1. Sonchus arvensis Linnaeus, Sp. Pl. 2: 793. 1753.
Field sow-thistle, laiteron des champs
Perennials, 0–150(–200) cm, usually rhizomatous or stoloni-ferous. Stem bases hard, some-times ± woody. Leaves: blades of mid cauline oblong to lanceolate, (3–)6–40 × 2–15 cm, bases auriculate, auricles straight or curved, rounded, margins usually pinnately lobed, lobes ± deltate, not constricted at bases, terminals usually larger than laterals, dentate or entire. Peduncles sessile- or stipitate-glandular. Involucres 10–17+ mm. Phyllaries sessile- or stipitate-glandular. Corollas: ligules ± equaling tubes. Cypselae dark brown, oblanceoloid to ellipsoid, 2.5–3.5 mm, ribs 4–5(+) on each face, faces transversely rugulose to tuberculate across and between ribs; pappi 8–14 mm. 2n = 36, 54.
Subspecies 2 (2 in the flora): introduced; Europe; introduced also in South America, Asia, Africa, Pacific Islands (New Zealand), Australia.
Sonchus arvensis is introduced in temperate regions of all continents. Plants of the species prefer relatively cooler, moister climates and are more abundant in the northern part of North America.