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The problem with Rhododendron ponticum: a garden guide

A threat to Scotland’s landscapes and biodiversity: the invasive Rhododendron ponticum
In 2020 the Trust began a nationwide project, Project Wipeout which aimed to free our natural environment of invasive plant species. One of the plant species targeted by the project is Rhododendron ponticum.

Why is Rhododendron ponticum a problem?

The Trust has some wonderful collections of rhododendron cultivars (def: a variety of plant produced by selective breeding) within their garden properties across Scotland. It is only the vigorous hybrid of Rhododendron ponticum (R. ponticum) that has become a major problem.

Rhododendron ponticum is a non-native invasive species in Scotland.

It is neither vigorous nor invasive in its natural habitat, parts of the Iberian peninsula, eastern Europe and west Asia. However, cross breeding with frost-hardy rhododendron species from north east America has produced hybrid vigour and helped the plants adapt to colder temperatures. Together with its fertility and tolerance of shade, the plant has been able to thrive in our climate and out-compete native flora.

Rhododendron became popular in policy woods around estate houses during the Victorian period. It was widely planted as a garden ornamental, and used as a shelter belt and for game cover. It was also a popular root stock for the many hybrid varieties of rhododendron. The pattern of invasion usually centres around these estates and some settlements.

“Almost half of the core areas of Scotland’s rainforests are being choked with R. ponticum.”

How does Rhododendron ponticum spread?

This invasive shrub spreads by prolific seeding and then layering of branches. This eventually forms a closed canopy that shuts out most light and destroys the rich biodiversity of our woodlands. It threatens and/or causes trouble for a variety of landscapes and habitats (including ours!):

  • Scotland’s rainforests – The Trust is one of more than 20 voluntary partners that represent the Alliance for Scotland’s Rainforest. This coastal temperate rainforest is globally rare and Scotland has some of the most diverse and best remaining woodland of this type in Europe.
  • Woodland habitats – Protecting and restoring the native woodland is a major conservation priority. One of the greatest threats to these woods is the highly invasive R. ponticum.
  • Waterways – Along watercourses the overhanging bushes cast dense shade and suppress algal growth. Slow decomposing leaf litter degrades the soil and effects invertebrate abundance and biodiversity, so damaging the ecology of freshwater ecosystems.
  • HeathlandR. ponticum seeding out from woodlands rapidly spreads to the heath and bogs beyond. The damp acidic soils here provide a perfect seed bed for the invader!
  • Grazing and livestockR. ponticum is toxic to most herbivores such as sheep and deer. Hungry deer have been known to die after browsing its leaves. R. ponticum will colonise pastures and reduce grazing opportunity for livestock.
  • Biosecurity – Rhododendron plays host to Phytopthora pathogens that can spread and kill a number of tree species.
  • Public spacesR. ponticum encroach on footpaths, riverbanks and other beauty spots.
“There have been cases where people have had to be rescued after becoming lost in a rhododendron jungle – here in the British uplands!”
R. ponticum will soon swallow this bus shelter | Image © Manta Ecology

Is Rhododendron ponticum bad for your garden?

Not only can it rapidly overruun your garden, but gardens with R. ponticum also provide a huge seed source for invasion.

To help control and eradicate these sources, the National Trust for Scotland is offering some residents a free ‘Community Garden Scheme’ to remove their R. ponticum with an option to replace them with other hardy and beautiful plants.

Torridon house | Image © Manta Ecology

One garden owner’s experience

A householder had planted R. ponticum as a shelterbelt over 20 years ago. The plants rapidly overran much of their garden and began spreading up into the heathland above the house.

Control works were carried out (stem and stump treatments). Some of the cut plants were chipped and the rest used for firewood. The invasive plants were replaced with sterile rhododendron cultivars chosen by the householder.

The garden is now clear, with the burn free to flow and providing a lovely feature. Surveillance and follow up will ensure regenerating R. ponticum will not re-infest.

“[It] was very effective and unobtrusive... and the work has made a big difference. We are very happy with the results.”
Torridon householder

How to control Rhododendron ponticum

Years of hard won experience have given practitioners a toolbox of techniques for tackling R. ponticum in any given situation. Different techniques have been assessed and honed for different plants, ground conditions and habitats.

  • Stem treatment – The Trust has been one of the main advocates of stem treatment. This is our preferred method and we have promoted the technique as widely as possible by sharing good practice and training. It has many advantages and works well in combination with grubbing or rooting out of smaller plants. Best for medium-mature bushes with dense mature stands, you apply a small amount of herbicide into a reservoir drilled into the woody stem of the bush. It can be very effective, with reports of 99% kill from a single treatment, ground disturbance is minimal, and there is minimal damage to non-target plant species and low risk to animals. The investment in tools and training is lower than for any other method.
  • Grubbing up – Where drilling stems is impractical, i.e. where the plant is too small or layered, the plants and root system can be ‘grubbed up’. Best for seedlings and small plants, it requires minimum training and is doable in all weather conditions. It’s only downside is that it’s easy for some root stock to get left behind.
  • Stump treatment – This method is best used for heavily infested areas with impeded access, where the bushes are medium-mature with dense mature stands. It’s particularly useful for previously cut plants with multiple stems. After cutting plants down to near ground level, stumps are drilled on all sides and solution injected into the reservoirs. This method quickly clears areas required for amenity or grazing. However, it is common for stumps to regenerate, so they must be revisited regularly. It also requires a chainsaw license and skilled operative, is labour intensive, and cannot be applied in wet weather.
  • Foliar spraying – The foliage of plants is sprayed by knapsack: herbicide is applied directly to the leaves ensuring complete coverage. This works best on very dense, low growing multi-stemmed plants, and it covers large areas quickly. There are disadvantages though: collateral damage to other vegetation, and the risk of run-off into ground water or water bodies. And for the person doing the spraying, careful use is imperative, and it’s a heavy knapsack when you’re navigating rough terrain.
  • Lever and mulch – Even some of the larger plants can be removed by hand, using the technique known as ‘lever and mulch’. Best for medium-large plants, the technique involves dismantling the rhododendron bushes using the plant’s own stems as levers, and then covering the place they were growing with their remains to exclude light and prevent re-growth. There’s no use of chemicals with this method, but it is very labour intensive. Piles of mulching plants can also hide regenerating R. ponticum.

Any control programme must also set aside resources for a follow up phase.

The first phase of control will always miss layers and seedlings tucked away! There are always new seeds waiting to grow. Remaining plants will be easier to spot after the first phase. Typically, follow up surveillance and control needs to be kept up for at least five years.

Sustainable solutions

Treated plants can be stacked until they are dry enough to cut for firewood. It has also been used to make charcoal and biochar.

The Trust has turned a problem into a solution by adopting a sustainable land management system at Inverewe. Wood from the invasive rhododendron is being used to produce biochar in a special retort kiln. The matrix structure and hydroscopic qualities of biochar trap moisture and nutrients, increase microbial activity, and lock carbon. Mixed with compost the biochar is then fed back to improve the soil that the invasive rhododendron has impoverished.

Read more: From invasive threat to environmental opportunity – Inverewe Garden’s biochar project

The special retort kiln used to produce biochar from R. ponticum at Inverewe Garden

Inverewe Garden is also producing ‘eco charcoal’ with the help of the Highland Conservation Volunteers and local volunteers, under the supervision of Estate Supervisor Aidan Bell. The charcoal kiln can produce up to 25 bags, using an ingenious sorting tray built and invented by the Trust volunteers.

Charcoal was first produced in the 1600s in vast amounts to supply the nearby ‘Red Smiddy’ blast furnace on the River Ewe. This work represents a return to local charcoal production after 400 years, but will most likely be used to fire the humble barbecue, and will be using invasive R. ponticum rather than the native oak trees that were once used.

Project Wipeout

As well as the ground work eradicating and controlling R. ponticum, Project Wipeout’s success also depends on collaboration and coordination. Educating and engaging with local communities is one of the most important elements of any successful control programme. Face to face contact, publicity materials, community workshops and school visits give a sense of involvement and ownership to local people.

Follow our work across Scotland

Project Wipeout: controlling invasive plants

Project Wipeout spreads south

Project Wipeout has arrived in the North East

This story was based on the presentation: Stemming the Tide, Protecting Scotland’s native habitats from the invasive non-native Rhododendron ponticum, by Nature Conservation Advisor Rob Dewar. With thanks to Manta Ecology.

Project Wipeout is supported by players of People’s Postcode Lottery, NatureScot and Baillie Gifford.