Annuals, (15–)30–80+ cm (tap­roots fibrous-rooted). Herbage puberulent (hairs abundant, curly). Stems single. Leaves evenly distributed; petiolate; blades obovate to oblong, 3–7(–12) × 1–3(–4) cm, usually 1–2-pinnate, bases tapered, ultimate margins dentate (distal leaves similar, clasping, bractlike). Heads 12–24 in corymbiform arrays. Calyculi 0 or of 1–5+ linear to filiform bractlets. Phyllaries ± 13 (± 21), 4–7+ mm, tips greenish or minutely black. Ray florets 0 or 1–8+; corolla laminae usually 1–2+ mm (barely surpassing phyllaries). Cypselae hairy (especially on angles). 2n = 40.

Flowering late spring–summer (northern areas of cool, climate); winter (southern areas). Mildly disturbed woodlands, open, sandy sites; 100–300 m; introduced; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., P.E.I., Que.; Calif., La., Mass., Mich., N.J., Ohio, Oreg., Pa., Wash., Wis.; Eurasia.

Senecio sylvaticus is a Eurasian weed that favors cool, wet climates. It is well established in coastal areas of the Pacific Coast and in parts of Newfoundland and Quebec; elsewhere in the flora, it appears to be sporadic.