Chris Evans, University of Illinois, Bugwood.org
Lespedeza, also known as Japanese clover, is a summer annual weed that grows low to the ground and produces small, purplish flowers and trifoliate leaves. It has hairs along the leaf margins and prefers to stay close to the ground. This weed thrives in compact, dry soil with low fertility and spreads out and wide as it matures.
It's best to prevent germination by using a pre-emergent herbicide in the spring. The herbicide should be labeled for lespedeza, which also means it will help to prevent the spread of other broadleaf and grassy weeds. If it grows in thin grass, try to raise the mower height to allow for thicker, taller grass to form. This will help to keep Japanese clover from establishing and spreading.