Yellow woodsorrel (oxalis)

Yellow woodsorrel (Oxalis stricta), commonly called oxalis, is a low to upright broadleaf weed easily confused with clover at a glance because of its trifoliate leaves divided into three leaflets, but each leaflet is distinctly heart-shaped (notched at the tip) rather than rounded, and the leaves often fold along their midribs and close at night. The clearest distinguishing feature is its small, five-petaled yellow flowers, which separate it from white-flowered clover. The plant has an upright to sprawling habit, with slightly hairy stems that branch at or near the base, and it produces erect, narrow, cylindrical seed capsules that, when ripe, burst and forcibly eject seeds several feet. Taproots and rooting at the nodes of creeping stems help it establish and spread.

Yellow woodsorrel degrades turf by establishing in thin or open areas and forming patches of clover-like foliage topped with small yellow flowers and upright seed capsules that interrupt the uniform appearance of the lawn. Its upright, branching growth and contrasting texture and color make it conspicuous, and because each plant produces abundant, explosively dispersed seed, infestations spread quickly and recur, steadily increasing the weedy fraction of the stand. While it does not form dense smothering mats, persistent populations reduce turf uniformity and density and reseed themselves season after season.

Yellow woodsorrel may behave as an annual or a perennial depending on conditions and biotype, reproducing prolifically by seed and also vegetatively through rhizomes and stems that root at the nodes. Plants develop a taproot and can produce thousands of seeds each, and the explosive seed capsules can fling seed as far as a dozen or more feet, so a single plant rapidly seeds a wide area and builds a persistent seed bank. The combination of heavy seed production, ballistic dispersal, and the ability of perennial biotypes to regenerate from rhizomes makes it a recurring, difficult-to-eradicate weed.

Yellow woodsorrel is a widespread, adaptable weed of lawns, landscape beds, nurseries, and disturbed sites, and it invades thin, open, or stressed turf where competition is reduced, as well as moist, fertile areas. It tolerates a range of light and moisture conditions, growing in both sun and partial shade, and its prolific, far-flung seed lets it colonize new openings quickly. Thin turf, recently disturbed soil, and bed edges adjacent to lawns are common entry points, from which the explosively dispersed seed spreads it across the surrounding area.