Wood-sorrels
What are they?
The wood-sorrels are low-growing, herbaceous perennial plants with five-petalled, pink, yellow or white flowers and leaves typically comprising of three leaflets like those of clovers. We have just one native species which is often considered to be an indicator species for ancient woodland on acid soils. The flowers are usually carried in small clusters and are followed by elongate seed pods that are usually carried pointing vertically upward.
Where are they found?
Our one native species, Common Wood-sorrel, is found in shady places on acid soil. All other species are garden ornamentals that spread readily by seed, by vegetative parts such as small bulbils or by rooting readily at the nodes and forming spreading carpets. Species that form carpets are often found as urban weeds, growing from cracks in pavements or walls, or on gravel paths. The larger, pink-flowered species are often found where garden waste is dumped on roadsides, rough ground and grassy places.
Identification
Flower colour will be a first step to narrowing down the identification of a plant. Leaf detail can be important in the pink species, while flower detail is important in the yellow species.
Procumbent Yellow-sorrel Oxalis corniculata
Introduced from the Mediterranean region. An abundant species in urban areas and often considered an invasive weeds in gardens (especially driveways and patios) and garden centre stock. Flowers June to September. A creeping plant that can cover extensive areas of ground. Leaves may be green but more often the entire plant (except the flowers) is flushed deep reddish-purple. Green-leaved plants can be difficult to tell from Least Yellow-sorrel but flowers have all 10 stamens with anthers (look closely as five are more prominent from a distance) and the flowers are typically in clusters of two to eight (though the flowers not all opening at once).
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Least Yellow-sorrel Oxalis exilis
Introduced from Australia and New Zealand. Much less common than Procumbent Yellow-sorrel but still widespread and perhaps increasing. A weed of garden borders, lawns and drives. Flowers June to September. A creeping plant that can cover extensive areas of ground. Leaves bright green. Can be difficult to tell from Procumbent Yellow-sorrel but flowers have only 5 of the 10 stamens with anthers and the flowers are typically solitary.
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Upright Yellow-sorrel Oxalis stricta
Introduced from North America. Rare as an urban weed. Flowers July to October. An upright plant which is often confused with Procumbent Yellow-sorrel, but usually easily identified by the covering of patent white hairs on the stems and the upright (not reflexed) stalks to the seed pods.
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Slender Yellow-sorrel Oxalis dillenii
Introduced from North America and occasionally recorded as an arable weed. Flowers August to October. An upright plant which is often confused with Upright Yellow-sorrel, but easily identified by the covering of white hairs that lay flat to the stems.
This species has been much confused with Upright Yellow-sorrel in the past, perhaps because both have been labelled as Oxalis stricta by various taxonomists. It is unclear to me whether this species has truly occurred in East Anglia.
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Cape Sorrel Oxalis pes-caprae
Introduced from South Africa and now a pernicious and problematic weed in the Isles of Scilly, as it also is in parts of mainland Europe. Fortunately only recorded once in our region, in Kings Lynn, but may spread. Flowers May to June. Spreads by means of underground bulblets to form tight colonies of plants. Flowers larger than other yellow-sorrels, 15-25mm, vivid yellow; sometimes double-flowered. The leaves often have small, dark spots.
The erroneous name 'Bermuda Buttercup' is commonly used for this plant, but it is neither a buttercup, nor from Bermuda. I have used the more accurate name that is widely used elsewhere.
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Common Wood-sorrel Oxalis acetosella
Native. Found in shady places on acid soils in woodland and occasionally hedgebanks and cemeteries. Flowers April to May. The white flowers with pale lilac veins and the fresh green leaves in creeping mounds are distinctive.
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Common Pink-sorrel Oxalis articulata
Introduced from southern South America. Common and widespread as a garden escape on grassy banks, roadsides and rough ground. Flowers May to October. A tussock-like plant with low mounds of green leaves, the leaflets bearing small, orange spots on the underside towards the tip. Flowers bright carmine pink with a darker centre, rarely all white.
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Broad-leaved Pink-sorrel Oxalis latifolia
Introduced from northern South America. Rare as a garden escape. Flowers May to September. A more spreading plant than Common Pink-sorrel forming looser and more extensive patches of leaves. Leaflets without orange spots on the back. Flowers pink to pale lilac in colour, usually with a whitish centre; occasionally all white. Leaves rather angular in appearance with the three leaflets typically bent downward from their bases.
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Pale Pink-sorrel Oxalis incarnata
Introduced from South Africa. Frequent as a garden escape and seeming to favour dry, gravelly places or the foot of walls. Flowers May to July. An upright plant to around 30cm in height (though often less), that forms spreading patches of yellow-green leaves. Flowers delicate, very pale pink, almost white.
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Four-leaved Pink-sorrel Oxalis tetraphylla
Introduced from Mexico as a garden ornamental. Occasionally found as a garden escape or where garden waste has been dumped. Flowers May to September. A spreading species that forms low carpets of leaves that bear four leaflets. Some forms have all green leaves and pale pink flowers but more recently, the cultivar 'Iron Cross' has proved popular in gardens and is finding its way into the wider countryside; it has dark bases to the leaflets and deeper red flowers.
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