Evening Primrose (Oenothera elata ssp. hirsutissima)
Aliases: Hooker's Evening Primrose, Hairy Evening Primrose
Family: Evening Primrose (Onagraceae)
This is a big, bold flower, with blooms three or four inches across on a plant up to six feet tall. Most people miss it, though, because it spends its days with its flowers closed, sleeping off nights spent carousing with hawk moths (particularly Hyles lineata, the White-lined Sphinx), its favored pollinators. It's common in Yosemite Valley and spectacular enough to be worth hunting for around dusk, when the flowers open up for the evening. I've seen it along the fringes of the Yosemite Village day use parking lot and along Northside Drive approaching El Capitan.
Blooms: June - September
Lifespan: Perennial
Origins: Native (see distribution maps for California and US/Canada)
Oenothera elata etymology: Oenothera is a Greek term meaning "wine-scented," derived from the root word oinos (wine). Elata derives from the Latin word elatus, meaning high, elevated, or haughty, and presumably refers to the plant's height, since it doesn't exhibit obvious signs of hubris.
This Photo: In Yosemite Valley along Northside Drive near El Capitan, mid June
Other Resources: CalFlora · CalPhotos · USDA
Watching the Flowers Open
One of the more remarkable things about Evening Primrose is how quickly the flowers unfold. If the sepals are all in agreement about opening time, a flower can unveil itself in just a few seconds, and even the slow ones usually take less than a minute. Once open, a flower lives for a single night and dies the next day. The video below shows Evening Primrose blooms opening in real time on a July evening a few miles from the Glacier Point Road's western terminus.