SOW-THISTLE, a genus (Sonchus) of herbs of the order Compositæ. The species, of which there are about 30, are natives of the Old World, but some have become widely naturalized in temperate climates. They have alternate leaves, blue or yellow flowers in heads and beaked seeds crowned by fine white down. The common sow-thistle (S. oleraceus) is an annual which grows about 30 inches tall, has yellow flowers, and is found usually in rich soils. Its young leaves and tender tops are used as a substitute for spinach in northern Europe. The alpine sow-thistle (S. or Lactuca, alpina) is a native of mountainous Europe. It has beautiful blue flowerheads. The beach sow-thistle (S. maritimus) grows in saline soils in southern Europe, and is specially attractive because of its yellow flower-heads. These species are sometimes cultivated in gardens.