Sonchus oleraceus aka Common Milk Thistle

Sonchus oleraceus produces small yellow flower heads, occasionally tinged orange or cream, arranged in flat-topped clusters at the tops of the stems. Each head is 6-14 mm across and is built entirely of ray florets — there are no disk florets — with more than 50 florets per head. The flowers are hermaphroditic and bloom mainly from early summer into autumn, attracting bees and flies. After flowering, each floret produces a small achene topped with a feathery white pappus that carries the seed on the wind.

Common sowthistle is one of the easiest plants to grow outdoors — to the point that it grows itself, often unwanted. It thrives in full sun on disturbed, fertile ground such as gardens, fields, roadsides and waste places, and tolerates light sandy, medium loamy and heavy clay soils with mildly acid to mildly alkaline pH. It avoids strongly acid soils and shaded positions. Seed sown in situ in spring germinates readily, and once established the plant will self-seed prolifically. It is not frost-tender in the temperate range it occupies and is naturalised across most of the world's temperate zones, including Europe, North Africa, the Americas, Australia and New Zealand.

Propagation is almost entirely by seed. Seeds are sown in situ in spring; germination is rapid and the plant will self-seed enthusiastically once flowering begins. In the wild, seeds are dispersed primarily by wind on a fine white pappus and secondarily by water. A single plant can produce a very large number of seeds in a season, which is one of the reasons sowthistle becomes weedy so quickly.

The flowers of Sonchus oleraceus are hermaphroditic and chiefly pollinated by insects — bees and flies are the main visitors. The plant is also reported to be self-fertile, which together with insect pollination helps explain its prolific seed set even on disturbed sites with sparse pollinator activity.

Sonchus oleraceus is a long-recognised wild edible. The young leaves can be eaten raw in salads or cooked like spinach and have a mild, agreeable flavour; PFAF rates its edibility at 2/5 while noting that this species has the best-tasting leaves in the genus. Older leaves grow noticeably bitter but blanching or steaming softens the bitterness. Peeled stems can be cooked like asparagus or rhubarb, the milky sap has historically been used as a chewing gum, and young roots are edible cooked but tend to be woody. The leaves are a useful source of vitamin C (about 30-40 mg per 100 g) along with calcium and iron.

Traditional medicinal uses for Sonchus oleraceus broadly parallel those of dandelion. PFAF rates its medicinal value at 2/5. Documented folk uses include emmenagogue and hepatic preparations, infusions taken for delayed menstruation and diarrhoea, leaf poultices applied to inflammatory swellings, latex dabbed on warts, and a febrifuge or tonic root infusion. Various cultures have additionally used it as a cathartic, diuretic, sedative or stomachic, and indigenous North American groups recorded uses ranging from a remedy for opium habit to a teething aid. These are traditional uses; clinical evidence is limited.

Beyond food and folk medicine, Sonchus oleraceus has a few minor utilitarian uses. PFAF rates its other uses at 2/5. The milky stem latex contains about 0.14% rubber — far too little for commercial extraction. The plant is widely used as livestock fodder, particularly for rabbits and pigs, and is occasionally encouraged as a companion plant near onions, tomatoes, corn, cucumbers and squash, where it can act as a sacrificial host that draws aphids away from the main crop and supports populations of hoverflies and other aphid predators.

The most consistent pest issue with sowthistles is aphids — the genus Sonchus is a well-known host for several aphid species. This is sometimes turned to advantage in vegetable gardens, where sowthistle is left in place as a sacrificial trap to draw aphids away from crops and to feed predators such as hoverfly larvae. PFAF lists no known hazards or significant diseases for the species. Sowthistle does, however, function as a reservoir for a number of viruses and fungal pathogens of cultivated lettuce and other Asteraceae crops in agricultural settings, which is part of the reason it is targeted as a weed.