White clover is a broadleaf perennial that used to be included in grass seed mixes. Also called white Dutch clover, it's distinguished by three-lobed leaves with a crescent-shaped white band. The plant spreads by creeping stems and thrives in sparse, undernourished turf with excessive moisture.
Control it by watering well, applying nitrogen fertilizer and avoiding excessive applications of phosphorus. Spot-treat with a selective broadleaf weed killer; a second treatment often is needed.