Any of several varieties of a hardy perennial herb belonging to the buckwheat family, all with some degree of acidity and sourness resulting from the presence of oxalic acid. Sorrel has grown wild for centuries throughout Asia, Europe and North America. The most strongly flavored of the sorrels is the garden or belleville sorrel, also called sour dock and sour grass. The mildest variety is dock sorrel, also called spinach dock and herb patience dock. As all sorrel matures it becomes more acidic. Sorrel leaves are shaped much like those of spinach and range from pale to dark green in color and from 2 to 12 inches in length. Fresh sorrel is available in limited supply year-round with a peak season in the spring. It should be chosen for its bright green, crisp leaves. Sorrel with woody-looking stems or leaves that are yellow or wilted should be avoided. Refrigerate fresh sorrel in a plastic bag for up to 3 days. Gourmet food stores sometimes carry cooked sorrel in jars and cans. The more acidic sorrels are used to flavor cream soups, pureed as accompaniments for meats and vegetables or used in omelets and breads. In the spring, when at its youngest and mildest, sorrel is used in salads or cooked as a vegetable. It's high in vitamin A and contains some calcium, phosphorus, potassium, magnesium and vitamin C.