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. 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 or longer than the petals and the stigma is nearly as wide as the spreading petals.

The 4 sepals behind the flower are ½ to about 1 inch (12 to 22+ mm) long and bend back away from the flower as it develops, pairs typically connected along one edge (connivent) until the flower opens, are variously hairy, sometimes with glandular hairs. Color may be green, yellowish, tinged reddish, or flecked red. A small knob or perpendicular ridge at the tip of the sepals at the point where the sepals separate is a key diagnositic of this species, 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 ¾ to 1½ inches (20 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 4 to 12 inches (10 to 30 cm) long and 3/8 to 1½ inches (1 to 4 cm) wide, pointed at the tip, tapering at the base, hairless to sparsely hairy, 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. Color is usually bright green, the midvein red or white.

Stems are erect, branched or not, stout, green or red, sparsely covered in 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 ¾ to 1½ inches (20 to 40 mm) long, tubular but tapering some at the tip end, variously hairy across the surface and 4 flaring lobes at the tip. Inside are angular seeds 1.1 to 1.8 mm long. The capsule dries to nearly black.

Notes:

Northern Evening Primrose is easily and often confused with Common Evening Primrose (Oenothera biennis). A key diagnostic feature is the small ridge or knob just below sepal tips. O. biennis also generally has larger flowers but there is some overlap on flower sizes so this is not always a reliable difference. At one time there were two varieties of O. parviflora which have been split into separate species. Oakes' Evening Primrose (O. oakesiana, formerly O. parviflora var. oakesiana) is more densely silky-hairy overall, leaves are dull green to gray-green, flowering stems typically exhibit a pronounced curve to one side at the tip, and capsules dry to rusty brown.

The types and density of hairs on the similar Oenothera species are all rather variable: stiff appressed hairs, longer spreading to ascending hairs with or without a pimple-like base, and with or without glandular hairs. In the coming seasons we will be seeking out good field specimens and working out better or more consistent distinctions, as well as clues on how any of these characteristics may change over the course of the season.