Rhubarbs
What are they?
This is very much an artificial collection of plants, brought together on this page simply because their leaves are rather similar, earning them all the name 'rhubarb'. However, their flowers are all completely different from each other, revealing that they are not closely related. The true rhubarb is related to the docks and knotweeds in the family Polygonaceae and this is given away by the flower structure and by the membrane at the base of the leaves.
The Gunneras are an interesting group of plants from the tropical and subtropical zones of the Southern Hemisphere. They are rather variable in appearance, though most have distinctly rounded 'peltate' leaves, with species ranging from creeping plants with leaves just a few centimetres across, to absolute giants, whose leaves may reach over three metres across! It is these giant species in particular that have become popular as garden plants and to which the name 'giant-rhubarb'.
Finally, the Indian-rhubarb has showy heads of pink flowers that are typical of the saxifrage family that it belongs to. It is, however, rather unusual for that family in that the flowers appear on their own, well before the leaves emerge.
Where are they found?
These are all garden introductions, so occasional cast-outs may turn up on waste ground or roadsides, but some are naturally plants of wet places and may turn up in a range of wetland habitats. In Ireland, Gunneras are now a major invasive species that are threatening native plant communities and the appearance of these plants (beautiful as they are) on the margins of the Norfolk Broads should give cause for concern.
Identification
Although these three species have rather similar leaves, they are readily told apart by their very different flowers, though other species of Gunnera grown in cultivation and not yet recorded in the wild in East Anglia will complicate things if they eventually get out!
Culinary Rhubarb Rheum x rabarbarum
A plant of hybrid origin, introduced as a culinary plant and popular on allotments, from where it sometimes spreads onto neighbouring land. Flowers May to June. Large plants with shiny leaves that typically have reddish stems. Flowers tiny, creamy yellow in a tall spike above the leaves.
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Chilean Giant-rhubarb Gunnera tinctoria
Introduced as a garden ornamental and occasionally found spreading into neighbouring wetland areas. Flowers May to August. Huge plants with leaves growing to 2.5 metres in height from a central rootstock. Leaves die down in winter, leaving peculiar, gnarly stumps covered in dense hairs. Individual flowers are tiny, but appear in huge, fingered spikes up to a metre in length, with the fingers being relatively short and compact.
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Hybrid Giant-rhubarb Gunnera x cryptica
Arose in cultivation as a garden ornamental and occasionally found spreading into neighbouring wetland areas. Much rarer than Chilean Giant-rhubarb, though some early records may need checking to confirm identification. Flowers May to August. Huge plants with leaves growing to 3 metres in height from a central rootstock. Leaves die down in winter, leaving gnarly stumps covered in dense hairs. Individual flowers are tiny, but appear in huge, fingered spikes up to a metre in length, with the fingers being relatively long and open.
Note: Plants once (and still) sold in cultivation as Brazilian Giant-rhubarb (Gunnera manicata) have been shown to be this hybrid and research has shown that the Brazilian species is no longer in culitvation in the UK, having probably died out due to hybridisation and a lack of ability to survive cold winters as long ago as the late 19th Century. This is described in a comprehensive paper by Shaw, Edwards & David (2022) in British & Irish Botany.
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Indian-rhubarb Darmera peltata
Introduced from Asia as a garden ornamental. Rare as an escape or garden throw-out on damp ground at Thorpe St Andrew, Norwich. Flowers May. Spreads from a creeping root to form extensive patches. Flowers appear well before the leaves, pushing up from below ground on long, upright stalks. Leaves appear after the flowers, dieing down again in the autumn.
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