Oenothera perennis

Also known as Nodding Sundrops. A native wildflower throughout much of Eastern North America, this is an easy-to-grow plant for the sunny border or rock garden. It forms a mound or tuft of narrow green leaves, the branching stems bearing small golden-yellow flowers for weeks on end, which open during the daytime and close at night. Sometimes acts as a self-seeding biennial. Said to have a spreading tendency in warm-summer regions, though the seedlings are easy enough to pull out or transplant to a new site. Tolerates dry or moist sites.