Yellow Evening Primrose

Looks like a primrose to me as well. Two years ago we had one grow in our garden. We sat and watched the flowers bloom one evening, was neat to see :). If I can find the pictures of it, I'll post them.

Its about 2 inches in diameter, and growing about 3-4 off the ground. A few of them sprouted in the middle of my backyard. Not a tree at all :-)

Impossible to say which species with no sense of scale and absence of any other plant parts.