Flower:

Flowers are usually about 1 inch across but are occasionally up to twice that size; they cluster around the ends of the main stems, usually only a few are open at a time. Each flower has 4 bright or pale yellow heart-shaped petals and several prominent stamens protruding from the center. The flowers open in the evening and close up during the day but may remain open longer on cloudy days.

Leaves and stem:

Leaves are up to 8 inches long and 2 inches wide, tapering to a point at the tip, with no leaf stem. They are slightly rough to the touch, can have smooth edges or small teeth, the edges are often somewhat wavy, and they often fold up some from the prominent central vein. The color is olive or light green and there are often small leaves sprouting from the leaf axils.

The stem is reddish or light green and covered in white hairs.

Notes:

Common Evening Primrose can become quite bushy when it's out in the open. It doesn't seem too particular about where it grows—I've seen it growing in moist woods as well as dry fields.