Great Dixter's Christopher Lloyd wrote 'it is not grotesque, neither is it coarse, and those who think otherwise have clearly never lived in close proximity to one' in his description of Gunnera manicata; the largest perennial hardy enough to survive winter temperatures over a significant area of the British Isles. It has been a popular garden plant since its introduction to British horticulture in 1867, and is valued for the architectural qualities of its enormous leaves.
The genus Gunnera contains about 40 species, distributed across much of the southern hemisphere from South America and the Antarctic Islands, through South Africa and Madagascar to New Zealand. Gunnera manicata is the largest species, its leaves reaching a width of three metres. In contrast, Gunnera monoica, found in New Zealand, has leaves only three centimetres wide.
Gunnera manicata is native to southern Brazil. It is a pachycaul perennial, producing tiny flowers in dense, cone-like inflorescences in early summer (memorably described by Lloyd as looking like 'a cross between a vast fleshy fir cone and a fertility symbol'), on which the small seeds ripen inside startling orange fruits. In Britain, its dormant buds can be damaged by late-spring frosts therefore plants are protected from these frosts by a mulch of old leaves from the previous season. The plants grow to their largest in proximity to open water, although they can be grown successfully in drier soils as their roots penetrate to depths of over one metre.
Gunnera manicata's common name is an allusion to the shape of the large, palmate leaves of cultivated rhubarb. In Chile, consumption of the young, peeled petioles of Gunnera tinctoria is reminiscent of rhubarb consumption in Britain. Non-edible uses of Gunnera manicata include root tannins being used as a black dye, while the leaves have apparently been used as coverings for roofs and as makeshift umbrellas. Despite their superficial leaf and ethnobotanical similarities, Gunnera and rhubarb are not closely related to each other.
All Gunnera species form a symbiosis with Nostoc, a genus of nitrogen-fixing cyanobacteria which also form symbioses with some cycads and the water fern, Azolla. This relationship is the only one of its kind known among angiosperms. Nostoc enters the plant through glands below the cotyledons and inhabits the intracellular spaces in the petioles. High soil nitrogen is thought to be the explanation of why intercropping with Gunnera macrophylla can produce impressive yield increases in brassicas.
Fern K 1997. Plants for a future: edible & useful plants for a healthier world. Permanent.
Lloyd C 1970. The well-tempered garden. Collins.