A weed with pale green leaves divided into three heart-shape leaflets invades thin areas of the lawn. The leaves are 1/4 to 3/4 inch wide and are similar to clover. Small, bright-yellow flowers are 1/2 inch long with five petals. Cucumber-shaped, light-green seedpods develop from the fading flowers. Plants may be 4 to 12 inches high. Oxalis, also called yellow woodsorrel or creeping woodsorrel, thrives in dry, open places but may also be a problem in moist, well-fertilized lawns. It often invades lawns that are beginning to thin from insect, disease, or maintenance problems.
Check the soil pH level and follow recommended lawn care practices for a healthy, vigorous lawn. A healthy lawn helps smother the oxalis.