Ontario Weeds: Yellow evening primrose

Table of Contents

1. Name
2. Other Names
3. Family
4. General Description
   • Photos and Pictures
5. Stems and Roots
6. Leaves
7. Flowers and Fruit
8. Habitat
9. Similar Species
10. For more information...

Name: Yellow evening-primrose, Oenothera biennis L.,

Other Names: onagre bisannuelle, Evening-primrose, onagre commune, herbe aux ânes

Family: Evening-Primrose Family (Onagraceae)

General Description: Biennial or short-lived perennial, or rarely annual, reproducing only by seed.

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Photos and Pictures

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Stems & Roots: Taproot, especially of biennial plants, becoming thick, fleshy and deeply penetrating; stems erect, up to 2m (6½ft) 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 & Fruit: Flowers in long spikes on stems and branches; each flower in the axil of a small leafy bract which resembles the upper stem leaves and is usually much shorter than the flower; flowers stalkless but mistakenly appear short-stalked, the lowermost portion of the apparent "stalk" being the ovary and the thinner portion above this being the hypanthium (floral tube) at the top of which are 4 narrow green sepals, 4 large yellow petals 1-2.5cm (2/5-1in.) long, and 8 stamens; edges and tips of sepals united in unopened flower bud; seedpod 1-3.5cm (2/5-1½in.) 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.

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.

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Similar Species: 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.

For more information...

... on weed identification, order OMAFRA Publication 505: Ontario Weeds

... on weed control, order OMAFRA Publication 75: Guide To Weed Control

...on weeds in field crops, contact Mike Cowbrough (mike.cowbrough@omafra.gov.on.ca), Weed Management Specialist (Field Crops), OMAFRA, Guelph
...on weeds in horticultural crops, contact Leslie Huffman (leslie.huffman@omafra.gov.on.ca), Weed Management Specialist (Hort Crops), OMAFRA. Harrow

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