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 and style are typically shorter than the petals.

The 4 sepals behind the flower are 1/3 to about ¾ inch (9 to 18 mm) long and bend back away from the flower as it develops, pairs typically connected along one edge (connivent) until the flower opens, are often red striped or tinged on the outer surface, and are variously hairy, sometimes with short glandular hairs. The floral tube, connecting the ovary nestled in the leaf axil and base of the flower, is up to 1¾ inches (23 to 44 mm) long and resembles a flower stalk. The ovary is often covered in longer hairs with a red or green pimply base (pustulate). 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 ½ to 1½ inches (1.2 to 4+ cm) wide, pointed at the tip, tapering at the base, and stalked, becoming smaller and stalkless or nearly so as they ascend the stem. Edges are flat to wavy, toothless to minutely toothed, the teeth sometimes widely spaced; surfaces may be covered in short, stiff hairs giving a gray-green cast. Stems are mostly erect, branched or not, stout, usually reddish at least towards the base, usually densely covered in stiff appressed hairs usually mixed with softer spreading hairs; hairs may have a red or green pimple-like base.

Fruit:

Fruit is an erect to ascending capsule up to 1¾ inches (20 to 43 mm) long, tubular but tapering some at the tip end, with 8 tiny erect lobes at the tip and variously hairy across the surface. Inside are angular seeds 1 to 2 mm long.

Notes:

A fairly common species, Hairy Evening Primrose (formerly Oenothera biennis var. canescens) is easily confused with the closely related Northern Evening Primrose (Oenothera parviflora) and Common Evening Primrose (Oenothera biennis). O. biennis usually has somewhat larger flowers but the sizes and other characteristics overlap between the three species so distinguishing them can be difficult, especially when fresh sepals are not present. For O. parviflora, look for a small ridge or knob at the tip end of the sepal, which O. biennis and O. villosa lack. Distinguishing O. villosa from O. biennis can be more complicated.

There are two subspecies of O. villosa: subsp. strigosa is the most recognizable, with sepals that are striped or tinged red, at least some long hairs with a red pimply base, short glandular hairs on the upper stems into the flowers, and tends to be loosely flowered; subsp. villosa is more densely covered in stiff, straight hairs giving a dull green to gray-green cast, rarely has glandular hairs, sepals are green to yellow-green, and tends to be more densely flowered. By comparison, O. biennis is less densely hairy overall, green to pale green, usually also glandular hairy, sepals are rarely (if ever) tinged red, and any pimply-based hairs are green, not red.

The types and density of hairs on all 3 species are rather variable: stiff appressed hairs, longer spreading to ascending soft hairs with or without a pimple-like base, and with or without short glandular hairs. Flora of North America notes that when flowers fade, O. villosa petals turn orange where O. biennis turn more whitish and somewhat translucent, but we aren't yet convinced orange is limited to O. villosa. Leaves of O. biennis are also said to be more flat along the edges where O. villosa tend to be more wavy. 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 during the course of the season.