Yellow oxalis (Oxalis corniculata)
A very aggressive perennial weed, yellow oxalis (also called yellow wood sorrel) is happy in sun or shade, and spreads quickly
Seedlings start out from a single taproot, which soon develops into a shallow, spreading, knitted root system. Tiny yellow flowers are followed by elongated capsules that can shoot seeds as far as 6 feet.
Dig out small plants early. If you have a lawn, keep it vigorous to provide competition; water deeply but infrequently, since frequent light watering encourages this shallow-rooted weed.
You can also use a preemergence herbicide on turf and around ornamentals listed on the label. Spot-treat oxalis in garden areas with glyphosate.