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Evening Primrose: Shows Nightly At Nine

Yellow evening primroses (oenotherea flava)
Jerry Pierce (CC-BY-NC-2.0)
Yellow evening primroses (oenotherea flava)

In the summer, folks come home from work at their jobs and hang out in their yards - they mow the lawn or weed the garden or have barbecues. I don't cook much, but just before dark I like to have a couple of friends over. We get out the beach chairs, face them toward the garden and get ready for my favorite summer evening entertainment. We watch my evening primroses open. It's positively thrilling. They’re yellow evening primroses – oenotherea flava. During the day, the flower buds are tightly closed, forming little cones, but each evening the buds burst and four beautiful pale yellow petals unfurl in about sixty seconds. It's like watching Disney time-lapse photography. The flowers then stay open all night, but are closed up again before noon the next day. Why are the flowers opening at night and how do they know when it's getting dark?

Plants like my evening primrose are adapted for pollination by night-flying moths. The strong scent and large flowers attract moths that then carry pollen between plants. The flowers close in the day to prevent other insects from taking the nectar and pollen without affecting cross-fertilization. Other plants are open only during the day, presumably because they are adapted to day-flying pollinators. For example, Montana has two species of large flowered blazing star. They are quite similar to each other, but the bright yellow-flowered species is open only during the day, while the white-flowered one opens only at night. These two species are using internal biological clocks to flower at different times of day, thereby attracting different pollinators and avoiding hybridization.

Many plants exhibit various daily cycles. Some open and close their flowers. Others change the position of their leaves, like my potted shamrock, whose leaves fall down at night and open back up in the day. These daily rhythms originate within the plants themselves, but it is daylight that keeps that biological clock accurately synchronized to the constantly changing lengths of day and night. The mechanism responsible for the biological clock is not well understood, but it is known that plants sense the presence or absence of daylight using a light-sensitive pigment called phytochrome. So as summer progresses and darkness comes earlier each day, my primroses know enough to change their rhythms a little bit every day, too. They open their flowers a little earlier each day because each day is a little shorter.

People also have daily biological rhythms that are adjusted by day length. During the summer, I wake up earlier and don't seem to need as much sleep. And it's a good thing, because I have lots to do. I might even go dancing downtown. But only after my primroses have opened.

"Field Notes" is produced by the Montana Natural History Center.

(Broadcast: "Field Notes,"  6/28/01, 6/28/16, 7/7/20 and 7/10/20. Listen weekly on the radio Tuesdays and Fridays at 4:54 p.m., or via podcast.)

Annie Garde is the Children's Program Director and host and producer of the Pea Green Boat and the Children's Corner.
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