And I quote from an on-line source:

"...a novelty plant that many people find so cool i.e. watch these pale yellow blooms pop open before your very eyes!....".

Then my neighbor came over and asked specifically if I had seen any evening primroses in that particular bed. She explained that the flowers popped open just as the sun was going down, and stayed open all night.

Turns out that she had remembered many years earlier before I lived there, that that particular bed was full of them, and the one she had, and all the others that she had given to members of her family, had all come from my garden.

They still come up, here and there, and they are completely fascinating to watch. It takes about 5 seconds for them to open, so you have to kind of sit there staring at the bud around sunset!

(Neighbor did order some from a nursery and was very disappointed that they sent her the wrong type of Oenothera, even though she had specified the triloba).

There are some youtube videos of them opening. Endlessly pleasing and kids love them. I usually move one from the garden each year and put it in a pot on the patio so I can watch them!

Kevin