Oxalis is often called yellow wood sorrel or sour grass because of the tart taste of the leaves. Oxalis stricta has bright yellow flowers and green leaves. It grows upright. O. europaea (also called O. corniculata) is a perennial with seeds and rootstocks so that it sometimes appears to be a creeping vine. The leaves and stems are often purple or reddish. The seed capsule goes under tension as it dries and then explodes to throw the seed several feet. A serious problem in container nurseries and in newly seeded turf.