Sundrops is the common name of oenothera pratensis. The flowers are bright yellow and grow clustered at the end of the stems. The flower can be found from Maine to Iowa and Arkansas.
GalleriesFlowers and Shrubs: S
L. H. Bailey Standard Cyclopedia of Horticulture (New York, New York: The MacMillan Company, 1917) volume 4, page 2331