Winter Hill: Rhododendron

Rhododendron removal 14th January 2024


There are an estimated 15,000 invasive species in the UK, 49 of those species are considered to be harmful to wildlife, and one of those species makes up one of out bread and butter tasks – Rhododendron ponticum. People ask ‘Why do you kill those plants, they’re nice, they have pretty flowers?’ Pretty flowers.. hmm… let’s see

The name Rhododendron translates as “Rose Tree”, ponticum refers to the plant’s home territory around the Black Sea (Latin name Ponticum Sinum). It is a member of the Ericaceae family, the same family as heather and bilberry, which explains why it likes acid soils.

Rhododendron was first introduced to Britain in 1763 from Gibraltar and was soon firmly installed in throughout the country in the ornamental gardens of stately homes. The fashion for this insidious threat continued up to the mid 1890s with R. ponticum becoming the favoured rootstock for grafting and cultivation. But a series of harsh Victoria winters wiped out many evergreen species, including some of the less hardy rhododendron. The remainders soon reverted back to their wild state, and as many country manors fell into disuse and management of stately grounds declined, the rhodys spread. Armed with an array of phytochemical weaponry and unimpeded by our native wildlife, Rhododendron ponticum soon began to dominate the countryside. BCV have seen the effects of this first hand at sites such as the Anderton Centre and Ravenden Clough, both sites were once the grounds of country estates, both sites were overrun with rhodys once left to their own devices.

Rick 2007
2007 first Anderton resi.

What makes our enemy so deadly? To start with its roots, stems, leaves, flowers, pollen and nectar are loaded with grayanotoxins, a cocktail of phenols and diterpenes that have a range of toxic properties including anti-viral and anti-fungal properties. How R. ponticum employs these chemicals is still open to debate. One theory is that the grayanotoxins kill the mycorrhizal fungi the grows on the roots of any competing plants, preventing those plants absorbing essential nutrients such as phosphorus. Another theory is that the toxins kill soil fauna such as worms and microbes which consequently inhibits nutrient cycling and availability of nutrients to other species. The theories surrounding methods of dispersal for grayanotoxins are equally as diverse. Researchers argue that toxins enter the soil via roots; by leachate from decaying leaf litter; by runoff from the plants leaves and stems and also by air. There are other theories that claim the toxic soil effects of rhodys are overstated and the likely cause of their dominance is that they shade out other species and out-compete them for nutrients and pollinating insects.

The toxicity of rhodys is beyond question. In addition to their damaging effects on habitats no native insect can survive nibbling rhody leaves, sheep and horses can die from eating them, and even humans have been harmed. There are records, dating back to 400 BC, of honey made from rhododendron nectar adversely affecting people who eat it. Apparently it has both hallucinogenic and laxative effects on the sufferer, so it’s not something you want to spread on your toast in the morning. Surprisingly the bees are immune to rhody toxins.

What other survival strategies to rhodys use? Well, they can spread both vegetatively, such as rooting at points where branches touch the ground, and also by seed. It takes a rhody between 12 and 20 years to mature and produce flowers, each of those flowers can generate as many as 3000 seeds each, that means a good sized shrub can produce around 7,000,000 seeds per year. The seeds are distributed by wind and can travel up to 500 metres from the parent. Winter Hill near Belmont shows just how effective this method of propagation can be, with rhody seedlings scattered across the hillside many hundreds of metres from the nearest mature individuals.

And the bad news doesn’t stop there. Rhododendron also spread a deadly fungus, Phytothora ramorum or Sudden Oak Death. First identified in California the fungus appeared in the UK in 2003 and is arguably now the greatest threat to our woodlands and heathlands. Rhododendron is responsible for the extinction of 150 native British species in the last 100 years. As more habitat is destroyed by the encroachment of rhododendron the more species suffer- trees, mosses, ferns, insects, amphibians, mammals, birds, nothing is unaffected. Pretty flowers? No.

This task we joined up with the British Mountaineering Council on Winter Hill to do battle with our arch-enemy. We were last up here around 8 or 10 years ago but BMC have been keeping up the fight on their own for the last few yeas. It’s a big job as rhodys have spread extensively across the hillside, and unfortunately the fires that ravaged Winter Hill in 2018 missed this area otherwise our work would have been done.