Evening-primroses
What are they?
These plants with often large and showy flowers are members of the Onagraceae and related to the willowherbs, which they somewhat resemble in having four-petalled flowers, narrowly ovate to linear leaves and seeds carried in prominent, elongate seed capsules. Evening-primroses in the UK have an intriguing yet confused history; a number of species were introduced from North America, primarily as a crop for the oils they produce, with these crop species then starting to appear as occasional strays on roadsides and rough ground. From these, a number of hybrids occurred spontaneously, some of which are probably endemic to the UK since their parents would not typically meet in the wild. Some such hybrids have, over time, formed stable populations that breed true to type and are now recognised as true species (called nothospecies). The identification of some plants from hybrid populations can be difficult or sometimes impossible if they do not match closely to known species or hybrids; however, the situation currently is a lot easier since evening-primroses are no longer grown as a crop (at least in East Anglia) and these plants are now much less common than they were in the 1980s and 1990s.
Evening-primroses get their name from their flowers, which each open for about 12 hours before fading. Individual flowers will open in late afternoon or early evening, stay open through the night, then fade the following morning. Thus, plants found in the afternoon will typically have no flowers open.
Where are they found?
Small populations of evening-primroses still occur in a few places on grassy roadsides, field edges and rough ground. Occasional plants may still arise from wildflower or bird seed mixes or may be found in urban environments where they have spread their seeds beyong the garden.
Identification
The identification of evening-primroses can be difficult due to the possibility of hybrids and even the possibility of some species in cultivation which may escape into the wider countryside but which have not yet been recorded. Most important are details of the flowers and it is good to note the length of the petals and to note the length of the four-lobed stigma relative to the length of the stamens. Also note the covering of the stems and seed capsules - especially the presence or absence of reddish, bulbous bases to some of the hairs.
Large-flowered Evening-primrose Oenothera glazioviana
Introduced from North America. The commonest species by far and still quite frequent on light, sandy soils on coastal dunes, along railways and on disturbed ground in urban environments. Flowers June to September. May grow to 1.5m in height. Flowers large with petals to 5cm in length and with sepals clearly stained red on the outside; stigma clearly much longer than the stamens. Seed capsules widest near base and with glandular hairs (sometimes red-striped); stems with many, bulbous red bases to the stem hairs.
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Intermediate Evening-primrose Oenothera x fallax
Probably native, seemingly having arisen as a hybrid between Large-flowered and Common Evening-primroses within the UK. Recorded in small numbers on light soils throughout most of the region. Plants with Large-flowered Evening-primrose as the female parent seem to have formed stable populations that come true from seed. Flowers June to September. May grow to 1.5m in height. Flowers fairly large with petals to 3cm in length and with sepals having some red streaking on the outside; stigma about the same length as the stamens. Seed capsules widest near base and with glandular hairs (and often red-striped); stems with many, bulbous red bases to the stem hairs.
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Common Evening-primrose Oenothera biennis
Probably first introduced as an ornamental but also formerly grown as a crop for its essential oils. Flowers June to September. May grow to a metre in height. Flowers small to large with petals typically 2-3cm in length and with sepals usually green but sometimes with light red streaking on the outside; stigma about the same length as the stamens or a little shorter. Seed capsules widest near base and mostly without glandular hairs; stems with no (or only a few) scattered, bulbous red bases to the stem hairs. A variable specie with a number of named forms, which may be recognised as separate species by some taxonomists.
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Fragrant Evening-primrose Oenothera stricta
Introduced from North America. Currently popular as a garden plant and increasingly appearing when self-seeding beyond gardens into streets and roadsides. Flowers June to September. Typically grows to around one metre in height. Flowers fairly large with petals to 3.5cm in length and with sepals red on the outside. Petals brilliant yellow, turning peach-coloured as they fade. Seed capsules widest towards the top and thickly hairy; stems hairy and reddish in colour. Leaves darker green and narrower than those of other evening-primroses recorded in the region.
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Small-flowered Evening-primrose Oenothera cambrica
Introduced from North America. Scattered records mostly from the sides of major highways after extensive roadwork has been carried out. In such cases, quite large numbers can occur but these plants are very transitory and often disappear after just one year. Flowers June to September. May grow to a metre in height. Flowers small to medium with petals 2-3cm in length and with sepals green on the outside. Seed capsules widest near base and with glandular hairs; stems with many, bulbous red bases to the stem hairs. Note that there are currently a number of different schools of thought on the identification and origin of plants that we know in the UK as Oe. cambrica and some people now simply consider this to be a small-flowered form of Common Evening-primrose and not the true Oe. cambrica at all.
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Oakes's Evening-primrose Oenothera oakesiana
Introduced from North America as a crop for essential oils. Probably long extinct in our region with both Suffolk and Norfolk listing just single records from the 1980s. Flowers June to September. May grow to 1.5m in height. Flowers small with petals to 2cm in length and with sepals green or only lightly stained with red on the outside. Seed capsules widest near base and with many hairs; stems green or with a few bulbous red bases to the stem hairs. Previously known as Oenothera perangusta in UK floras.
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Large-flowered Water-primrose Ludwigia grandiflora
Introduced from the Americas as an ornamental. A plant that favours the margins of ponds and still or slow-moving water, from where it grows out onto the water surface in floating mats. Flowers June to September. Flowers like those of evening-primroses but with five or six petals. Currently only three or four records for the region, but this highly invasive species is a notifiable alien and should be reported to the Environment Agency for eradication if found.
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