Sow thistle (less commonly hare thistle or hare lettuce) is the common name for a number of related annual herbs in the Genus Sonchus, the ancient Greek name for these plants. Sow thistles are common roadside plants, and while native to Eurasia and tropical Africa, they are found almost worldwide in temperate regions. In some regions sow thistles are known as "milk thistles", although a true milk thistle is placed in a different genus.
All are characterized by soft, somewhat irregularly lobed leaves that clasp the stem and, at least initially, form a basal rosette. The stem contains a milky sap. Flower heads are yellow and range in size from half to one inch in diameter; the florets are all of ray type.
Mature sow thistle stems can range from 30 cm to 2 m (1 to 6 feet) tall, depending upon species and growing conditions. Colourations range from green to purple in older plants. Sow thistles exude a milky latex when any part of the plant is cut or damaged, and it is from this fact that the plants obtained the common name, "sow thistle", as they were fed to lactating sows in the belief that milk production would increase.
Widely adaptable to soils, full sun.
Sonchus asper - spiny sow thistle
Sonchus arvensis - field or perennial sow thistle
Sonchus oleraceus - common sow thistle
Sonchus wilmsii - milk thistle
The plant has been used as a stockfeed and particularly as a feed for rabbits, hence the other common names of "hare thistle" or "hare lettuce". The plant is also edible by humans as a leaf vegetable; old leaves and stalks can be bitter, but young leaves have a flavour similar to lettuce. Going by the name puha it is frequently eaten in New Zealand as a vegetable, particularly by the native Māori. When cooked it tastes similar to chard.
In many areas sow thistles are considered a noxious weed, as they grow quickly in a wide range of conditions, and their wind-bourne seeds allow them to spread rapidly. Sonchus arvensis, the perennial sow thistle, is considered the most economically detrimental, as it can crowd commercial crops, is a heavy consumer of nitrogen in soils, and can regrow and sprout additional plants from its creeping roots.
Sow thistles are easily uprooted by hand, and their soft stems present little resistance to slashing or mowing. Most livestock will readily devour sow thistle in preference to grass.
Pests and Diseases
Sow thistles are a common host plant for aphids. Gardeners may consider this a benefit or a curse; aphids may spread from sow thistle to other plants, but alternatively the sow thistle can encourage the growth of beneficial predators such as hoverflies. In this regard sow thistles make excellent sacrificial plants.
- Low, Tim. Wild Herbs of Australia and New Zealand. Rev. ed. Angus and Robertson, 1991. ISBN 0-207-17001-0.
- Washington State Noxious Weed Control Board
- Perennial Sowthistle Fact-Sheet