The discovery of a new Gunnera hybrid means that large-leaved Gunnera (giant rhubarb) are now considered invasive and are set to be banned. What does it mean for historic gardens and gardeners?
The UK Government is likely to impose a ban on the sale and cultivation of giant rhubarb, or Gunnera, because it was discovered that a vigorous new hybrid has been growing in gardens undetected for many years.
Until now, it has been believed that the large Gunnera plants growing in the UK were either Gunnera manicata or Gunnera tinctoria (already classed as an invasive non-native plant). Recent research has found that plants that were thought to be Gunnera manicata are actually a hybrid of Gunnera tinctoria, called Gunnera x cryptica. As it is a hybrid of Gunnera tinctoria, it is also likely to be classed as an invasive non-native plant.
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- More on why gunnera may be banned in the UK
- What to do if you have gunnera in your garden
- What’s a non-native/invasive plant
- What does a gunnera ban mean for nurseries?
Gunneras are much loved for their huge, exotic-looking leaves that can measure up to 4m in height in summer. Introduced by the Victorians, these moisture-loving bog plants are a familiar sight in many historic gardens, parks and botanical collections, and are often found growing alongside lakes, ponds and streams. Their ban will have implications for historic gardens, nurseries and gardeners.
I have a gunnera in my garden. What should I do?
If an invasive non-native plant is already growing on your land, the view of DEFRA (the Department for Environment, Food and Rural Affairs) is that the plants are not considered to be intentionally kept or cultivated. So you’re not committing an offence if a large-leafed Gunnera is already growing on your property. However you should refrain from planting or cultivating new plants.
Guy Barter, chief horticulturalist at the RHS, explains: “It is not illegal to grow Gunnera and if it poses no risk of spread, it can remain. Where it poses a risk of spread, near streams, say, or in areas of boggy ground, it would be best to eliminate it, taking care to destroy it by burning or composting all plant material in the garden, or less environmentally soundly by adding to landfill waste. Where this not feasible, removing seedheads before seed matures would be a good practice.”
Reports in the press have suggested watering plants on dry soils amounts to “cultivation”, but Guy says: “The notion of withholding water to kill existing Gunnera can be disregarded as a journalistic misinterpretation.”
Small species of Gunnera, such as Gunnera magellanica (devil’s strawberry), a marginal pond plant, will not be affected by the ban.
Why is the new Gunnera ban being imposed?
Until now, it has been believed that the large Gunnera plants growing in the UK were either Gunnera manicata or Gunnera tinctoria. Both plants were introduced in the 1800s. Gunnera tinctoria was listed as an invasive non-native plant in 2017 as it has become a serious problem in wetter areas such as the west coast of Ireland and Scotland, but Gunnera manicata, which hails from Brazil, was not considered problematic – until now.
Recent studies by the RHS, which involved molecular and morphological analyses, as well as a historical investigation, revealed that Gunnera manicata appears to have been lost from cultivation not long after it was introduced in the 1860s. In its place, the researchers found a hybrid between Gunnera manicata and Gunnera tinctoria, which has been named as Gunnera × cryptica. The researchers concluded that all large-leaved Gunnera species in Britain and Ireland today are either Gunnera tinctoria or Gunnera × cryptica, the previously overlooked hybrid plant.
The two parent species and the hybrid look very similar, which may explain how the hybrid went undetected for so long. As Gunnera manicata disappeared from cultivation, gardeners were unknowingly selecting the hybrid plant as it was hardier and more vigorous. DNA research conducted by scientists at the RHS has now uncovered the true identity of the plants. The RHS will now be changing its advice to gardeners concerning Gunnera manicata, which may either be the hybrid or Gunnera tinctoria.
Read more about why plant names keep changing
What is an invasive non-native plant?
There are over 35 plants that are deemed to be invasive non-native plants in the UK as they spread rapidly and threaten ecosystems, habitats or native species. They include Japanese knotweed, Himalayan balsam, water hyacinth and giant hogweed. It will be an offence to sell or plant Gunnera tinctoria and Gunnera × cryptica or cause them to grow in the wild. To find out more about invasive plants and the rules surrounding them, go to www.gov.uk/guidance/invasive-non-native-alien-plant-species-rules-in-england-and-wales
What does the Gunnera ban mean for historic gardens and plant nurseries?
Currently the law prohibits the importation, selling and cultivation of any hybrids of a listed invasive species and therefore the discovery of Gunnera × cryptica means that there will be significant implications for the horticultural industry and plant collections in the UK.
The RHS is in the process of identifying any Gunnera x cryptica plants within its five gardens and where it feels there is a strong case to retain the plants, will be positioning appropriate information nearby to explain the status of the plants and advise gardeners not to plant it in their gardens. It is also removing any plants labelled as Gunnera manicata from its garden centres. We’ll keep you posted on developments.