Yellow Evening-primrose Oenothera biennisOnagraceae—Evening-primrose familyby Tom Reaume © 2010 Nature ManitobaGrant:

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 wide, reduced above, falling as fruit ripens; flowers per-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; 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 microscopic thread; ovary hairy, 7–20 mm long by 2–3 mm wide, cylindrical, ridged; style 4–4.5 cm long; stigma 4-cleft, 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 3–7 mm wide, 4-valved, ascending and arranged in a spiral 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 on the faces, walls c. 0.1 mm thick.l LEAVES (in first year rosettes 8–56 cm wide with 12–80 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 (dorsal) surface downy and lighter green, midrib slightly red-dish above; petioles hairy, 0–20 mm long (to c. 7 cm long for leaves of rosettes), reduced above; stipules absent.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.l RANGE: (CAN) 10 provinces; (USA) 44 states; natural-ized.

A group of Yellow Evening-primrose to 160 cm tall along the gravelly slope of a railway in Winnipeg, Manitoba

Plant 140 cm tall showing lower floral branches; stem leaves spreading

Upper plant with short branches from base of terminal spike

Flower x1; daytime; c. 6 cm long Left flower is c. 6 cm long

ovary

floral tube

sepal

petal

branch

branch

bract

Stamen x3

stigmas

style

ovary

floral tube

sepals

Flower bud x1

Pistil x1

bract

filament

anther

Reddish flower buds adorn the top of the lengthening terminal inflorescence

Yellow Evening-primrose flower from above; 4 cm wide with the 4 petals over-lapping

stigma

anther

petal

filament

anther

sepal

floral tube

ovarybract

Flower with petals removed show-ing stamens and stigma

stigma

Sepals x2

Stem leaves change color in late summer; some with a partial twist at the tip; petioles short; Dorsal; Ventral side

petiole

midrib

twist

Flower with petals and sta-mens removed

stigma

style

sepal

floral tube

Cross-section of taproot 15 mm wide from a 23 cm wide rosette in fall

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

1 2 3 4 5

sepal

seed

ovary

bud

wilted

petal

seed

apex

V

D

Seed x20

thick top side

Fruit (capsule) x1

Fruit x5; cross-section

seed

placenta

wallcapsule

open apex

rachis

ridge

First year rosette 35 cm across with c. 35 leaves

Yellow Evening-primrose green fruit each with a subtending bract

Ripe fruit opening at apices; sub-tending bracts have fallen

fruit

bract

Woody taproot 15 cm longOpen capsule, seeds, placenta

Open capsule from above

seed

seed

valve placenta

Plant in early evening with flower buds about to open, wilted flowers, and green fruit (capsules)

budwilted flower

fruit

stem base

taproot sideroot

Hairy leaf margins; Ventral, Dorsal; slightly toothed

V

D