Rhododendron, the foreign invader smothering our native woodlands

Rhododendron, arguably the most destructive non-native plant in the British isles

The common Rhododendron, Rhododendron ponticum, is widely regarded to be one of the most destructive and widespread non-native terrestrial plant in Britain and has been responsible for the degradation of many of our native habitats, including our temperate rainforest, of which there is very little remaining to begin with.

It was introduced, as were many of our non-native plants, benign or otherwise, around 1760 in the so called ‘age of enlightenment’ when Britain was realising its place in an increasingly globalised world, and became very popular in Victorian times when the fashion amongst the intellectual elite (and financially well off) was to display how worldly one was by commissioning gardens full of exotic plants from far off places.

The Rhododendron, naturally found in the Iberian peninsula, (Spain and Portugal) is one of these plants and proved to be very well suited to our climate, thriving in the woods and landscaped grounds of our country houses and estates, so it became very popular as a quick and easy way of developing a showcase 18th century garden. It grows very fast, out-competes pretty much anything else and found next to no resistance from pests or disease to its take over of our countryside.

Indefatigable and undefeatable

It is rather triffid-like in its behaviour, creeping inexorably across the forest floor it blankets out woods, it also recreates the acidic soils of its homeland too, and the nightmarishly tangled mass it’s branches form block any sunlight from reaching the woodland floor, all in all making it an indefatigable and undefeatable foe to our native plants.

Herbivores, such as deer, which are not usually at all picky about what they graze, even eating Holly leaves if they are hungry, find Rhododendron unpalatable, as do other mammals and invertebrates, this is due to the presence of phenols and diterpenes in the plant, both of these being toxic and bitter tasting natural compounds, (diterpenes give coffee it’s bitter flavour), studies have found that in mature woodland with a significant undergrowth of Rhododendron bird numbers are much lower than without.

Pathogenic vector

Rhododendron also hosts a serious plant pathogen; Phytophthora ramorum, which is a fungus with the potential to attack a wide variety of native woody plants, especially Oaks, where it is the main cause of ‘Sudden Oak Death’ syndrome.

The first indication that Rhododendron has this disease is wilting of the shoots, this then develops into a spreading dark brown infection that moves along the green twigs and into the leaves, which blacken at the base and tips, Defra (the Department for Farming and Rural Affairs) has identified this species as likely to cause significant damage to trees and landscapes if it establishes widely and it is being closely monitored at the moment.

Mad Honey

Cases of human poisoning are also known to be associated with Rhododendron, with most being caused by the consumption of honey produced from Rhododendron flowers. This is known as ‘Mad Honey Disease’, or ‘Honey Intoxication’ and cases of this have been recorded from as far back as 400 BC. It results in relatively short-lived intestinal and cardiac problems and is rarely fatal, the severity of symptoms depending on the amount of contaminated honey consumed. This means that beekeepers have to be very careful about the siting of bee hives if Rhododendron is common in an area.

Removal and eradication

As well as out-competing and poisoning our native plants Rhododendron is also a very prolific seed producer, however, a naturally seeded plant does not flower and produce seed until it is at least 10 years old so this provides a long window of opportunity for preventing a serious infestation.

Several methods can be utilised for getting rid of the plant but as it regrows vigorously when cut some method of stump killing or removal is always necessary, these are the usual procedures;

Firstly stems will be cut with a chainsaw, cutting as close to the ground as possible to remove above ground growth, however, the removal of this alone does not prevent regrowth as the plant will regrow from cut stems and stumps.

One method of stopping this regrowth is by digging the stumps out, this can made more effective by removing all viable roots too.

Another method is to directly treat the stumps with a herbicide such as Glyphosate in a 20% solution, some people will try a variation on this called ‘drill and drop’, where a smaller amount of a stronger solution (25%) is injected into the stump via a hole drilled with a hand drill.

Finally any regrowth and seedlings can be effectively killed by spraying with herbicides such as Glyphosate, of course best practice spraying protocols should always be carefully followed.

A B-H