Alissa MacMillan
Q: How worrisome is it to have rhododendron in the garden? I’ve heard it can be quite an invasive plant.
A: There are about 1,200 species and hybrids of rhododendron, says Dr. Therese Higgins, lecturer in Wildlife Biology at MTU Kerry. But only one is problematic: Rhododendron ponticum.
“There are Rhododendrons native to a large part of the world,” Higgins explains, and R. ponticum is found from Turkey to Georgia, along the Black Sea, and in Spain and Portugal. There are many species native to the foothills of Himalayas, which “are dripping with fabulous, lush native forest with rhododendrons. They co-exist with other native vegetation.”
In Iberia, “Rhododendron ponticum is protected because it’s endangered and very rare,” she explains, the drier conditions much less favorable to its growth. “It’s not an evil plant; there is no such thing as an evil plant, only a right plant in right place.”
Rhododendron ponticum is a pest for forestry and horticulture in places like the Balkans – and, as it happens, in the UK and Ireland, where it can take over hillsides and woodlands.
Rhododendron was introduced to Britian in 1763 by horticulturist Conrad Loddiges, the man who brought in rhubarb, which was a big deal because it was a source of vitamin D and fought scurvy, says Higgins. At first Rhododendron ponticum was “loved by the horticulture set because it was more frost-hardy,” although its flowers were considered “a bit gawdy.” Its rootstocks were used to graft less-hardy plants, it eventually becoming popular in its own right, winning horticulture medals.
Rhododendron also turned out to be frost and wind tolerant, animals didn’t eat it, and it was great for shelter. It’s evergreen, which means it can grow in the winter months and “outcompete everything else.”
Where there’s dense vegetation, its tiny seed doesn’t compete well when establishing new plants, but in a woodland grazed by deer or cattle, and especially very humid areas, like in the west of Ireland, it can establish quickly, going toward the light and leaning on mature trees.
It doesn’t flower well unless the soils are acidic, she notes, but will do so happily in those situations, or with bare soil or mossy carpets, which is why “it’s gone bonkers in some parts of Ireland and to an extent, Scotland and Wales.” There are very few insects that eat rhododendron, none to the point of having a negative impact. Bees do feed on its nectar, especially at the gap before summer’s start, but research is raising questions about whether the nectar is good for them.
Either way, Higgins says, too much rhododendron in the wrong habitat can spell the end of biodiversity.
“If we let it grow, there is no food for the birds and it tends to make the ground beneath it a bit drier – even mosses don’t do well under it. Flowery things and grasses are shaded out, mosses are lighted out, tree seedlings can’t establish because it’s so dark, so there are no young trees under there, and trees can’t replace themselves.”
‘KILL THE STUMP’
You might have heard of the rampant rhododendron in Killarney Park, first planted near hunting lodges and woodland entrances. It probably took off, Higgins explains, because deer-hunting patterns changed in the 1800s, when the Herbert family at Muckross House tried to woo Queen Victoria to visit “to save a fortune in taxes and gain a title and seat in parliament.” It took 30 years to get her to come, but they introduced a lot of deer into the park to mimic the style of deer-stalking Prince Albert did in Balmoral. When the hunting ended in the 1900s, “the deer population went mad and opened the way for rhododendron.”
“Not that the rhododendron wouldn’t have expanded,” Higgins explains, “but it wouldn’t be the out and out devastation of rhododendron and nothing else.” As with sheep and cattle, “if you graze down hard any habitat, it makes it much more vulnerable to invasion; a habitat that’s healthy and functional is harder to disturb.”
Rhododendron is difficult to manage, Higgins says. From the “how to kill a plant perspective,” if you cut it down but leave the live stump, it will regrow. As with plants like hazel, it will grow back smaller stems but flower very happily, so, “you have to somehow kill the stump.”
“We used to dig the stumps out of the ground,” she explains. “If it’s small and there are only a few it’s absolutely the best way to do it, with no chemicals, and it works.” The stump might continue to grow but can be burned. The more modern method is using chemicals, says Higgins: you drill into the stumps and spray in glyphosate. Once you damage a plant it will scar, so apply the chemical to a fresh cut.
Spraying the leaves of an existing plant isn’t recommended because you’ll have to kill every branch and often the rain washes away the spray, killing whatever else is around it and putting chemicals into the environment.
If you have the odd plant you might also try stem injection, Higgins suggests: you hatchet the stem and inject glyphosate, leaving behind the dying stick. This works if you have a few plants but too many sticks in the field are a fire risk.
MANAGE THE LAND
“Definitely, as with all invasives, prevention is better than cure,” she adds. “If you can get it before it goes mad, you will have a much easier time.” Rhododendron also doesn’t recognise boundaries, so watch nearby land. As with all invasives, “you can’t just manage the plant, you have to manage the land as well,” Higgins says. “If you have a site that’s heavily grazed it will be harder.”
Officially, Rhododendron ponticum is off the market thanks to an invasive species amendment to the Wildlife Act in 2012, but the one catch is enforcement. Proper garden centres should know their plants and discourage sales of this type.
“The fact that we’re putting more emphasis and value on wildlife is good for the battle against invasive species generally,” Higgins adds. While you “have to know the biology of what you’re dealing with before you touch it,” at this point, “we are much better at sharing knowledge, practical skills, and know-how than ever.”
Because, if they aren’t suppressed, leaving space for a healthy, thriving, diverse range of plants and wildlife, “fast forward 200 to 300 years, in certain places, we will just have rhododendron.”
