- Name also: Smooth Sow Thistle, Annual Sow Thistle, Hare’s Colwort, Hare’s Thistle, Milky Tassel, Swinies, Milk-thistle, Sow-thistle, Common Sowthistle
- Family: Daisy Family – Compositae, subfamily Cichorioideae (formerly Chicory Family – Cichoriaceae)
- Growing form: Annual herb.
- Height: 40–80 cm (15–32 in.). Stem lower part branched, soft, juicy, hollow. Containing abundant latex.
- Flower: Single flower-like capitula 2–2.5 cm (0.8–1 in.) broad, surrounded by involucral bracts. Capitulum flowers pale yellow, tongue-like, tip 5-toothed. Stamens 5. Gynoecium composed of 2 fused carpels. Involucral bracts (max. 35) overlapping in 3 rows, dark green. Capitula borne in a corymbose cluster.
- Leaves: Alternate, basal leaves stalked, stalks channelled, narrowly winged, sparsely toothed, teeth tipped, stem leaves stalkless, amplexicaul, basal lobes surrounding stem. Blade pinnate, glabrous, thin, top dull bluish green, underside bluish grey, margin blunt-toothed, with small bristles–bristleless, lateral lobes narrow, turning towards stalk, terminal leaflet large, triangular.
- Fruit: Flat, ridged with protuberances, brown, 2.5–4 mm (0.1–0.16 in.) long achene, crowned with approx. 8 mm (0.32 in.) long unbranched hairs.
- Habitat: Gardens, flower beds, soil heaps, arable land (occasionally in fields), around old inhabited areas, wasteland, rubbish tips.
- Flowering time: July–September.
Smooth sow-thistle likes manure and nitrogenous garden soil, and the same habitat is also very appealing to its close relative spiny sow-thistle (S. asper). The plants are so much alike that they were originally classed as the same species. The can be told apart by spiny sow-thistle’s thicker, spiny-edged leaves, which are only rarely lobed, and any lobes are more shallow than smooth sow-thistle’s. The latter also has larger capitula so it looks more impressive, although both plants’ flowers are a rather pale yellow. If there is still some confusion the number of involucral bracts can be counted or if the achene is ripe: spiny sow-thistle’s have glossy ridges and smooth sow-thistle’s are ridged with protuberances.
Smooth sow-thistle is clearly more at home in a culturally-influenced environment than spiny sow-thistle and it is not often found among crops as a weed. It varies to a certain degree: in the type variety (var. oleraceus) the lateral lobes are entire and the terminal leaflet is large, while the rarer alien variation (var. lacerus) grows in rubbish dumps and urban environments and has lobes that are long and paired. The terminal leaflet is no larger than the lateral ones.
Smooth sow-thistle arrived in Finland with people. In the Middle Ages the young plants were eaten cooked as a vegetable. The plant’s scientific name means that it is an edible vegetable. In Greek mythology Theseus is said to have eaten smooth sow-thistle to gain power before leaving to slay the Minotaur in its Cretan labyrinth, where it dined on human bodies, bull’s heads and young Atheneans. The plant was also believed to empower domestic animals – it makes great fodder for sheep, pigs and chickens. It also used to be used medicinally.