Flower:

Elongating spike at the top of the stem, dense with leafy bracts and flowers opening from bottom to top. Flowers are stalkless but a narrowed neck (hypanthium) between the ovary, nestled in the leaf axil, and the base of the sepals can be over an inch long giving the appearance of a flower stalk. Flowers are ¾ to 1½ inch across with 4 yellow heart-shaped petals and 8 yellow stamens with elongated tips (anthers) around a bright yellow cross-like stigma in the center that is nearly as wide as the spreading petals.

The 4 sepals are light green, narrow lance shaped and hang tightly down below the open petals, often in two pairs still attached along one seam (connivent). 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.

Leaves and stems:

Leaves are lance-like with small, shallow teeth or toothless, bright to gray-green with red or white veins. Basal leaves are stalked and wither away by flowering time (deciduous), 3 to 10 inches long and 1 to 1½ inches wide. Stem leaves have little to no stalk, 1½ to 7 inches long, the surfaces smooth or with scattered short hairs.

Stems are mostly unbranched to sparsely branched, often reddish with sparse, short hairs or mixed short and long hairs, some glandular tipped.

Fruit:

Fruits is a tubular capsule nestled in the leaf axil, 1 to 1½ inches long and ¼ inch wide, tapered upwards but with four conspicuously flared tips. The capsule dries to nearly black (var. parviflora) or a rusty brown (var. oakesiana).

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. There are two varieties of O. parviflora found in Minnesota: var. parviflora has bright green leaves with conspicuous veins and is typically unbranched, var. oakesiana (formerly Oenothera oakesiana) leaves are gray green with inconspicuous veins, is toothless or nearly so on the leaf edges, does not have glandular tipped hairs, and is more often branched. It also typically exhibits a pronounced arching to one side of the flowering stems. This nodding or arching is of special note, as many Oenothera parviflora references mention the “spike nodding at the tip” yet I found no clear illustrations of this in either botanical drawings or photos. We think we have found an example of this—see the photo below.