Rhododendron ponticum: the silent killer of Scotland's rainforest

Wednesday, 01 February 2023

Scotland is home to a range of rare temperate rainforests.

These are woodlands native to the west of Scotland which are made up of birch, hazel, ash, oak and pine trees. They're rich in biodiversity and capture vast amounts of carbon.

Yet they're under threat.

In the late eighteenth century Rhododendron ponticum (referred to simply as rhododendron in the rest of this blog) was introduced to the UK as an ornamental plant that provided cover for game and produced vibrant flowers.

To most, they're pretty and add colour to the West Highlands. However, these woody plants are creating ecological deserts in our semi-natural woodlands and on some open habitats. They are destroying biodiversity across Scotland, but they're especially hitting the rainforests of the west coast.

Rhododendron overpowers native rainforest vegetation, robbing other plants of nutrients, moisture and, most importantly, light. The result is a cold, dark and bare woodland floor. Many rare plants in these rainforests live on the bark and branches of native trees, which are killed by heavy shade.

Stem Injection 6

The dark understory of a Rhododendron bush showing no vegetation

How we're tackling rhododendron

We are a proud member of the Alliance for Scotland's Rainforest, a coalition of partners from NGOs and government agencies working together to help save this important habitat. Together, we're trying to remove rhododendron from our woodlands, which is no easy task.

A strategic approach is needed to control rhododendron at the landscape scale, and all neighbours need to be involved for this to be effective, as we need to remove any seed source from the area.

Killing a rhododendron bush can also be challenging, and this tenacious species is capable of regrowing after being controlled. We need to kill every part of the plant above the roots. Otherwise, the plant will return to dominate the habitat just as before.

One method of control we're becoming increasingly interested in is stem injection, which is being promoted and effectively used by the National Trust for Scotland.

As the name suggests, this technique involves drilling a hole in every stem and injecting a small amount of herbicide. This method is a practical and targeted way to tackle this invasive species that doesn't harm the surrounding habitat.

Stem Injection 1

Drilling a hole into the steam of a rhododendron plant for a controlled injection of herbicide

How you can help

Rhododendron ponticum is still available in garden centres. Try not to plant it in your garden if it's anywhere near semi-natural habitats – particularly if you live in the west of Scotland and other parts of the UK.

Many people are not aware that Scotland is home to hugely biodiverse rainforests, so sharing them with others is also a great way to help spread awareness of them and make others value this precious habitat the way we do.

For more ideas, visit the Alliance for Scotland's Rainforests website.