Oxalis corniculata is a perennial herb, to 20 cm tall (usually less), from a taproot, plant usually purple or purple-tinged, stems in tufts, colonial from creeping stems, horizontal stolons, rooting at nodes, branching; hairs not septate (use lens).
Leaves alternate, leaf stalk with swollen joint near base, (stipulate; use lens); blade 3-parted, leaflets 1-2 cm wide, heart-shaped, notched at tip, midrib conspicuous, bright green to dark purple green, somewhat succulent, sour to taste; leaflets folded downward at midday and during night.
Flowers yellow, radially symmetrical, 5-parted, petals 0.4-1 cm long, sepals 5; stamens 10, in two cycles, the outer 5 shorter than the inner, stalks (filaments) of each cycle all fused; ovary superior, 5-parted, each part long, narrow with a separate style and stigma, difficult to distinguish from the inner ring of anthers; inflorescence more-or-less umbrella-shaped, but usually with only two rays; self-pollinating.
Fruit stalks strongly bent downward, with capsule held erect (deflexed), capsule narrow, 5-parted, ribbed, 0.4-2.8 cm long, cupped by old sepals, tipped with old stigmas; fruit stalk often bent sharply downward; dispersal occurs by sudden rupture of the fruit that ejects seeds forcibly from slits in the capsules (Lovett-Doust et al. 1985); seeds red-brown, sticky, numerous, about 0.1 cm long, ridged; eaten by birds and probably by small rodents (Martin et al. 1951).
Wetland status: FACU.
Frequency in NYC: Very infrequent in natural areas.
Origin: Probably originally from tropical Asia, but found widely in the tropics and S. U.S.A. (Lovett-Doust et al. 1985).
Habitat: Common garden and greenhouse weed.
Notes: Wood-sorrel is attacked by the maize rust fungus Puccinia sorghi and other species of Puccinia (Lovett-Doust et al. 1985). Edible raw in small amounts. Sour taste is from oxalic acid which binds to calcium. Can cause a serious drop in blood calcium level. May have some medicinal uses (Fern 2004).