Oenothera sp. (Sundrops and evening primrose)
Oenothera biennis  or Evening primrose

Zone: 3 to 9

Soil: Sand to loam

Light: Full sun to part sun

Bloom colour: Yellow

Bloom period: late spring to early summer

Height: 2 to 6 feet

Moisture: Dry to medium

Attracts: Bees and sphinx moths.

Notes: It is hard to generalise about the members of this genus because there are so many species. Typically, the species seen in Ontario do well on freely drained soils. The moisture requirements vary considerably by species. Sundrops are great plants for a garden as they often have a carpet of flowers that cover relatively short stems. This makes these plants highly desirable in an ornamental garden.

Unfortunately, common evening primrose, which is the species in nature most often seen, is not that great for a suburban garden. Only a few flowers are open at a time and the plant, growing to over 6 feet, is rather lanky looking. Being a true evening primrose, the flowers are fully open in the evening when we are not actually looking at our flowers. It is reported that these plants are pollinated by sphinx moths although I have not been able to verify this personally.

Better plants for a garden include Oenothera pilosella and Oenothera fruticosa. These plants are particularly showy and I use them as accent plants. As they are only in flower for a few weeks, they are not the mainstay of the garden. The former plant is also called prairie sundrops and grows well in dry sandy as well as moist soil.

Oenothera biennis is a true evening primrose in that its flowers do not open until nighttime. This suggests that the pollinators of this plant are either crepuscular or nocturnal.
Oenothera pilosella
Sundrops are often separated from evening primroses by their tendency to open their flowers during the day; however, this is not a hard and fast rule. This is prairie sundrops (Oenothera pilosella), which is native to Ontario.
Sundrop flowers
The flowers of these plants are easy to identify. They have four petals and the stigma has four lobes.
Oenothera with Verbena simplex
Plant Verbena simplex in front of Oenothera pilosella
Lassioglossum oenotherae
This is Lassioglossum oenotherae, which is oligolectic on Oenothera sp. A small minority of bees specialise in collecting pollen from a particular genus of plants.
The flowers of sundrops
The flowers of sundrops are absolutely delightful.