Yellow evening primrose, Oenothera biennis

Life Cycle

Biennial or short-lived perennial, or rarely annual.

Propagation

Reproducing only by seed.

Stems

Stems erect, up to 2m high, usually somewhat branched, the branches spreading and becoming erect, often hairy, either green or purple-tinged.

Leaves

Leaves of first-year plants in a rosette, their short stalks gradually broadening into the elliptical to oblong leaf blades, green or with a reddish cast, midrib often pinkish to reddish, margins irregular or weakly toothed; stem leaves alternate (1 per node), stalkless, similar to the basal leaves but gradually smaller upwards, with wavy or toothed margins.

Flowers and Fruit

Flowers in long spikes on stems and branches; each flower in the axil of a small leafy bract (b) which resembles the upper stem leaves and is usually much shorter than the flower; flowers stalkless but mistakenly appear short-stalked, the lowermost portion (c) of the apparent "stalk" being the ovary and the thinner portion (d) above this being the hypanthium (floral tube) at the top of which are 4 narrow green sepals (e), 4 large yellow petals 1-2.5cm long, and 8 stamens; edges and tips of sepals (f) united in unopened flower bud; seedpod 1-3.5cm long, nearly cylindrical but tapering towards the tip, the inside divided into 4 chambers by 4 lengthwise partitions, and the outer wall splitting downwards from the tip into 4 valves to release the seed; seeds irregular in shape, dark reddish-brown to black with rough surfaces. Flowers from July to September.

Roots and Underground Structures

Taproot, especially of biennial plants, becoming thick, fleshy and deeply penetrating.

Habitat

Yellow evening-primrose occurs throughout Ontario in waste areas, roadsides, lakeshores, river valleys and occasionally in fields of winter wheat or fall rye where its long spikes of bright yellow flowers are very conspicuous.

Distinguishing Features

It is distinguished by its tall, erect stems with long spikes of large, bright yellow flowers followed by short seedpods splitting downward on 4 sides; its rosette plants distinguished by the usually pinkish to reddish midribs of their elongated, elliptic to oblong leaf blades.


Figure #1.

B. Top of flowering stem.


Figure #2.

A. Base of second-year plant. C. Seedpod.


Figure #3.


Figure #4.


Figure #5.

Unopened flower bud.


Figure #6.

Yellow flower of evening primrose.


Figure #7.

Stem of green-purple tinge of yellow evening primrose.


Figure #8.

Underside of leaf from yellow evening primrose.


Figure #9.

Underside of leaf from yellow evening primrose.