Ontario Weeds: Yellow
Table of Contents
- Other Names
- General Description
- Stems and Roots
- Flowers and Fruit
- Similar Species
- For more information...
Name: Yellow evening-primrose, Oenothera
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.
Photos and Pictures
Yellow evening primrose.
A. Base of second year plant.
Yellow evening primrose.
B Top of flowering stem.
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.
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 (firstname.lastname@example.org),
Weed Management Specialist (Field Crops), OMAFRA
...on weeds in horticultural crops, contact Kristen Callow (email@example.com),
Weed Management Specialist (Hort Crops), OMAFRA
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