Oenothera is a genus of New World herbs, mostly in the U.S.A.
Leaves alternate, usually narrow, simple.
Flowers yellow (ours), mostly open only one night, 4-parted, radially symmetrical, sepals, petals and stamens arising from top rim of calyx tube (hypanthium); petals 4, free; sepals fused in bud, splitting as flower opens and becoming curled back (reflexed); stamens 8; ovary inferior, 4-chambered; crowned by a slender, tubular hypanthium that connects the ovary with the upper floral parts, hypanthium deciduous with floral parts; style elongate, surrounded by hypanthium from ovary to flower, stigma 4-lobed; flowers mostly solitary in leaf axils or in a terminal raceme or spike.
Fruit becoming dry, an angled capsule, splitting open to release numerous seeds (Radford et al. 1968; Hall et al. 1988).