Giant Rhubarb

Gunnera are big. Very big. Well, not all of them: Gunnera herteri stands only 6 cm tall (1). But the big ones are very big. They're old too, with the fossil record showing they have been in existence for at least 95 million years. It is their size, however, that is most obviously impressive. G. mantica has a leaf diameter of 3 m (1). Its not alone in this, with many other of the c. 50 species of this monogeneric family towering, in some cases 6 meters, over any other vegetation.

Gunnera tinctoria

This height has posed a problem in Ireland where the species G. tinctoria was was introduced from its native Chile some time in the past 100 years, probably as a ornamental. While not as big as enormous as some of the Gunnera giants, it still clocks in at an impressive 2 meters tall with leaves 2 meters in diameter. It tends to grow in dense colonies that out-compete and displace native vegetation. It was first recorded wild here by the renowned Irish botanist Robert Lloyd Praeger in 1939 on Achill Island, Co. Mayo and it is here today that the plant colloquially called 'Giant Rhubarb' shows the extent of its dominance (2). Large tracts of the island are now almost entirely covered with G. tinctoria. As such, it is considered an invasive in the west of Ireland where it has colonised many habitats such as grassland, waterways, roadside verges, bogs, heaths and coastal cliffs (3). While its sparse distribution throughout the rest of the country may be reflective of the different climatic conditions there, it may also may just be evidence of the beginning of its invasion. 

Gunnera tinctoria leaf

While the size of G. tinctoria accounts for its adverse effect on other plants, it is its methods of reproduction that account for its invasive nature. As a perennial rhizomatous plant, it can spread easily from year to year once established. It also produced many seeds which may be carried to new sites by birds feeding on them (3). Ireland's wet climate also assists these damp loving plants, which accounts for their status as an invasive in similarly inclement New Zealand. The rhizomes mean mechanical removal may not always be effective, so herbicides such as Round-Up are a valuable tool in controlling the plant. 

Gunnera tinctoria flowering parts

Other than their height, Gunner species also possess a distinct adaptation that singles them out from all other flowering plants. They form symbiotic relationships with Nostoc cyanobacteria (4). The cyanobacteria are uniquely harboured internally by the plants which surround the Nostoc filaments with membranes where they fix nitrogen for the plant. Entry is made via specialised glands on the Gunnera stems. Interestingly it has been shown that G. manicata grown on substrate with sufficient nitrogen did not develop these glands (4).

References:

1. Wilkinson, 2000. Botanical Journal of the Linnean Society 134 pp. 233-266
2. Praeger, 1939. Proceedings of the Royal Irish Academy 45B pp. 231-254
3. Armstrong et al., 2009. Giant Rhubarb (Gunnera tinctoria) Invasive Species Action Plan
4. Chiu et al., 2005. Plant Physiology 139 pp. 224-230