Rhododendron invades areas both vegetatively and via seed. Established plants spread by lateral horizontal growth of the branches. A single plant may eventually end up covering many metres of ground with thickly interlaced, impenetrable branches. Where the horizontal branches touch the ground, they will root, continually extending the area of Rhododendron cover.
Rhododendron is capable of extending into areas which it would otherwise be expected to fail. For example it has been known to dominate wet land with its extensive canopy while the roots remain on dry land.
Toxicity of Rhododendron
Potentially toxic chemicals, particularly ‘free’ phenols, and diterpenes, occur in significant quantities in the tissues of plants of Rhododendron species. Diterpenes, known as grayanotoxins, occur in the leaves, flowers and nectar of Rhododendrons. These differ from species to species. Not all species produce them, although Rhododendron ponticum does.
These toxins make Rhododendron unpalatable to most herbivores. Phenols are most concentrated in the young tissues, such as young emergent leaves and buds. This provides a primary defense against herbivores, before the tissues have acquired the added deterrent of physical toughness found in older tissues. Young emergent leaf buds have the additional protection of a sticky exudate which also contains phenols. This physically discourages small invertebrates from eating the buds, because they get stuck in the exudate. Its poisonous nature must act as a further discouragement.
Grazing animals are discouraged from eating Rhododendron foliage because of its toughness and unpalatability. The unpalatability is learned and cases of poisoning may result in animals such as sheep and cattle if they ingest sufficient quantities because of extreme hunger or inexperience. The general toxicity of Rhododendron to herbivores means that it cannot generally be controlled by grazing.
Cases of human poisoning are also known. Most are caused by the consumption of honey produced from Rhododendron flowers. This is known as ‘Mad Honey Disease’, or ‘Honey Intoxication’. 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 depends on the amount of contaminated honey consumed. It is worth thinking carefully about the siting of bee hives if Rhododendron is a prominent feature of the area.
It is a threat to native species of animal and local fauna. IWM have carried programmes of treatment and eradication for the forestry commission as well as private contractors.