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The UK government has added the Brazilian giant rhubarb to the list of banned species, which means that it will soon be illegal to sell or cultivate it. The Guardian reports that it will also be the responsibility of those who have it in their gardens to prevent it from spreading. It has very large leaves with feathery edges and long thick stems, and often grows at watersides.
Giant rhubarb is native to South America, and is known by the botanical name of Gunnera. There were thought to be two large leafed species of Gunnera in British gardens, the Brazilian giant rhubarb (Gunnera manicata) and the Chilean giant rhubarb (Gunnera tinctoria). They are difficult to distinguish but the tinctoria, which has spread into the wild and colonised extensive areas of Ireland and the wetter areas of the west coast of the British Isles, has been banned as an alien invasive species since 2017 due to its very vigorous nature.
It has been assumed until recently that all large leaved Gunnera cultivated for sale were the Brazilian variant, however following recent research the RHS has now declared that this species is likely to have been lost to cultivation a few decades after it was introduced in Victorian times, and that what were thought to be Gunnera manicata are in fact a new undescribed hybrid of the manicata and tinctoria which they have named Gunnera x cryptica.
Under the invasive species regulations a species includes any form of that species, including any hybrid with it, so a hybrid is automatically classified as invasive and included in the ban. Consequently any Gunnera manicata is assumed to be the new hybrid, x cryptica, and is now banned from sale or cultivation.
Invasive species can, in most cases quite rightly, be a cause for alarm. However, in the case of the manicata/x cryptica you should not panic if you have one in your garden as they produce few seeds, are rarely found outside gardens and are not known to spread vigorously.
The RHS has advised that the species should no longer be bought or cultivated -though they are seeking clarification from Defra as to the precise meaning of cultivation in this context. Existing plants can remain so it is only new plantings that must be avoided. Any excess material from the plant, for example when it is cut back in the autumn, should either by burnt on site or taken to your local green recycling centre.
John David, head of horticultural taxonomy at the RHS, said: “When we began our research we were using Gunnera to look at the difficulties in differentiating between an invasive species and its non-invasive close relatives.”
He added: “It was therefore 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.”
“This would not have been possible without the help of botanists in Brazil and Chile, where these plants originate, as well as many others who provided material that enabled us to prove the disappearance of one species and the discovery of the new hybrid.
As noted above this newly classified hybrid is unlikely to be a problem as its growing characteristics appear to derive more from the manicata than the tinctoria. As a general rule invasive plants are non-native species that quickly disrupt and outpace the growth of native vegetation. These plants grow profusely to the point where they choke out the oxygen and sunlight from native species and threaten ecosystems and wildlife habitats. They may have no natural control mechanisms such as disease, climate, or a source of food for herbivores.
If left unchallenged, invasive species can block waterways, making them difficult for vessels to navigate and leading to flooding and pollution. They can choke fish and disrupt their spawning beds, leading to declining populations and stifling the growth of native plants.
When growing near water, invasive non-native plants can also damage soil by soaking up too many nutrients and altering the chemical composition, making it more difficult for native species to grow. When they die back in winter, they can leave river banks vulnerable to erosion.
It may sometimes take many years for it to become apparent that a non-native plant is invasive, by which time it has taken a stranglehold and is difficult, expensive, or even impossible to eradicate. According to the Royal Horticultural Society (RHS) there are 1,402 non-native plant species in the UK, of which 108 have a negative environmental impact.
There are a number of regulations in place to control the introduction and spread of invasive species in the UK. In England and Wales, the Wildlife and Countryside Act 1981 lists non-native alien species that are covered by the restrictions. It is illegal to import, sell, keep, breed, transport or cultivate any of the species on the list.
Some of the most common invasive aquatic species include the following:
As the name suggests, this plant is native to the Himalayas. It has hollow stems that are a pinkish red colour and long slender green leaves. Between June and October, it produces pale pink or purplish pink flowers, and the plant can grow up to three metres tall.
It spreads rapidly and can overtake whole stretches of waterway banks, leaving them vulnerable to erosion when it dies back in winter. It can be removed by pulling it up from its roots and it should be burnt or composted if not in flower.
Floating pennywort (Hydrocotyle ranunculoides) is native to the south of the USA, and was introduced to the UK as an ornamental pond plant. It has shiny kidney shaped leaves with crinkled edges that grow to a width of around 7cm, and sometimes produces tiny white flowers at the height of summer.
When left unchecked, it forms dense mats on the surface of the water and can deplete oxygen levels and choke out native plant life. It grows profusely, spreading by up to 20 cm per day during the summer months. Even tiny fragments of plants can lead to new growth, so it is very difficult to control because even when removed new growth can easily take hold.
Other aquatic plants that are banned include Parrot’s feather (Myriophyllum aquaticum), Water hyacinth (Eichhornia crassipes), Water fern (Azolla filiculoides), Curly leafed waterweed (Lagarosiphon major), Water primrose (Ludwigia grandiflora/ uruguayensis), Water lettuce (Pistia stratiotes) and Australian swamp stonecrop (Crassula helmsii) .
If you find that you have unintentionally got a banned species growing in your garden, you will not face any legal action if you dispose of it correctly. This means removing every fragment of the plant that you can, and depositing it in a sealed garden waste bin or taking it to a recycling facility. Never dump the plant in the wild or give it away to anyone else.