Flower:

Elongating leafy spike of yellow flowers at the tip of the stem, with flowers blooming at or near the tip and fruit forming below, the stem and branch tips usually with a pronounced curve like a sideways “S”. Flowers are ¾ to 1½ inches across with 4 yellow heart-shaped petals and 8 yellow stamens surrounding a style with a cross-shaped stigma in the center. The stamens are about as long as the petals and the stigma is nearly as wide as the spreading petals.

The 4 sepals behind the flower are up to about 2/3 inch (9 to 17 mm) long and bend back away from the flower as it develops, pairs typically connected along one edge (connivent) until the flower opens, usually covered in long silky hairs. Color may be green, yellowish, tinged reddish, striped or flecked red. At the tip of the sepals at the point where the sepals separate is a small knob or perpendicular ridge, though it may not be pronounced on all sepals. The floral tube, connecting the ovary nestled in the leaf axil and base of the flower, is ½ to 1½ inches (15 to 40 mm) long and resembles a flower stalk. The flowers open in the evening and close up during the heat of the day.

Leaves and stems:

Leaves are basal and alternate, the basal and lower stem leaves 3 to 12 inches (8 to 30 cm) long and 3/8 to 1+ inches (1 to 3 cm) wide, pointed at the tip, tapering at the base, stalked, becoming smaller and stalkless or nearly so as they ascend the stem. Edges are mostly flat, sometimes a bit wavy, and minutely toothed, the teeth sometimes widely spaced; surfaces are variably covered in short appressed hairs. Color is green to gray-green, the midvein red or white. Stems are erect, branched or not, stout, green or red, mostly covered in long silky hairs, sometimes a mix of hairs that may be spreading to appressed, stiff or soft, long with or without a pimple-like base (pustulate), or short with or without a glandular tip.

Fruit:

Fruit is an erect to ascending capsule ½ to 1½ inches (15 to 40 mm) long, tubular but tapering some at the tip end, usually silky hairy across the surface, with 4 flaring lobes at the tip. Inside are angular seeds 1.1 to 1.2 mm long. The capsule dries to rusty brown.

Notes:

Oakes' Evening Primrose, formerly Oenothera parviflora var. oakesiana, is primarily found in disturbed soils such as roadsides, gravel pits and railroad rights-of-way, and mostly in the northeastern counties where it reaches the western edge of its range. It might be confused with related Oenothera species with similar 4-petaled yellow flowers, but has a couple distinctions that should make it easier to separate this from the pack: the pronounced curve at the tip of the stem and branches, and (usually) the lack of any glandular hairs, which the others typically have in varying quantities.