Hi Elaine,
I think you have an Evening Primrose, although unless I could see it I can't know for sure. But what you discribed sure makes me think that it is. Evening Primrose - Oenothera Family (several different ones)
The one that opens at night is called Missouri Primrose or Ozark Sundrops -missourensis
They have large yellow flowers and tolerant of poor soil in sunny locations.
Use that name and see if you can find a picture for I.D.
Your library may have a perennial book called Taylor's Guide to Perennials. I really like this book because it has a large selection with pictures categorized by colors. It really helps for finding things.
Happy Gardening, Diane