Rhododendron ponticum: a garden beauty and an ecological disaster

An evergreen shrub with colourful flowers varying from light pink to shades of purple seems an attractive purchase to brighten up gardens. The Victorians certainly thought so as Rhododendron ponticum, introduced to the UK from Gibraltar in 1763, bloomed in popularity as an ornamental shrub. Little did they know that they were planting what would grow to be one of the greatest threats to temperate rainforests across the UK. This continues to be the case as it is still imported into the UK and is readily available to buy from garden centres, but illegal to allow it to escape into the wild.

There are over 1000 rhododendron species, but only R. ponticum is classed as invasive in the UK for its negative environmental, ecological and economic impacts on semi-natural woodland, some open habitats and temperate rainforests. It is costly and time consuming for landowners to manage, costing the UK £6.2 million each year. Invasive species are one of the top five drivers of biodiversity decline, and the presence of R. ponticum in some of our most delicate habitats is no exception.

The rapid growth of thick, dense stands of a R. ponticum bush outcompete native species for light and space, inhibiting natural tree regeneration and leaving the surrounding ground bare of any native plants, including internationally rare species of ferns that live in temperate rainforests. R. ponticum seeds are very easily spread in the wind as the seeds are amongst the smallest and lightest of any plant species, and a single Rhododendron bush can produce over 1 million seeds per year. The potential for shading and outcompeting rare species that make our habitats unique is becoming more than a concern, it’s directly impacting the biodiversity crisis.

It is also a host to the tree disease Phytophthora ramorum which is responsible for killing thousands of larch trees and Phytophthora kernoviae, a potential threat to pedunculate oak. Additionally, grayanotoxanes in the leaves, roots and stem of the plant are poisonous to humans and animals. Visiting bees can carry the toxins back to the hive, potentially leading to short-lived intestinal and cardiac problems under the name of ‘mad honey disease’ for those who consume the honey.

R. ponticum has taken a hold of the wetter west of the UK, including the rare and already vulnerable fragments of temperate rainforests which once covered up to a fifth of the country and now exist only in small pockets occupying less than 1% of Britain.

Here R. ponticum chokes vegetation and blocks the characteristic plethora of lichens and bryophytes that this habitat usually exhibits. The consequences of this are devastating for biodiversity and greatly reduce the woodlands capacity to capture carbon and mitigate climate change. Our precious pockets of lush temperate rainforests could be reduced to a mere memory if action isn't taken.

Landowners are incentivised to remove this plant and can receive payments from the Countryside Stewardship Higher Tier Grants between £3,500 to £5,500 per hectare, depending on the slope of the site and average height of the plant. However, these government capital resources will be wasted if control methods are not coordinated and delivered at large enough scale without a clear plan to prevent re-invasion in the future. Even if rhododendron has been effectively suppressed in one area, if neighbouring land contains untreated rhododendron, it will spread back into the treated area. Rhododendron is characteristically persistent and after initial treatments the area will need monitoring and re-treatment as time goes on.

Managing the existing pressures of R. ponticum and other invasive species such as deer and squirrels will help improve woodland resilience and give these important habitats the best chance of survival against other threats including chronic overgrazing and climate change.

To give rainforests the chance to thrive, UK Government must:

• Recognise that INNS need to be addressed at landscape or catchment scale rather than by estate or landowner levels and provide the funding for this. For example, providing funding for projects that involve multiple landowners, and long-term funding to allow for multiple rounds of control.
• Develop a coordinated National Strategy to manage R. ponticum.
• Fund National projects that address the threat of rhododendron ponticum on a wide scale
• Conduct research on the impacts of R. ponticum in the UK. For example, the extent that this plant can escape gardens and spread into the surrounding environment, and to assess to what extent the invasive ponticum in the UK might be different to that present in other regions.
• Make powers of enforcement more widely known and used where necessary.

Individual members of the public can help to protect rainforests by Being Plant Wise and choosing not to buy or plant Rhododendron ponticum in their gardens. Removal of this plant, and even avoiding it all together, need not bring down the wonder of your garden. Beautiful garden alternatives include any hardy shrub such as: Azalea, camellia, flowering currant, daphne, viburnum. Any of these are worth their weight in species richness and for the benefit of the natural environment. 

Lisa Manning is a Policy & Engagement Officer at WCL and supports the Invasive Non-Native Species group and the Trees and Woodlands Group. Erin Shott is Communications and Policy Officer at Plantlife. Follow @wcl_news and Plantlife @Love_plants

The opinions expressed in this blog are the authors and not necessarily those of the wider Link membership.