Oenethera biennis flowers

The bisexual yellow flowers of Oenothera biennis are fairly long-stalked, produced in a tall raceme. A flower has four broad petals, notched at their tips. The nectar guides on the petals are invisible to the human eye but apparent under ultraviolet light. These guides are visible to the pollinators, mostly moths, butterflies, and bees, serving their purpose. Flower diameter is up to 5 cm.

Flowers open briskly at dusk, lasting until around noon on the following day, earning the plant the common name of evening-primrose. Blooming lasts from late spring to late summer.

The fruit is a capsule up to 4 cm long and 6 mm broad, containing numerous seeds, up to 2 mm long (iNaturalist; Wikipedia).