A very aggressive perennial weed, yellow oxalis (also called yellow wood sorrel) thrives throughout most of North America. Happy in sun or shade, it spreads quickly by seed. Seedlings start out from a single taproot, which soon develops into a shallow, spreading, knitted root system. Tiny, five-petaled yellow flowers are followed by elongated seed capsules that can shoot seed as far as 6 feet.
Dig out small plants before they set seed. Keep lawns vigorous to provide tough competition; water deeply but infrequently, since frequent shallow watering encourages this shallow-rooted weed.
For chemical control, use a pre-emergence herbicide containing oryzalin or pendimethalin to prevent seeds from germinating and becoming established. Spot-treat oxalis in garden areas with glyphosate, taking care to avoid contact with desirable plants.