Prairie Sundrops (Oenothera pilosella)
A member of the Evening Primrose family, it is actually a day-flowering species. It spreads semi-aggressively and forms dense mats, but can be removed easily from a garden setting because of its shallow roots. The bright yellow, fragrant flowers and photo-worthy stamens appear early summer. Prairie Sundrops prefer moist soils and near full sun but can tolerate periods of drought and light shade.