- Name also: Horse Thistle, Milk Thistle, Wild Lettuce
- Growing form: Usually biennial (sometimes annual) herb.
- Family: Daisy Family – Compositae, subfamily – Cichorioideae (formerly Chicory Family – Cichoriaceae)
- Height: 40–80 cm (16–32 in.). Stem lower part short-spiny, to a certain extent glabrous.
- Flower: Single flower-like 1–1.5 cm (0.4–0.6 in.) capitula surrounded by involucral bracts Capitula flowers pale yellow, tongue-like, tip 5-toothed. Stamens 5. Gynoecium composed of 2 fused carpels. Capitula in a dense, cyme-like group.
- Leaves: Alternate, apparently vertically side-by-side in two rows, stalkless. Blade long, with sagittate base, pinnately lobed, with unevenly toothed margins, rigid, bluish green, midrib on underside spiny, green.
- Fruit: Wide-edged, greyish green, 6–8 mm (0.24–0.32 in.) long achene, crowned by a pappus of grayish-white unbranched hairs.
- Habitat: Harbours, ballast soil deposits, industrial areas, waste ground, heaps of earth, road cuttings, roadsides, yards, flower beds, new lawns.
- Flowering time: July–August.
- Harmfulness: Potentially or locally harmful alien species.
Genus Lactuca includes about a hundred plants, most of which are of the northern hemisphere. Prickly lettuce takes its name from its thin spines which run along the central vein and margins of the underside of the leaves. In Finland it is commonly referred to as “compass plant” because the leaves align themselves in a north-south direction while the surfaces run east-west. This special leaf position protects the blades from the strong midday sun so that they do not heat up so much and thus reduces evaporation. The leaves of prickly lettuce that grows in the shade do not align themselves like that. The species grows commonly on stony ground and in dry parts of southern Europe, where avoiding the direct glare of the sun is an obvious advantage. In Finland it perseveres as an established alien only in south-westernmost Finland around inhabited areas, but it appears to be becoming more common.
Prickly lettuce is thought to represent an older form of its close relative garden lettuce because they cross-breed easily. On the other hand both could have been descended from the same unknown original hybrid stand. Lettuce is certainly familiar to everyone as a vegetable, but not so many people are aware that it is related to dandelions and many other very common Finnish members of the Chicory subfamily. The species is originally native to the Near East where its original weed-like form was probably developed into animal feed and perhaps also for its oil-rich achenes. The lettuce that we all know today has been achieved by breeding the least spiny plants which also have the least amount of bitter-tasting latex. The end result has been a completely spine-free and mild-tasting cultivar.
Lettuce has been widely used at least 4,500 years ago in Egypt, from where it spread with the Romans to the whole of Europe. Nowadays most of the lettuce that ends up in shops is grown by large farms, but some is also still grown in Finnish gardens. Some lettuce varieties are able to flower in the Finnish summer and can spread to the wild and grow around e.g. rubbish dumps and harbours, in yards and from the outskirts of planting sites as escaped weeds.