- Name also: Spiny Milk Thistle, Spiny Milkthistle, Spiny Sow Thistle, Spiny Sow-thistle, Spiny Sowthistle, Spiny-leaved Sow Thistle, Sharp-fringed Sow Thistle, Prickly Sow-thistle
- Family: Daisy Family – Compositae, subfamily Cichorioideae (formerly Chicory Family – Cichoriaceae)
- Growing form: Annual herb.
- Height: 30–80 cm (12–32 in.). Stem branched, glabrous, hollow. Containing abundant latex.
- Flower: Single flower-like capitula 1.5–2 cm (0.6–0.8 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 (35–45) overlapping in 3 rows, green. Capitula borne in a corymbose cluster.
- Leaves: Alternate, basal leaves stalked, stalks channelled, winged, teeth tipped, stem leaves stalkless, amplexicaul, roundish basal lobes surrounding stem. Blade usually ovate, basal lobes rounded with cordate base, glabrous, rigid, hard, shiny dark green, toothed margin with spiny bristle, sometimes lobed.
- Fruit: Flat, winged, glossy-ridged, brown, 2–3 mm (0.08–0.12 in.) long achene, crowned by unbranched hairs.
- Habitat: Gardens, yards, soil heaps, arable land (often also in fields), around inhabited areas, wasteland, harbours.
- Flowering time: July–September.
Spiny milk-thistle is a rather demanding plant that likes fertile places. It is most often found behind cow-sheds, in compost heaps, gardens and populated areas. In can also grow as a weed among root crops, and often it grows alone and does not harm its vegetable companions. Prickly sow-thistle has grown so long as a weed that its original habitat is difficult to define. It has arrived in Finland with people and doesn’t thrive in the wild outside culturally-influenced environments. Sometimes it can be found by the coast in among decomposed seaweed, but in that case it will still be close to an inhabited area. In Finland it grows mainly in the south-west of the country, although it can travel with traffic as far as Lapland and grow casually there. Apart from the species’ prevailing entire-leaved name variety (var. asper), the unevenly large-toothed and even lobe-leaved var. pungens can be found especially in urban areas.
Spiny milk-thistle’s close relative smooth sow-thistle (S. oleraceus) has similar demands with regards to habitat, so the two species can grow close to and even right next to each other. Smooth sow-thistle avoids fields and is thus the slightly rarer of the two. Prickly sow-thistle differs from the rest of its genus with its spine-tipped leaves: smooth sow-thistle’s leaves have no bristle at the tip, or a very small one at most. Although it is called a thistle, its spines are clearly less aggressive than true thistles (Cirsium), and in the event of any lingering confusion a look at the capitula will clarify matters: sow-thistles belong to the Chicory subfamily plants and their capitula are composed of tongue-like flowers, while true thistles are members of the Daisy family and have tubular flowers. Neither do true thistles contain any latex, while sow-thistles contain a lot, as can be verified by breaking the stem.