Rhododendron is a non-native shrub that has been cultivated and widely planted in gardens and woodlands since Victorian times. It has since spread extensively into the wild, particularly in the west Highlands, where it out-competes nearly all native trees and shrubs.
The impacts on the biodiversity of native woodlands are catastrophic, and here we discuss methods by which it can be controlled.
Rhododendron ponticum was first introduced to Britain in the mid eighteenth century. Often planted as a shelterbelt and as cover for game, it quickly forms dense, green barriers that can grow 8m high. It really announces its presence in late spring when the huge pink and purple flowers explode into the landscape. It is this time of year when its true extent becomes spectacularly obvious, for the shrub is no longer just in gardens, but can be seen along roadsides, extending up rocky screes and cliffs, creeping through pine woodlands, snaking along river margins and speckling moorland and mires.
Control of rhododendron can only be achieved if work is carried out and coordinated at a landscape scale and over a period of many years. It is expensive and time-consuming but it is important that we tackle it now. Estimates suggest that, left unmanaged, control costs double every 20 years.
Methods of removing rhododendron
Hand cutting is an energetic team activity which makes an immediate visual impact and leaves sites clear of debris. It is very labour intensive but a fire is always popular with both contractors and volunteers, especially in wet or midgey weather. Follow-up management is needed as new shoots will quickly and abundantly sprout from the cut stumps.
Cut stumps are generally treated with herbicide (usually glyphosate – known to gardeners as Round-Up), applied to the freshly-cut stump. Applied properly, and in the correct mix, this should cause the root to die back.
Some operators drill holes and apply herbicide into the reservoirs created. This has been shown to kill the plant in situ. However, the dead plant will remain for many years unless physically cut or mulched.
A popular method is to let the cut rhododendron regrow for 18 months, during which time it will produce abundant leaf cover. The fresh growth is then sprayed with herbicide. The new leaves absorb the herbicide and kill the root system which has already expended much of its energy on regrowth.
Where access and site conditions permit, rhododendron can be flailed using a hydraulically-powered flail on a tracked vehicle. The mulched chippings are automatically spread across the area, leaving little above ground to deal with. The smashed stumps provide opportunities for water, frost, insects and fungi to get in, causing some stumps to die off. However, many rhododendron shoots are likely to emerge from the root system and these will require further control.
Rhododendron is a fairly shallow-rooted shrub. Young plants can be uprooted, especially if the roots are loosened before pulling. Again, this is a very labour intensive approach but it can be very rewarding. This doesn’t work for regrowth from previously cut plants, as the root system will still be extensive.
Again, where access and site conditions permit, a tracked excavator fixed with an uprooting tool can be used to uproot mature rhododendron bushes. The uprooted bush still needs to be dealt with (cut and burned, chipped or mulched). This process can also leave an unsightly and potentially hazardous hole in the ground. However, this ground disturbance can be beneficial for other plants and seeds. Holly is a good, early coloniser of land cleared of rhododendron.
Cut rhododendron has a number of uses. Larger stems can be used as ‘dead hedging’ to protect regenerating vegetation from grazing. Where access allows, cut material can be chipped and used for surfacing paths or as a mulch. Alternatively, rhododendron has a very high calorific value and makes excellent firewood, and it can also be used to make high-grade charcoal which is used both for fuel and to improve soil.
Rhododendron is still actively spreading in many parts of the Highlands and continues to be a huge problem which threatens biodiversity, forestry and other land management. Scotland’s Wildlife & Natural Environment Act (2011) provides a significant step forward, introducing powers that require landowners to take action on non-native species, and further enabling partnerships to control invasive rhododendron. However, it is important that the public do their bit as well. Despite there being hundreds of non-invasive hybrids to choose from, Rhododendron ponticum plants are still sold in some horticultural nurseries, garden centres, non-specialist retail outlets and on the internet. So if you want to plant a colourful shrub in your garden, please help to protect our native forests by choosing something other than Rhododendron!