Composite or aster (Asteraceae family):
Lactuca serriola L.
EPPO code:
LACSE
Other names:
Wild lettuce, compass plant

Species information

Lifecycle:
Winter annual, annual or biennial.
Propagation:
Reproduces by seed.
Emergence:
The majority of seeds will germinate in autumn with seedling plants often in the rosette stage in the spring.
Habitat:
Prevalent throughout the province in waste areas, pastures and roadsides and in agricultural crops under minimum till or no-till cropping systems.
Competitiveness:
Ontario studies have shown yield losses as high as 80% when no-till soybean was competing with prickly lettuce at a density of 50 plants/m2 while no yield losses in winter wheat were observed even though prickly lettuce densities were as high as 200 plants/m2 (Weaver et al., 2006).

Identification clues

Seedling

Cotyledons:
Rounded and tapering to the base.
First leaves:
A basal rosette with club-shaped leaves having finely-toothed margins.
Mature leaves:
Alternate orientation on the stem, variable in size and shape ranging from deeply lobed to without lobes but having spiny toothed margins and a prominent single row of stiff, sharp prickles on the midrib of the leaf’s underside; exudes a milky white juice (latex) when broken.

Mature plant

Stems:
Erect, hollow and hairless; ranging from 30-150 cm tall; green to reddish-green; exudes latex when broken.
Flowers:
Flower heads are made of yellow ray florets and are 8 to 10 mm in diameter; the flower heads are borne at the end of long fine stalks and arranged in a pyramidal panicle.
Seeds:
Yellowish-brown to brown, 2–4 mm long with prominent ribs on each side and attached to a white feathery pappus that most refer to as “fluff”.
Roots:
Fleshy taproot.

Often mistaken for

I know it's not Annual or perennial sow-thistle because prickly lettuce’s mature leaves have a row of spines on the underside of its leaf midrib.

A fall germinated seedling rosette in late April
A fall germinated seedling rosette in late April.
A spring germinated plant in June
A spring germinated plant in June.
The distinct row of spines on the central vein of the leaf underside
The distinct row of spines on the central vein of the leaf underside.
A pre-bloom plant in winter wheat during late June
A pre-bloom plant in winter wheat during late June.
The small yellow flowers of prickly lettuce
The small yellow flowers of prickly lettuce.