Each year, the simple soap plant puts up a long shoot with many branches. Each branch contains many buds. Deer love to eat the branches down to the ground; if they survive, each bud blossoms for one night and one night only, then curls into a spent flower, and eventually a seed pod.
Cindy says the flowers open at 5 pm on the dot (PST?) -- you are allowed to take a drink if you see a soap plant flower. Rodolfo asks, who pollinates the flowers? Here is an answer. After dusk, many small flying insects congregate on the flowers. If you wait patiently, you can see honeybees and other (bumblebees?) come too.
I was there from about 7:30 pm to 8:20, after the Jasper Ridge docent party. Just at sunset and thereafter. I can't vouch for the visitors earlier in the afternoon, or later in the night and towards morning. By morning, the flowers have curled up, and some of the small flying insects are still hanging around.
In June 2010 (June 11), I also captured some images of insects on soap plant flowers. Also in "Adventures on a Smaller Scale".
Please leave a comment if you have an ID or other info!