Purple Loosestrife & Allies
What are they?
The family Lythraceae is a difficult one to define easily as there is much variation in the appearance of the species. However, we have just a handful of species in the UK which helps to make things a little more straightfoward. Commonest by far among these is Purple Loosestrife which, despite the name, is not related to the yellow loosestrifes (which are in the primrose family). Plants in this family have complicated flowers that typically have six petals but which also have an extra layer of sepals known as an epicalyx. In some cases, the epicalyx lobes can be mistaken for petals, while the petals themselves may be absent!
Where are they found?
These are mostly plants of wet areas, either on seasonally flooded, winter-wet areas that dry out in summer, or beside more permament waterways.
Identification
The small number of species are quite easily identified by their general features. Purple Loosestrife is a common and distinctive species, while the others are local and easily overlooked, or rare.
Purple Loosestrife Lythrum salicaria
Native. Widespread and often common along the margins of rivers, lakes, ponds and wet ditches. Flowers June to August. Grows to 1.5m in height. Its square stems and opposite leaves may cause confusion with members of the dead-nettle family, but it can be told from them by the distinctive arrangement of the veins on the leaf (best viewed from the underside) and by the six-petalled flowers.
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Grass-poly Lythrum hyssopifolia
Native. Extremely rare with only two established locations for the species in the entire country, one of which is in Cambridgeshire in a seasonally flooded, arable field. Formerly occurred in Suffolk and may occasionally appear from spilt birdseed. Unexpectedly, this species turned up at a wetland restoration project in North Norfolk in 2020 and again in South Norfolk in 2024. Flowers June to July. Grows to 25cm in height, typically rather low and sprawling. The leaves are distinctly narrower than those of False Grass-poly. Flowers usually have six petals but some may have four or five.
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False Grass-poly Lythrum junceum
Introduced from southern Europe and occasionally appears as a contaminant of birdseed, self-seeding where the seed is spilt. Flowers June to July. Typically a little more upright and taller than Grass-poly, growing to around 70cm in height. The leaves are distinctly broader than those of Grass-poly and the flowers a little larger. Flowers usually have six petals.
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Common Water-purslane Lythrum portula
Native. Uncommon in seasonally wet spots such as hollows and wheel ruts near more permanently wet locations. Flowers June to September. Typically a prostrate species, growing on wet mud or in shallow water. The flowers are unusual in having up to six petals (sometimes none) that are less than 1mm long, so it is the slightly larger (to 2mm long!), pinkish epicalyx segments that often appear to be the petals (see picture).
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