Do Primroses Only Bloom During the Daylight Hours?

Primula Spp.

• Common primroses are valued in part for their cold-hardiness. Not only do they bloom during the day, but they also may even flower throughout the winter in warmer hardiness zones. In general, common primroses grow best in U.S. Department of Agriculture plant hardiness zones 3 through 8, but this can vary according to species. Hot summers are too much for these plants and will wilt the blooms and leaves. Home gardeners in areas outside of USDA zones 3 through 8 may grow them as annuals.

Oenothera Spp.

• Evening primroses (Oenothera spp.) are not in the Primula family, but they get their common name from the fact that their pink, white or yellow flowers are shaped much like the blooms of common primroses. Unlike Primulas, however, these plants bloom at night for the most part -- although they will remain open into the morning hours and all day if the sky is overcast. Hardy in USDA zones 4 through 9, this perennial has a pleasant fragrance, which makes it highly desirable to many home gardeners.

Primula Culture

• Light and heat are important blooming triggers -- and they work in opposite ways on the flowering of evening and common primroses -- but they aren't the only factors affecting blooming. Common primroses grow best in light or even full shade in the hottest climates. Home gardeners in northern USDA zones can grow common primroses in sunlight as long as the soil is kept moist, according to the University of Vermont. In fact, primroses grow best in continually moist and even wet soil, depending on the species.

Oenothera

• Evening primroses are not quite as picky when it comes to culture as common primroses. In fact, not only will these plants thrive in poor soils, but if the soil is kept continually moist and fertile, the plants may act as weeds and become very invasive, according to Floridata. For that reason, it's best to grow them in locations where they won't overwhelm nearby plants. Evening primroses enjoy high humidity and full sunlight, although they will keep their flowers tightly closed in the sun.