The researchers from the James Hutton Institute, the University of Aberdeen and Scottish Natural Heritage studied plots in the Atlantic oak woodlands of Argyll, Kintyre and Lochaber on Scotland's west coast, comparing plots that had never been invaded with others covered in dense rhododendron thickets, plus a time-series of sites cleared of rhododendron at different periods between 1984 and 2014.
In the study, published in the Journal of Applied Ecology in August, ecologists showed that even 30 years after rhododendron removal, the native understorey normally found in the Atlantic oak woodlands they studied had not recovered. Instead of primroses, violets, wild garlic, ferns and grasses, as would be expected in that environment, only dense mats of mosses and liverworts have returned.
They warn that reseeding may be necessary for the natural environment to have a chance of getting back to its pre-rhododendron state.
Waiting decades for the land to recover, or even reseeding and waiting for natural regrowth, is not an option in our public gardens that have had to remove rhododendron. So how can head gardeners ensure that the estates in their care are not left scarred for years?
National Trust property Glendurgan Garden in Cornwall is one of those that had to take action to clear rhododendron infected with sudden oak death (Phytophthora ramorum) 10 years ago. "We were one of the early gardens where it was cleared," says garden manager John Lanyon, who also manages nearby Trelissick. "It was one of the areas where it was first recognised. We cleared about 120 plants, some covering big areas."
Rhododendron ponticum, which was originally from Armenia and came to England via Spain, was planted across the great estates of the UK in the 18th and 19th centuries, often for the purposes of hunting. The shrubs provide good pheasant cover and a windbreak under the trees. "They planted big areas of it. It nearly spread everywhere," adds Lanyon.
Although the job was nothing like the hundreds of hectares that were subsequently cleared in the South West in natural and semi-natural woodland areas, removing the rhododendron still had a marked effect on the garden.
"It had quite a big impact on how the garden looked," says Lanyon. "There were big areas of ponticum, they’d been there for a number of years and they’d just wiped everything out.
"It created a barren landscape, it’s a bit like the moon. There was no species diversification. If you haven’t got any infection you’ve still got a big crusty layer, it’s so dominant. The crust goes 30cm deep if not more. It’s better to get rid of it."
Recovery "is about cultivating the ground, getting it into the condition where we can re-establish plants", he adds.
Gardeners follow a typical horticultural approach, says Lanyon. "The indigenous soil is often buried under layers of leaf mould and roots. It’s like a big crust of organic matter. If it’s an infected layer, it’s important to get rid of that layer and burn it."
Head gardener at Goodwood Luke Sargent agrees that removing the infected or simply affected topsoil is a good idea. "They do tend to produce a lot of leaf fall. You need to remove several inches of topsoil. They make conditions for themselves," he says.
Lanyon recommends cultivating the ground and then digging in bird, fish and bone. "It was a big job in the past, but these days you do it with a mini-digger," he adds.
Gardeners who remove Rhododendron ponticum infected with Phytophthora ramorum are not allowed to replant with anything on the list of species susceptible to the disease. Luckily, the palette of plants the National Trust uses at both Glendurgan and Trelissick is diverse, says Lanyon.
Gardeners also cleared areas of laurel in the estate’s more natural areas. "Laurel was often planted as a windbreak too, it’s really good at it," he says. "The trouble is it is too good at getting established. It’s just as bad if not worse than ponticum and it doesn’t allow anything else to regenerate."
Lanyon recommends felling the laurel and then using tree taps with glyphosate to kill the laurel stumps in a targeted fashion. "Replant with native woodland, hazel, holly and hornbeam. If you plant them at the right time of the year they get going," he adds.
Chief executive of conservation charity Buglife Matt Shardlow also recommends native species to replace invasive rhododendron as good for both the soil and the local wildlife, including invertebrates. "Look at what’s native, find out what species of plant live in nearby woodlands and sites of specific scientific interest and find out what type of plants are there," he advises.
"See if you can source some of these from a local seed bank." Shardlow recommends using locally sourced seeds — "stuff that will grow a much richer herbaceous layer with lots of flowers".
The Journal of Applied Ecology report has also had an unintended effect, according to senior garden consultant Alan Sargent, who recommends "putting a lot of lime on the ground to try and make it more pH neutral".
He says he has been getting requests from wealthy private clients to clear rhododendron from their land after the story was published in The Times.
"I think it would be wrong for the public to get the idea that all rhododendron is dangerous though because they are not. It is ponticum that is the problem," adds Sargent.
Head of gardens and landscape at English Heritage John Watkins agrees that rhododendron can be fine if well-situated and managed. The National Trust has cleared thousands of hectares in the South West where conditions are warm and humid, but English Heritage has not had many problems because many of its properties are located in the drier east.
"Most of our properties that do have rhododendron are quite important historical collections and so they are managed. At Kenwood and Chiswick and even Wrest Park we have them and they are not a problem.
"We’ve got hybrids planted in the early 19th century and they grow quite slowly. This is in ornamental areas and we are trying to maintain that hybrid for its cultural history."