Not to be semantically confused with : Rhubarb [a plant with similar name]
Easily mis-identified as : Brazilian Giant-Rhubarb, but that is larger still, with leaves often larger than 2m across, and has reddish-tipped bristles and spines on the leaf stems rather than green. The flowering spikes (really upright panicles) of Brazilian Giant-Rhubarb are also longer at up to 1.2m rather than less than 1m.
The large leaves have some similarities in shape to those of Giant Hogweed for which it may be mistaken, but the stems of Giant Hogweed lack the short stubby curved spines.
The leaves bear some some similarities to those of : Butterbur, but are far larger. Leaf stalks are hollow, but this has no diagnostic value.
It is native to Brazil, but is grown in the UK as a garden plant, mainly for big gardens such as parks, in which setting you are much more likely to see it. When planted in the UK, it does spread a little, but does not behave rampantly like it does in its native country, where it is in places out of control! Little else can take root under the darkness cast by its enormous leaves.
Like Brazilian Giant-Rhubarb the plant produces huge erect catkin-like panicles of flowers, which are usually a mixture of male, female and bisexual flowers. But un-like Brazilian Giant-Rhubarb the panicles are greater than 4 times as long as wide, although their total length at less than 1m long are shorter than those of Brazilian Giant-Rhubarb which are less than 1.2m long.
As the scientific name, tinctoria, suggests, the plant yields a dye, in this case it is the roots from which a black dye can be extracted which was used for dying leather black. The root contains 9% tannin, and it is likely that the black dye is indeed a mixture of tannins.
The roots contain glands that contain a cyanobacterium called Nostoc, which fixes nitrogen for the plant. It is thought to be the only flowering plant in the world which has entered into a symbiotic relationship with a cyanobacterium; all other nitrogen fixing plants rely on eukaryotic bacteria to fix nitrogen rather than prokaryotic bacteria. The plant gives the cyanobacterium carbon compounds in return for soluble nitrogen extracted from the air.
The seed head produces about 80,000 seeds.