Common Evening Primrose of North America - Oenothera biennis

by cazort

The common evening primrose of North America, Oenothera biennis, is an easy plant to grow in a sunny garden.

There are many plants in the Evening primrose genus, bearing the name "Evening primrose". In Eastern North America, the most common of these plants is Oenothera biennis, native to the region.

Oenothera biennis is an attractive plant that is exceptionally easy to grow, provided that you have enough sun in your garden. It has an upright growth habit, neat green foliage that turns a striking red color as it dies, and showy yellow flowers.

Here you will learn how to grow this plant, as well as its benefits to the local ecosystem.

Growing Requirements & Life Cycle

A biennial plant requiring abundant sunlight, but tolerating poor soils.

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.

Basal rosette of first-year plant
Basal rosette of first-year plant
Photo by Alex Zorach
Tall plants growing in dense vegetation
Tall plants growing in dense vegetation
Photo by Alex Zorach
Closeup of Blooms
Closeup of Blooms
Photo by Alex Zorach
Closeup of Blooms
Closeup of Blooms
Photo by Alex Zorach

Identification

Comparison to similar looking or easily-confused plants

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.

The central plant here is Oenothera biennis. Top-left is a similar-looking and easily-confused plant, Erechtites hieraciifolius.
A plant that has not bloomed yet
A plant that has not bloomed yet
Photo by Alex Zorach

Propagation and Cultivation

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.

This photograph shows what this plant looks like in dry conditions--it often drops its lower leaves.
Evening Primrose Growing in a Crack in the Pavement
Evening Primrose Growing in a Crack in the Pavement
Photo by Alex Zorach

Ecological Relationships: Supports Lots of Wildlife

Attracts sphinx moths, hummingbirds, bumblebees, supports a variety of other moths, beetles, and goldfinches.

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.

References and Further Reading

Learn more about this plant from these websites.

More Native Wildflowers of Eastern North America

Shade tolerant asters, which grow, thrive, and bloom in part shade to full shade, in Eastern North America.
Lobelias of Eastern North America, including Cardinal Flower, Great Blue Lobelia, the annual Indian Tobacco, and many others.
Clasping Venus' looking glass, or Triodanis perfoliata is an annual flower native to Eastern North America with small, distinctive leaves, and a cute purple blossom.
Updated: 07/01/2016, cazort
 
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blackspanielgallery on 06/12/2015

When a plant supports wildlife it gets my vote.

AngelaJohnson on 10/10/2014

I haven't seen the yellow evening primrose, but I've seen plenty of pink primroses. They grow in early spring here in south Texas, but I've seen them in Illinois and Tennessee, too.

cazort on 10/03/2014

Yeah! I haven't even started adding anything about the medicinal properties of this plant...I have heard that evening primrose has traditional medicinal uses (and food uses) in native American culture, and I know it is used in modern complementary medicine too...but I haven't researched it enough yet.

Perhaps this is something I can add in the future!

Mira on 10/01/2014

I was looking to learn more about this plant. Evening primrose oil has many therapeutic uses. I enjoyed your article and now remember seeing this plant in these parts as well. It was nice to learn more about it.

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