Day of the Triffids: Giant rhubarb plants nicknamed 'dinosaur food' because of its size will be BANNED after scientists realise it's an invasive hybrid similar to Japanese knotweed

With its prickly leaves spreading up to nine feet, giant rhubarb has long been a favourite in stately homes and big gardens.

Now, however, it is to be banned from sale in Britain after research found it is a danger to native plants.

Experts say the species from South America – nicknamed ‘dinosaur food’ because of its colossal bulk – ranks alongside Japanese knotweed as a ‘plant of concern’.

Gardeners who already have giant rhubarb in their flower beds must ensure that it does not escape, and it must not be newly planted or cultivated.

It had been thought that the giant rhubarb commonly grown in Britain was a variety called Gunnera manicata which did not spread. A type called Gunnera tinctoria is already banned.

But researchers from the Royal Horticultural Society have found that nearly all giant rhubarb in the UK is a hybrid variety, Gunnera x cryptica, resulting from a cross between manicata and tinctoria.

The hybrid has the potential to be highly invasive. In parts of Cornwall, Devon, Scotland and Ireland it suffocates other plants in mires and heaths and along waterways.

When it grows on soft coastal cliffs it can cause erosion because its great weight causes the cliff to crumble, carrying soil and rock with it.

It spreads via seeds and rhizomes – root-like growths that spread just under the surface of the soil to find new areas to colonise. Their roots grow around three feet deep.

Advice from the Department for Environment Food and Rural Affairs is to treat it as a banned species, so the RHS is changing its own advice to gardeners. The society’s research was published in the journal Sibbaldia.

John David, head of horticultural taxonomy at the RHS, told the Guardian: ‘It was a surprise to find that a plant that has been a firm favourite in our gardens for its impressive size and exotic appearance, turned out to be an undetected hybrid.’