Sundrops and evening primroses (Oenothera spp.) are popular perennial flowers that are easy to grow in most parts of the country. While there are more than 120 species available, only a handful are typically grown as garden plants. The saucer-shaped, bright yellow or pink flowers are a common sight in early summer, blooming for three to four weeks.
From 1998 to 2003 the Chicago Botanic Garden (USDA zone 5) evaluated 19 different species of Oenothera for characteristics such as flowering display, disease and pest resistance, cultural adaptability to climate and soil, and winter hardiness. While 16 of the species survived the trial period, there was a significant difference in flowering performance. Most of the species had only sparse blooms, but the evening primrose Oenothera speciosa 'Siskiyou' (pink flowers) and the sundrops O. fruticosa 'Fyrverkeri' (yellow flowers) had impressive numbers of flowers -- up to 100 percent flower coverage -- over the same period. They also showed healthy growth and winter survival.
For more information on this research, visit the Chicago Botanic Garden Web site.