So what the hell is this then ? (Photo by Tom Oates.The original uploader was Nabokov at English Wikipedia. (a.k.a. Tom Oates) – Transferred from en.wikipedia to Commons by Kafuffle using CommonsHelper., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=17669124)

Dear Readers, what a can of worms has been opened this week on the subject of Giant Rhubarb! Usually known as Gunnera, this plant has been a favourite of stately homes for many years. I briefly considered it for beside my pond, before realising that what I’d actually need was a lake about a hundred times bigger than my relative ‘puddle’. But a few days ago, Giant Rhubarb was declared an invasive species, and it is now forbidden to be sold or cultivated. What’s going on?

It was believed that there were two species of Gunnera in the UK. The first, Gunnera manicata, comes originally from coastal areas of Brazil, was believed to be a benign plant with enormous leaves and an  impressive flower stalk. It was widely sold, and I suspect that there are many, many parks with this plant spreading its mighty leaves beside the goldfish pon.

Gunnera manicata flower head (https://www.bethchatto.co.uk/conditions/plants-for-damp-conditions/gunnera-manicata.htm)

The second, Gunnera tinctoria, looks similar but is invasive. It’s been banned from sale in the UK since 2017, and has wreaked havoc in New Zealand.

Gunnera tinctoria (Chilean rhubarb) (Photo by By Stevage – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=5919241)

Well, Readers, it appears that ‘the enemy’ has been hiding in plain sight. Scientists now believe that all the plants we’ve been thinking are the lovely Gunnera manicata are in fact hybrids: Gunnera manicata crossbred with Gunnera tinctoria very early on in the history of the plant in this country to produce Gunnera x cryptica. Gunnera x cryptica is extremely invasive (and is already causing problems in wetter areas such as the west of Ireland and Scotland). Furthermore, there are probably very few Gunnera manicata left, as the hybrid has outcompeted its less vigorous sibling.

In ‘Alien Plants’, Clive Stace and Michael Crawley have this to say:

‘On coastal cliffs, the main impacts of colonies of Giant-Rhubarb are the threat of erosion and the loss of maritime species. Apart from the ecological impacts associated with the loss of biodiversity, there are also landscape impacts, including the reduction in the area of land that is suitable for agriculture and amenity purposes. Dense stands of Giant-Rhubarb growth may also lead to the blockage of drainage channels and increased risk of flooding. Due to the size of the plant, access to sites infested with Giant-rhubarb is difficult, making control measures problematic’ (Pages 470-471)

What to do? DEFRA says that any existing plants can be kept, but no more can be planted, so there will be a flurry of activity in nurseries and garden centres up and down the country. For existing plants who have hopped over the fence, it’s probably down to cutting back and blitzing with herbicide, in much the same way as people are trying to deal with Japanese Knotweed. The plant also needs a lot of water, so is likely not to do well in drier areas.

Yet again, it seems that something imported for its dramatic appearance has turned out to be rather too fond of life outside the garden, with the added complication, in this case, of the plant turning out to not be what it seemed. I shall watch with interest to see what happens next.

Gunnera tinctoria in Chile (Photo by By CARLOS TEIXIDOR CADENAS – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=128385105)