PlantFiles: Smooth Sow Thistle
Sonchus oleraceus

Smooth sow thistle, milk thistle, swine thistle (Sonchus oleraceus) is an introduced plant from Europe that can be found growing natively in Texas and numerous other states. It can grow in just about any type of soil; however, it prefers nutrient rich soils. Smooth sow thistle has a considerable variation in leaf form and flower color.

This annual/biennial sow thistle forms a winter rosette, grows from a long taproot and is from 8 inches to 5 feet high. The stout stems are hollow, usually 5-angled, hairless and exude latex when broken. The spined leaves are grayish rather than dark glossy green like prickly sow thistle. However, both species can have some plants that fall in the intermediate range in this regard. In the shade, the leaves may have a have purple blotching The upper leaves are not divided and have a broad-based triangular shape; whereas, the lower leaves are divided to the midrib into lobes with the end lobe being the largest. The lobes clasping the stem have arrow-shaped, pointed tips. Sonchus asper are ear- shaped.

Smooth sow thistle flowers usually from June to August. It can flower in April and continue until the first frost. The pale yellow flowerhead is about 1 inch in diameter when fully open and the flowers appear in loose clusters. Dry, stored seed remains viable for around 10 years.

The smooth sow thistle is used in Greece as a winter salad. The leaves also can be boiled like spinach, mixed with other pot-herbs or added to soups.