I saw this plant during a walk in a nearby sand prairie. It is just blooming now in Sept. I live in Eastern Iowa. Searched my books, and plant identification websites with no luck. Even sent some pictures to a local Nature Center, and they didn't know what it was either. Here are some pics (I hope!) Like some other plants, this one closes it's flowers at night and opens them during the day. Some of the pictures were taken first thing in the morning when the flowers were still closed, and some were taken in the afternoon when they were open. The whole plant was about six feet across, very open and scraggly, but the flowers - wow!
SOLVED: What (prairie?) plant is this?
The only species occurring naturally in Iowa is Mentzelia decapetala.
I wish it was occurring naturally here in my yard, what a pretty bloom! :D