Plant Story--Evening Primroses, Names and Relations

Evening primroses were one of the first midwestern wildflowers I learned. Partly because they are so spectacular

Missouri eveing primrose Oenothera missouriensis in the grassland

I quickly figured out they were not roses or primroses (more on that below). The evening primroses are some 145 species native to the Americas, in the genus Oenothera, in the evening primrose family, Onagraceae. The U.S.D.A. lists 80 species of Oenothera native to North America. With beautiful flowers!

Apparently this group evolved and diversified relatively recently, spreading all over the Americas. Their classification--scientific names and relationships--has been unstable as DNA studies reveal that many evening primrose species look more like their cousins than their siblings: plants classified by flower and leaf features as A and B together and C and D together often are actually related A with C and B with D. Thus, lots of western plants called suncups, previously in the genus Oenothera, have been moved to the genus Camissonia, making it much bigger. At the same time, all the Gaura species, called bee blossoms or butterfly plants, are now species of Oenothera (see my blog on "scarlet guara" link). Since there are easily 144 species of Oenothera and Camissonia just in North America and only a few of them have been DNA-tested, there will probably be additional surprises. (Current status see Onagraceae website).

The family of evening primroses, the Onagraceae, is scattered around the world. Willow herb and fireweed (Epilobium and Chamaenerion) and the clarkias (genus Clarkia) from California are other members of the family.

evening primroses

The plant family was named after evening primroses...and then it got weird. Philip Miller, 1691-1771, working in England, gave a couple of evening primroses the scientific name Onagra. Onagra is based on the Latin onager, a wild ass, referring to some, now forgotten, association of the plant with wild donkeys. But about the same time Linnaeus named other evening primroses Oenothera. Nobody knows why Linneaus chose oenothera either. Botanical linguists break it into oinos, wine, and theras seeker and speculate that was because the roots were used to flavor wine. Or they find in it onos theras, donkey catcher. At the time, the middle 1700s in Europe, the common evening primrose was widely grown as a vegetable, so my guess is that both Miller and Linnaeus were referring to donkeys eating the plants. Evening primroses were not known to Pliny (Rome, first century) so whatever Pliny called onos thera in his Natural History was some other plant. Linneaus often recycled plant names from old Rome as the scientific names for totally different, newly discovered plants from the Americas.

When botanists realized that both Onagra and Oenothera were plants of the same genus and wanted to put them into a single genus, they had to choose one name or the other. Oenothera won. Either it was older or more properly documented or both. All the Onagra species went into Oenothera: there are no longer any plants in the genus Onagra. By then, though, the scientific community was used to calling the evening primrose family the Onagraceae. The rule is that one of the genera (= plural of genus) in the family adds -ceae to its name to make the family name. With no Onagra species left, they might have made it Oenotheraceae--quite a mouthful--but agreed to keep the name Onagraceae. (That's what nom. cons. (nomen conservandum) means when you see it by a plant family, that, after discussion, this older name was kept.) So the family of evening primroses is named after a nonexistant plant genus.

evening primrose, likely common evening primrose
 Oenothera biennis

The iconic evening primrose is the common evening primrose, Oenothera biennis. This is a famous plant but it does not occur in Colorado where I live. Look at the USDA map link, Colorado is one of the few states where it is not found. Common evening primrose is a roadside weed across its range, but not a problem weed. It grows easily from seed and has been is widely used in ecological experiments testing important principles of ecology and evolution. In Europe, it has been grown as a garden flower and a vegetable for easily 400 years. (See Eat-the-Weeds, Plants for a Future.)

evening primrose flower

Recognizing most evening primroses is pretty easy. The flowers have four, usually rather large, petals at right angles to each other, stigmas and stamens that stick out (exerted), and the stigma, in the center, has a cross-shaped tip. In the picture above, the stigma is starting to wilt, but you can see how distinctive it was.

With a few exceptions the evening primroses are white or yellow. Many open their flowers in the evening to bloom during the night, pollinated by moths. Often in the morning they are attractive to bees and butterflies as well. My photos run to the yellow ones because they are more likely to be open when I arrive with my camera than the white, truly night-blooming ones.

As I said above, evening primroses are neither roses nor primroses and not closely related to either roses or primroses. Roses are the genus Rosa, in the rose family Rosaceae, with species native all over the world. Primroses are in the genus Primula, in the primrose family, Primulaceae, with species native from the Alps to mountainous Yunnan, China to the Rocky Mountains in North America. But one species, the common primrose, Primula vulgaris, was a showy early spring wildflower in Northern Europe. The common primrose is yellow. The evening primroses were discovered after colonization of North America and likely the first one seen was the common evening primrose.

So I imagine the names went like this:

Old England: "Those be roses."

roses, Rosa

Old England:  "That yellow wildflower resembleth a yellow rose, and 'tis the first flower of ye spring, 'tis ye prim'rose."

Prime meant first. The genus name reflects that Primula, "first."

Colonist: "Mercy, that wildflower looks like a primrose. But, how queer, the flowers only open at evening."

Evening primrose, Oenothera

So they called it evening primrose. (Genus Oenothera, as described above.)

Now all three grow all over the English-speaking world, confusing people with their similar names.

another pretty evening primrose

Names are critical to finding plants on the internet so this blog post has the practical function of helping navigate confusing names, as well as letting me indulge in obscure botanical history and share pretty flower pictures. I'll write about evening primroses' natural history, strange genetic system, uses and folklore in the future.


Comments and corrections welcome.

For more information, you might like my blog posts on:
Gaura coccinea now Oenothera suffructenscens  link
Primroses (Primula)  Reasons to like  More reasons to like  primrose color link
Plant families link

Kathy Keeler, A Wandering Botanist

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