Yellow Evening-primrose Oenothera biennis Onagraceae—Evening-primrose family by Tom Reaume © 2010 Nature Manitoba Grant: A biennial wildflower, solitary or in groups, 30–200 cm tall from a fleshy taproot 10–40 cm long by 4–35 mm thick with side roots 1–20 cm long by 0.5–5 mm thick; in sandy to gravelly sites of open woodlands, along shores of rivers, streams, lakes, roadsides, open slopes and railways. l FLOWERS yellow, blooming June–October; inflorescence a spike, terminal, 20–70 cm long by 6–9 cm wide, bracteate; floral branches 0–18 per plant, alternate and ascending, 2–71 cm long, but not longer than the central stem; bloom- ing sequence bottom to top; subtending bracts (of flowers) one, spreading, entire, lanceolate, 1–5 cm long by 3–10 mm A group of Yellow Evening-primrose to 160 cm tall along the wide, reduced above, falling as fruit ripens; flowers per- gravelly slope of a railway in Winnipeg, Manitoba fect, sessile, crowded above, 1.5–4.5 cm wide by 3–6.8 cm long; large buds which open at night are blooming the next morning, then wilt by evening and fall overnight; floral tube 2–4.5 cm long by 1.5–2 mm wide, lightly to densely hairy, some hairs glandular; sepals 4, light green, in 2 pairs, each pair united except for 1–4 mm at the pointed tips, reflexed or twisted, margins hairy, each 9.5–25 mm long by 2–5 mm wide; petals 4, ascending, 8–27 mm long and wide, often slightly wider than long, tapered, glabrous and overlapping, apices shallowly notched, slightly erose; stamens 8, yellow, 13–16 mm long, exserted at times; filaments 9–12 mm long, bent, attached to a green ring inside the bases of the sepals; branch anthers 6–10 mm long by c. 1 mm wide, curved, attached slightly below the middle to the filaments; pollen yellow, bluntly triangular, flattened, attached together in a line by a Upper plant with short branch microscopic thread; ovary hairy, 7–20 mm long by 2–3 mm branches from base of wide, cylindrical, ridged; style 4–4.5 cm long; stigma 4-cleft, terminal spike light green, 5–8 mm long, extending slightly above the an- thers; flower to fruit c. 3 months. l FRUIT a capsule, tan, cylindrical, sessile, 1–4 cm long by Plant 140 cm tall showing lower floral 3–7 mm wide, 4-valved, ascending and arranged in a spiral branches; stem leaves spreading along the spike, stiff, hard, opening from apex, glabrous to hairy (some hairs gland-tipped), 35–290 capsules per plant with an average of 22 per 10 cm of stalk, walls 0.2–0.3 mm thick; seeds 300–400 per capsule, in 3 or 4 rows in each of the 4 chambers (locules) of the fruit, medium brown, variable in shape and size, 5- or 6-sided, 1.3–2 mm long by 0.7–1.3 mm wide and thick, with corner ridges and a few lower ridges petal on the faces, walls c. 0.1 mm thick. l LEAVES (in first year rosettes 8–56 cm wide with 12–80 sepal leaves) are from 2.3–20 cm long by 1–6 cm wide; second year stem leaves alternate to subopposite, simple, entire or teeth shallow; blades spreading, 5–30 cm long by 0.7–7 cm wide, reduced above, often twisted or wavy, hairy both sides, hairs appressed and less than 1 mm long, edges and lower floral tube (dorsal) surface downy and lighter green, midrib slightly red- bract dish above; petioles hairy, 0–20 mm long (to c. 7 cm long for leaves of rosettes), reduced above; stipules absent. ovary l STEM erect, round, stout, hollow, green to reddish with appressed reddish hairs 0.5–2 mm long; 2–25 mm thick near the woody, naked base. Flower x1; daytime; l RANGE: (CAN) 10 provinces; (USA) 44 states; natural- c. 6 cm long Left flower is c. 6 cm long ized. stigma bud petal filament stigma petal anther wilted sepal anther floral tube bract Yellow Evening-primrose flower from ovary above; 4 cm wide with the 4 petals over- Reddish flower buds adorn the top of the lapping lengthening terminal inflorescence anther Flower with petals removed show- ing stamens and stigma stigma style seed filament seed sepal sepal Stamen x3 floral ovary tube 1 2 3 4 5 Flower with petals and sta- mens removed Left to right: (1) young flower bud (2) mature flower bud (3) open flower (4) green capsules (5) ripe capsule starting to open at apex stigmas apex midrib Sepals x2 sepals style D petiole floral tube twist V bract ovary Flower bud x1 Cross-section of taproot 15 mm Pistil x1 wide from a 23 Stem leaves change color in late summer; some with a partial cm wide rosette twist at the tip; petioles short; Dorsal; Ventral side in fall open apex seed placenta wall ridge capsule Fruit x5; cross-section thick top side rachis Seed x20 Fruit (capsule) x1 bract fruit First year rosette 35 cm across with c. 35 leaves V Yellow Evening-primrose green fruit each with a subtending bract D Ripe fruit opening at apices; sub- tending bracts have fallen Hairy leaf margins; Ventral, Dorsal; slightly toothed seed stem base wilted flower bud Open capsule from above seed side taproot root fruit valve placenta Open capsule, seeds, placenta Woody taproot 15 cm long Plant in early evening with flower buds about to open, wilted flowers, and green fruit (capsules).