Common Evening Primrose of North America - Oenothera biennis

This flower has a strong preference for sun.  It likes full sun exposures, but can tolerate light shade.  It is not terribly picky about soils, although it tends to prefer sandy soils.  It naturally grows in a wide range of habitats, including sunny areas in floodplains, prairies or meadows, and disturbed areas like vacant lots or edges of parking lots.

O. biennis, as its scientific name suggests, is a biennial, meaning that it will not bloom in its first year.  During the first year, expect a stubby plant, usually only growing a basal rosette of leaves.  It won't look like much, but it won't take up much space in your garden either.  Just make sure the plant gets enough sun that it can store up nutrients for next year.

During the second year, this plant will shoot up and get huge!  In poor conditions, it may only get 3 feet tall, but under the right conditions, it can reach heights over 7 feet; I've seen plants get even taller than this when growing in moist floodplains.  Large, healthy plants may branch at the top, producing many more blooms.  Less vigorous plants will usually only produce one spike of flowers.

As the common name suggests, this plant blooms at night, but in shady areas and on cloudy days, the blossoms will stay open during the day as well.  Even when the flowers are closed, they still provide a flash of bright yellow color.

Evening-primroses are easy to identify when in bloom, but before blooming the plants can look quite similar to other plants.  There are numerous other Oenothera species in North America, although they are much less common than this one, in most regions, and this guide will not cover how to tell them apart.

Oenothera biennis looks superficially similar to lobelias, especially Lobelia cardinalis and Lobelia siphilitica, but they rarely grow in the same habitat: O. biennis prefers dry, exposed conditions, and both these lobelias prefer very wet conditions, often sheltered or partly shaded.  In wet and shadier conditions, if they survive at all, evening primroses will grow very tall (looking for more light).

One plant that frequently shares its habitat is Erechtites hieraciifolius, often called "Fireweed", "Burnweed", or "Pilewort".  This plant is an annual, and not only has a similar growth habit and leaf shape, but it also shoots up at a similar rate to second year O. biennis plants.  The picture below shows both plants.

A distinctive feature to look at on the evening primroses is the coloration of the leaf: the leaf midribs usually have a distinctly pinkish hue.  This feature is not common on plants and is usually characteristic of evening primroses.  Also, the teeth on the leaf margin are smaller and more regular than on Erechtites hieraciifolius, which can sometimes have small, regular teeth, but often has larger lobes or irregular teeth.  As the plants get very tall, evening primroses typically have a tougher, more woody stem, whereas Erechtites hieraciifolius has a softer (but often thicker), more vegetative stem.  Also note in the picture below how on the primrose plant, the leaves spiral out in a radial pattern, like a rosette, near the top of the plant, whereas on Erechtites hieraciifolius, they often initially grow upright and then fold down as the plant grows.

Before growing this plant, make sure you have enough sun: this plant needs a lot of sun and will not do well in even part shade.  In very rich soils, it will also often get out-competed by other plants.  It is a good choice of plant when gardening in exposed areas with disturbed soil.

This plant is hard to transplant.  It grows a big taproot, and tends to be strongly rooted.  Even small first-year plants can be surprisingly hard to transplant without disturbing the root.  Its tendency to grow in poor, compacted soils contributes to this problem.  If you want to grow this plant, I recommend growing it from seed.

You can collect the seeds from seed pods which ripen in fall, around the very end of the growing season, although a few pods usually start opening earlier, such as by the end of September.  Scatter the seeds or sow very shallowly--the seeds need ample light and will not germinate if sown deeply.  Keep in mind that the seeds are attractive to birds. To minimize the risk of them being eaten by birds, you can cold-stratify the seeds yourself (either in a freezer or an area outdoors protected from things that might eat them) and then sow the seeds in the spring.

One final note on cultivation: this plant can sometimes become unsightly, depending on the conditions it is grown in.  Drought will cause the lower leaves on the stem to fall off, and competition from other plants in its second year can drive it to get very tall and leggy looking.  For the best appearance of this plant, grow it in an exposed location with ample moisture.

Evening primrose is pollinated primarily by sphinx moths, large moths with a hummingbird-like flight that come out at night.  But during the daylight hours in which these flowers are open, they will often attract ruby-throated hummingbirds and bumblebees.  There is even a bee, Anthedonia compta, which specializes in evening primroses.

The foliage of this plant supports the larvae of a number of moths and beetles.  Among these, the moths Endryas unio, or the Perly Wood-Nymph, Desmia funeralis, and Hyles lineata, the White-lined sphinx (which also is a pollinator), are quite striking.

Goldfinches eat the seeds in the fall.  Do not cut the stems of these plants until spring because you will be removing a valuable food source for birds!  If there are a lot of larger plants, they will support the finches all winter and some seeds will remain on the plants even through spring.

Although this plant is common and widespread, it still is much more beneficial to wildlife and to ecosystems than most of the non-native plants widely used in gardening.  By planting this, you not only will support more wildlife, but you provide a seed source of this plant, making it more likely that it, and not other invasive plants, will come up in weedy and unmaintained areas such as vacant lots, roadsides and railroad tracks, or other disturbed areas.

• Common Evening Primrose | Illinois Wildflowers - This website focuses on Illinois, but has lots of useful general information about the plant.
• Oenothera biennis | USDA Plants Profile - A government website with a lot of information on this plant, including range and distribution maps.
• Oenothera biennis | Wikipedia - An evolving article on this plant.