- Name also: Corn Sow Thistle, Dindle, Field Sow Thistle, Field Sowthistle, Gutweed, Swine Thistle, Tree Sow Thistle, Perennial Sow-thistle
- Family: Daisy Family – Compositae, subfamily Cichorioideae (formerly Chicory Family – Cichoriaceae)
- Growing form: Perennial herb. Root system broad, horizontal, yellowish brown.
- Height: 40–150 cm (15–60 in.). Stem usually unbranched, with glandular hairs, hollow. Containing abundant latex.
- Flower: Single flower-like capitula 4–5 cm (1.6–2 in.) broad, surrounded by involucral bracts. Capitulum flowers golden yellow, tongue-like, tip 5-toothed. Stamens 5. Gynoecium composed of 2 fused carpels. Involucral bracts with glandular hairs. Capitula borne in a corymbose cluster.
- Leaves: Alternate, basal leaves stalked, stalk channelled, winged, stem leaves stalkless, amplexicaul, roundish basal lobes surrounding stem. Blade usually pinnate, lobes turning towards base, sometimes more-or-less lobeless (var. maritimus), glabrous, shiny bluish green, sometimes fleshy (var. maritimus), with irregularly toothed margins, finely spined.
- Fruit: Flat, bristly, ridged with protuberances, dark brown or white–yellowish (var. maritimus), 2.5–3 mm (0.1–0.12 in.) long achene, crowned by unbranched hairs.
- Habitat: Fields, grass leys, gardens, yards, soil heaps, wasteland, roadsides, boat shores, rocky sea-shores, waterside meadows, seaweed piles, islets.
- Flowering time: July–August.
Field milk-thistle is a troublesome weed whose vigorous presence in fields is detrimental to cultivated plants. The plant’s deep yellow capitula attract flower flies, bumblebees and small beetles – successful pollination can result in the production of up to 6,000 flying-haired achenes, which help it spread with ease. The achenes do not just travel on the wind but also in combine harvesters’ threshing bins, with grain and hay-seed, and they remain capable of germinating for a long time. Field milk-thistle first grows a leaf rosette and a broadly horizontal rootstock, whose adventitious buds develop new shoots. Stands are often very wide and difficult to completely eradicate. The copious amount of latex that they produce is familiar to anyone who has ever had the plant attach to them.
Field milk-thistle has been growing in southern and central Finland since ancient times: its colonization of the north began later and is still in progress. A lack of crop rotation has probably played a part in the plant’s success. Apart from moving inland, the species has spread with people to coastal areas all around the world. Like many other weeds, field milk-thistle is probably native to the sea-shore. In Finland too it is native to seaweed piles and is lower-growing, bluer, and more glabrous with entire and fleshy leaves compared to its close relatives. It also has sparse capitula and pale acheness (var. maritimus). In some places it has spread to culturally-influenced environments, but it shows no tendency of becoming a weed, even in populated coastal areas. Some of the field milk-thistle that grow as weeds, however, are probably hybrids of different subspecies.