Sonchus arvensis is a perennial herb, a C3 plant; spreading root system to 1.8 m from vertical roots to 2 m, colonial from rhizomes, vegetative buds produced to 50 cm deep, sap milky, stems hollow, branched, plant often waxy pale green, upper stem and inflorescence glandular-hairy; new spring leaves reddish-purple, emerge late April-May.
Leaves alternate, lobed to entire, margins prickly, basal rosette leaves 15 cm long, 4 cm wide, tapered to a winged stalk, lower stem leaves to 30 cm long, 10 cm wide, basal lobes rounded, stalkless, clasping, crowded, upper leaves reduced, distant.
Flowers yellow, in heads to 3-5 cm wide, 150-235 per head, all rays (ligulate), basal bracts narrow, in three overlapping rows, gray-green, the outermost bracts to 1×0.1 cm; inflorescence branched. Flowers self-incompatible; insect pollinated, visited by bees and hover flies and blister beetles. Flowers open usually a few hours in the late morning (Mulligan and Kevan 1973; Lemna and Messersmith 1990); blooms July-Oct.
Fruit dry, achenes, to 0.3 cm long, 0.1 cm wide, 5 ribs per side, white-plumed, to 1.2 cm long. Dispersed by wind, but hooks at ends of pappus hairs may cling to fur or clothing. Seed development and maturation from flower to fruit about 10 days; Seed viability drops sharply after one or two years. Seedlings emerge mid to late May, soon forms a rosette. Plants usually do not flower until their second year.
Wetland status: UPL.
Frequency in NYC: Very infrequent.
Habitat: An agricultural weed of open areas. Prefers moist, neutral to slightly alkaline soils (pH 7.2), tolerates moderate salinity, intolerant of drought. Above ground parts very intolerant of frost.
Notes: Young leaves edible, latex contains 5% potentially useful oil and plants are a source of pentacyclic terpenes (components of essential oils) (Lemna and Messersmith 1990). Terpenes and many other chemicals considered to be produced by plants as protective agents (Raven et al. 1986). Perennial sow-thistle is attacked by the several nematodes including; the root-knot nematode Meloidogyne incognita and by the cyst-forming nematode Heterodera sonchophila, which specializes on sow thistle. Sonchus arvensis is also attacked by a number of fungi including three that are specialists; Marssonina sonchi, Septoria sonchi-arvensis and S. sonchifolia. It is also susceptible to the Aster yellows virus, Chologenus callistephi (Lemna and Messersmith 1990).