Common Evening Primrose
A robust, much-branched, leafy biennial, growing as a rosette of leaves the first year, and sending up a flowering stalk, then dying, the next. Flowers several to many in terminal racemes, light yellow, 4-petaled, subtended by narrow, recurved bracts. The petals are rounded. Blooms June–October. Leaves alternate, sessile or with very short petioles, lanceolate, light green, with insignificant teeth or without, to 6 inches long.
Similar species: Missouri has about 22 species of Oenothera. This is the most common and widespread of them, and it is very variable in form.