- Name also: Grundy Swallow, Ground Glutton, Simson, Sention, Old-man-in-the-spring
- Family: Daisy Family – Compositae, subfamily Asteroideae
(formerly Aster Family – Asteraceae)
- Growing form: Annual herb.
- Height: 20–40 cm (8–16 in.). Stem branchless–irregularly branching, bristly, usually quite swollen, usually sparsely haired–glabrous, underside often reddish. Lacking fragrance.
- Flower: Single flower-like approx. 4–5 mm (0.16–0.2 in.) capitula surrounded by involucral bracts. Capitula flowers yellow, ray-florets usually lacking (occasionally tongue-like, soon curving downwards), disc florets tubular, small. Stamens 5. Pistil of 2 fused carpels. Involucre cylindrical, involucral bracts 1 row, lanceolate, usually glabrous, with membranous margins, green, with black tips; outer bracts at base of involucre 8–10, small, triangularly lanceolate, with visibly black tips. Capitula borne in a dense, corymbose cluster.
- Leaves: Alternate, lower short-stalked, upper stalkless, amplexicaul. Blade pinnately lobed, thicket, usually shiny, sparsely haired–glabrous, lobes with toothed margins.
- Fruit: Cylindrical, ridged, hairy along grooves, dark brown, 1.5–2 mm (0.06–0.08 in.) long achene, tip breaking off easily, with white unbranched hairs.
- Habitat: Arable land, yards, gardens, flower beds, heaps of earth, wasteland, roadsides, sea-shores.
- Flowering time: June–October.
Genus Senecio is large and one of the most diverse. It includes tropical African mountain stout-trunked trees and large oddities with heads that look like cabbages as well as everyday weeds. Common groundsel is somewhat mundane-looking and goes easily unnoticed. It is small, a weak competitor, and is easily crowded out by other plants. It makes up for these short-comings, however, by quickly colonizing bare patches of ground, and it can produce two generations in a single year: it flowers throughout the summer and in mild southern winters, when there is not much snow, common groundsel that have just opened their capitula can be found long into the winter. It can self-pollinate and thus produce cypselas even if its potential pollinators are hibernating.
Common groundsel’s annual seed production is a respectable 1,500–7,000. Initially the involucral bracts that protect the capitula press tight against the flower and the involucre is narrowest at the top. As the cypselas ripen the involucral bracts turn downwards, allowing the mature cypselas to be carried off easily on the wind: they are light and downy and travel long distances on the air currents. It has also spread along highways from one population centre to another, and it grows in Finland as far north as the Oulu region – after that it becomes rare. Common groundsel favours rich, loamy, nitrogenous land, and it thrives especially well in gaps in the canopy and light-filled areas where the land has been tilled. Its reputation as a weed is well-deserved in planting beds, gardens and flower beds. It can also grow far from human habitation if a suitably fertile habitat can be found. It is still spreading and becoming more abundant in Finland.
Common groundsel looks a little like heath groundsel (S. sylvaticus), which grows in southern Finland and elsewhere as an established alien, and sticky groundsel (S. viscous), which likes the same kinds of habitats on railway banks and roadsides. These both have narrowly tongue-like and often coiled ray-florets, however, which groundsel is usually lacking.