Evening primrose flowers at dusk, fades in the day

By Jeff Mitton

I had hiked from my campsite to the rim of the Picketwire Canyon to watch the canyon fill with shadows and the sky fill with stars. Walking back in the dark, I noticed flowers that had not been open in the afternoon. Evening primroses had opened.

More specifically, these were Colorado Springs evening primroses,Oenotheraharringtonii.

I took a better look at them in the morning, as the sun was rising. I had stumbled on a small number of plants, perhaps a dozen, growing widely spaced in loose sand. The plants were less than a foot tall and each bore one or more flowers, with four petals forming a shallow cup, predominantly white but deep yellow at the center. The female part of the flower is a single column with four flaring lobes or stigmatic surfaces. Eight stamens are held apart from the stigmatic surfaces, and this separation of male and female structures favors outcrossing, or fertilization with pollen from other individuals.

Flowers open at dusk and release a heavy fragrance of gardenia. The flowers are in good shape at dawn, but they wilt quickly in the bright sun.

Colorado Springs evening primroses are sometimes annuals, and sometimes biennials that spend their first year as a rosette. In gardens they can be short-lived perennials, flowering for several years.

While some solitary bees attend the flowers, pollination is primarily by hawk moths. Hawk moths are large moths that appear, at first glance, to be hummingbirds. We see them most commonly at dusk, hovering at flowers and probing them with their long proboscis.

The hawk moth seen most frequently sipping nectar from Colorado Springs evening primroses is the white-lined sphinx,Hyles lineata. The forewings, head and thorax are dark olive with distinct white stripes. The upper side of the hind wings has a broad stripe of salmon, seen as flashes when it is hovering. The abdomen is tan with alternating dots of black and white.

The hawk moths that carry the pollen also eat the plant. White-line sphinx lay eggs on primroses and their larvae eat the buds, flowers and youngest leaves.

I had stumbled on a few primroses, but I could not find any more.

Oenothera is a large genus, with over 120 species, and some are widespread and locally abundant. But Colorado Springs evening primrose is found only from Picketwire Canyon, along the Arkansas River, to about Canon City, and it exists in a few sparse populations, widely separated.

This primrose is a good colonizer of disturbed sites, but it will only tolerate a moderate amount of disturbance. If a site begins to fill in, the primrose succumbs to competition. Increasing levels of disturbance can also push the population to extinction. The Colorado Natural Heritage Program lists it as an imperiled species.

Are populations of evening primrose isolated from one another, unable to move pollen among populations? Perhaps not.

Hawk moths are strong fliers and have been clocked at 30 mph. A field study of hawk moth pollination used fluorescent particles on flowers to trace the distances that hawk moths carried pollen. Some hawk moths moved pollen 20 to 30 miles, far greater than distances measured for honeybees or solitary bees.

So it is possible that the tiny populations of primroses are sharing genes, even if they are over the horizon from the next population.

Jeff Mitton(mitton@colorado.edu) is chairof the Department of Ecology and Evolutionary Biology at theUniversity of Colorado. This column originally appeared in the Boulder Camera.

March 22, 2010