Flower:

Leafy spike at the end of the stem. Flowers are yellow, ½ to 1¼ inches across, the 4 petals rhombic-shaped, tapered to a pointed tip, with 8 yellow stamens surrounding a style with a cross-shaped stigma in the center. At peak flowering, the style and stamens are same length or nearly so. The 4 sepals behind the flower are narrowly lance shaped,  ¼ to 2/3 inch long, folded sharply back at flowering. usually in 2 pairs still married along one edge (connivent). The calyx tube is 2/3 to 1¼ inches long and resembles a flower stalk, attached between the base of the sepals and the ovary nestled in the leaf axil. Flowers open at night and on cloudy days, from the bottom of the raceme-like spike up.

Leaves and stems:

Leaves are narrowly oblong to lance-linear, 1¼ to 3½ inches long and to ½ inch wide, with short white hairs on both surfaces, the lower surface more densely hairy, mostly stalkless or the lower leaves on short stalks, and often twisted or wavy. Leaf edges are toothless or with a few small, widely spaced teeth. Stems are erect or ascending, covered in fine appressed hairs, unbranched to strongly branched at the base, sometimes with upper branching later in the season,.

Fruit:

Fruit is an erect to ascending, 4-parted, ½ to ¾-inch long capsule, a curved cylinder in shape and covered in short appressed hairs.

Notes:

Cleland's Evening Primrose is easily distinguished from all other evening primroses by its diamond-shaped petals, with the exception of Oenothera rhombipetala (Rhombic or Four-point Evening Primrose). The 2 were originally described as a single species, O. rhombipetala, in the late 1800s but studies in the 1960s broke O. clelandii out as a separate species based mostly on pollination fertility characteristics. Since this is of little help in the field, the petal and sepal sizes are considered the most visually helpful aids, O. clelandii the smaller of the two, generally half the size of its cousin. Another reference notes that the stigma is well extended above the stamens in O. rhombipetala whereas they are about the same length in O. clelandii. Unfortunately there is some overlap in both of these characteristics and since the two share the same general range and habitat, field identification is not easy. O. rhombipetala is considered the more rare species but neither is especially common in Minnesota.