Yellow Wood Sorrel (Sheep Sorrel)
A herbaceous annual or perennial with taproots when young, developing rhizomes with age. Flowers in unevenly branched panicles on long stems, with 5 yellow, rounded petals. Blooms May–October. Leaves alternate, trifoliate (like clover), the leaflets heart-shaped, light to dark green or copper to purple, often recurved, sometimes with grayish hairs. At the end of each day, the leaflets droop or fold downward, parallel to the stem; they spread again the next morning. Fruit an upright, pointed capsule to about 1 inch long.
Similar species: There are 5 species of Oxalis in Missouri. One is violet wood sorrel, with pink or violet flowers. The other 4 all have yellow flowers. Of these, O. stricta is the tallest, most common, and the only one with flowers in panicles (a central flower flanked by a pair of branches bearing 2 or more flowers). The rest have the flower stalks umbellate (arising from the same point at the tip of a stem).