Perennial sow-thistle is a common introduced weed species in southern British Columbia that originates in Europe. It is found along roadsides and in other disturbed sites. It can be identified by the yellow composite (daisy-like) flower heads with strap-shaped flowers, lack of basal leaves, and prickly-margined stem leaves. Plants exude milky juice when broken. .

General:
Perennial herb from a long, vertical and horizontal, rhizome-like, deep-seated root; stems erect, solitary, few-branched above, glabrous below, usually glandular-bristly above, hollow, exuding milky juice when broken, 0.4-2.0 m tall.

Leaves:
Basal leaves lacking; stem leaves alternate, prickly-margined, deeply lobed or pinnately cut, 6-40 cm long, 2-15 cm wide, with clasping leaf bases, upper leaves reduced.

Flowers:
Heads with strap-shaped flowers, several on glandular stalks in an open, flat- or round-topped inflorescence, relatively large, commonly 3-5 cm wide in flower; involucres 14-22 mm tall; involucral bracts lanceolate, tapering to a slender tip at the apex, glabrous, obscurely woolly-hairy or more or less copiously covered with coarse, spreading, gland-tipped hairs and often with tufts of woolly-hair as well, outer ones often becoming ridged-thickened in age; ray flowers yellow.

Fruits:
Achenes 2.5-3.5 mm long, ribbed, cross-wrinkled; pappus of white hairlike bristles.

Notes:
Two varieties occur in BC:

1. Involucres and stalks of the heads with coarse, spreading, gland-tipped hairs................. var. arvensis

1. Involucres and stalks of the heads glabrous or obscurely woolly-hairy...................... var. glabrescens Guenth., Grab. & Wimm.

Source:  The Illustrated Flora of British Columbia

Synonyms and Alternate Names:
Sonchus arvensis var. glabrescens Guenth., Grab. & Wimm.
Sonchus uliginosus