SNPA Strapline

Invasive Species




In Snowdonia and other parts of the western British Isles, Rhododendron is a spectacularly invasive plant. From the bushes planted around a century ago in large gardens and as pheasant cover, it has spread to occupy over two thousand hectares in Snowdonia alone.  This has very negative consequences for wildlife. The bushes which can grow to grow to 3m or more have dense evergreen foliage. Very few plants can survive under the dense shade and the existing native vegetation is largely eliminated. Rhododendron itself is a poor substitute. The foliage is poisonous to most invertebrates and mammals and so it does not support an extensive food chain.

Its poisonous nature is one quality that makes Rhododendron so invasive, but it has several other strategies for survival. Bushes are long-lived; when they do eventually collapse, the branches layer and establish new plants. But the reason why Rhododendron is so such an effective coloniser is the vast number of seeds it produces. A single large bush can produce one million tiny seeds per year. The seed needs special moist conditions to germinate successfully but even if only a single seed a year from each bush successfully became established, the invasion rate would be enormous. Most seeds land close to the parent plant and the invasion is most prolific at the edges of existing stands. However, in strong winds some seeds are carried much further, even several kilometres. If germination conditions are suitable, these will become established plants and eventually form daughter colonies.

In the last thirty years, a considerable amount of effort has gone into trying to get rid of Rhododendron. The National Park Authority, the National Trust and other conservation organisations and private landowners have all been involved. In several places, we are making a real impact and Rhododendron is well on its way to being eliminated. Elsewhere the battle has hardly begun.

Control work undertaken in the past has recently been reviewed. In the all the most successful cases shared two aspects of management. First, there was a long-term commitment to control. Second, there were no remaining major sources of seed adjacent to the cleared areas. A long-term approach is necessary because it takes five years for a germinated Rhododendron seed to reach a size that can be easily seen. Many seedlings are effectively invisible in the first year or two after the initial treatment. Recently cleared sites are particularly susceptible to reinvasion. After a few years the bare ground grows a thin mossy layer that is particularly favourable for successful germination of Rhododendron.

Current work by SNPA is concentrated on two problem areas in the south of the Park.  Gardens where there are just a few Rhododendrons are turning out to be a particular challenge. It is extremely time consuming for Park staff to identify these cases, to enter discussions with owners to, if acceptable, arrange their removal. SNPA hope that gardeners in rural locations will take matters into their own hands and remove these bushes.

Rhododendron Control in Gardens

Advice for gardeners in Snowdonia National Park

Rhododendron ponticum, the common purple flowered rhododendron, thrives in the damp climate and acid soils of Snowdonia. An attractive and trouble-free plant, it has been deservedly popular in gardens. Many of the bushes planted in Victorian times are still with us.

Outside the garden, Rhododendron is a major problem. Once established it is almost indestructible. Small bushes gradually grow into an impenetrable mass of very large bushes that take over woods and farmland and smother native plants. There are several hundred hectares of solid Rhododendron cover in the National Park and thousands more where the infestation is at an earlier stage.

Rhododendron spreads mainly by seed. A single bush can produce up to million tiny seeds per year. The main areas of colonisation are near large established stands but even a few bushes in a garden can lead to significant invasion if  there are surrounding areas of woodland, heath or other unimproved grazing. Seeds can be carried on the wind for more than two kilometres.

Responsible gardeners should consider replacing Rhododendron ponticum with modern hybrid rhododendrons that are not invasive. This is particularly so if your garden is in the countryside.

How to eradicate Rhododendron bushes

There are three basic approaches. The easiest method to eradicate rhododendron is to use herbicides.

Method 1: Mechanical (blood-sweat-and-tears method)

If you do not want the stumps to remain in the ground, the bushes can be removed mechanically. Cut so as to leave branches long as this gives extra leverage. If you are lucky and the bush is branched right at the base it is sometimes possible to disarticulate the branches by repeatedly swinging on them violently. Otherwise dogged determination with a crow bar and mattock is necessary. Rhododendron does not have a tap root but the fibrous root plate is extensive. Winches and mechanical diggers can be useful.

Physical removal is the best method for small bushes. Plants up to a metre in height can be quite easily removed physically. Providing  that all of the stem is removed, the plant will not grow back from the roots.

Method 2: Starvation  

If you do not want to use herbicides but are not concerned about the stump remaining in the ground, it may be worth trying to starve out the bushes. Cut the bushes to the ground and then cover the stumps with an opaque, impenetrable material such as black polythene and weight down. The sheet will have to be in place for at least two years, possibly more. It is vital that no stump or remaining shoot is left uncovered.

Cut stumps covered with black sheet

Method 3: Cut and apply herbicide (the easy method)

1. Cut the bush down.
Each stem even including small ones, should be cut to as near ground level as possible. Any layered stems should be uprooted and detached.

2. Apply herbicide
Several products are available such as those containing glyphosate (e.g. Round-Up) or ammonium sulphamate.  There can be restrictions on use for amateur gardeners. Please check the manufacturer's label to ensure suitability and for safety requirements. The herbicide should be applied straightaway to a freshly sut curface.

Applying herbicide

It there is a delay, the stumps should be cut again to expose a fresh surface. Drilling holes near the edge of the stump and filling these with herbicide increases effectiveness.

Drilled stumps

If rain is likely in the following 12 hours, the cut stumps should be temporarily  covered .  Stump treatment appears to be most effective in the winter months.

3. Check bush one year later.
If there is any sign of regrowth expose a fresh surface and apply herbicide again.

If you have numerous bushes or a very large garden, there are additional methods employing herbicides which may be more appropriate. SNPA will be pleased to give advice.