Sheep-sorrel (Rumex acetosella) , a close relative of the Docks (Rumex spp.) is a common perennial forb in the Knotweed Family (Polygonaceae). As it is a cosmoplitan plant, be it vegetable or weed, its exact nativity is uncertain, but it is usually considered native to Eurasia, northern Africa, Greenland, Iceland, the British Isles, Taiwan, Japan, and Sakhalin.
It's tender new growths have long been used as a flavourful salad green, and in fact, there are cultivated forms of this plant. The sour taste derives from oxalates/oxalic acid in the leaves. While nutritious and rich in vitamins, it is still often recommended that plants with high levels of oxalates be eaten in moderation, as oxalates do bind to calcium and been known to cause kidney stones. Other vegetables that contian significant oxalates include Spinach (Spinacia oleracea cvs.), Chard & Beet greens (Beta vulgaris cvs.), Rhubarb (Rheum x hybridum), and of course the Wood-sorrels (Oxalis spp.).