Oenothera biennis (Common Evening Primrose)
|Also known as:
|Onagraceae (Evening Primrose)
|biennial, short-lived perennial
|part shade, sun; wet to dry soil; prairies, dunes, roadsides, railroads, waste places, woodland edges, cliffs, shores, river banks
|July - October
|2 to 6 feet
|Wetland Indicator Status:
|GP: FACU MW: FACU NCNE: FACU
|MN county distribution (click map to enlarge):
|National distribution (click map to enlarge):
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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 1 to 2 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 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 yellowish-green and variously hairy, sometimes with glandular hairs. 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 stem:
Leaves are basal and alternate, the basal and lower stem leaves 4 to 12 inches (10 to 30 cm) long and ¾ to 2 inches (2 to 5 cm) wide, pointed at the tip, tapering at the base, slightly rough to the touch, mostly hairless, and 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 often widely spaced. Color is olive to light green and there may be small leaves clustered in the leaf axils. Stems are mostly erect, branched or not, stout, reddish or light green, variously covered in stiff spreading to appressed hairs that may have a pimple-like base (pustulate).
Fruit is an erect to ascending capsule ¾ to 1½ inches (20 to 40 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 about 1 mm long.
A very common species, Common Evening Primrose is easily confused with the closely related Northern Evening Primrose (Oenothera parviflora) and Hairy Evening Primrose (Oenothera villosa, formerly O. biennis var. canescens). 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. O. villosa sepals can be striped or tinged red (but not always) where O. biennis sepals are green to yellowish-green.
The types and density of hairs on all 3 species are rather variable: stiff appressed hairs, longer spreading to ascending hairs with or without a pimple-like base, and with or without 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. Two subspecies of O. villosa with different characteristics make the distinctions even more challenging. 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.
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- branching Common Evening Primrose plant
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Photos by K. Chayka taken in Lake and Ramsey counties. Photos by Peter M. Dziuk taken in Anoka and Ramsey counties.
Have you seen this plant in Minnesota, or have any other comments about it?