle. Its leaves do not have a waxy feel to them nor are the leaf margins as prickly as those of spiny annual sow-thistle.