Aquatic and Wetland Vascular Plants of the Northern Great Plains
49. Asteraceae, the Aster Family
17. Sonchus L. -- Sow thistle1. Sonchus arvensis L. -- Field sow thistle
Stout, erect, milky-juiced perennial (3)5-15 dm tall, spreading by deep creeping roots; stems glabrous or with spreading gland-tipped hairs above. Leaves cauline, alternate, often larger and more crowded toward the base, lanceolate to oblong or oblanceolate, pinnately lobed to pinnatifid or unlobed, 10-30 cm long, 2-10 cm wide, acute to broadly rounded at the tip, prickly-margined, auriculate-clasping. Inflorescence containing several to many heads, open, corymbiform, the branches and involucres sometimes with spreading gland-tipped hairs; heads ligulate, yellow, many-flowered, 2-3.5 cm across; involucres campanulate, 10-22 mm high, sometimes thinly tomentose at the base, the bracts linear, imbricate, the outer ones much shorter than the inner; receptacle naked. Achenes reddish-brown, flattened, 2-3.5 mm long; pappus of numerous white capillary bristles, strongly accrescent. Late Jun--Aug. Wet meadows, shores, stream banks and other wet places; also fields, woods and roadsides; very common in the n, less so in the s; (A cosmopolitan noxious weed originating in Europe). S. uliginosus Bieb.
The great majority of plants in this region are subsp. uliginosus (Bieb.) Nyman (S. arvensis var. glabrescens Guenth.), characterized by glabrous stems and involucres. The subsp. arvensis is of limited occurrence, often appearing restricted to municipal areas. It differs from subsp. uliginosus in having spreading gland-tipped hairs on the upper portion of the stem, the inflorescence branches and the involucres.
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