Rhododendron on Exmoor

For many people walking through an Exmoor woodland during May or June, coming across a pink-flowering rhododendron bush is a beautiful sight and something they will remember as part of their experience. However, for many woodland owners and land managers, rhododendron can be a major problem because not only can it rapidly colonise whole landscapes if left unchecked but it can end up smothering native plants such as young trees, as well as flora like bluebells and primroses.

Where did rhododendron originally come from?

Rhododendron (Rhododendron ponticum) was originally introduced into Britain from southern Europe such as the Iberian Peninsula and Turkey. From 1763, it began to be planted widely in British country estates, parks and gardens as an ornamental plant and later, during the Victorian era, rhododendrons were planted extensively as game cover. Rhododendron became hugely successful as a game cover shrub in woodlands in particular, largely because it is such an efficient and rapid coloniser of open ground. Unfortunately, this feature has also made it unwelcome to many landowners today because it has continued to spread into many areas of high conservation value such as Atlantic oakwoods, moorlands and heathlands.

Why is rhododendron such a problem?

There are many reasons why rhododendron has become a big problem in woodlands and other habitats. Historically, when rhododendron first began to be cultivated in Britain, it was crossed with other rhododendron species forming a range of hybrid plants and these not only became very vigorous but were also very effective at colonising new areas of ground. Some ornamental varieties of rhododendron were grafted onto Rhododendron ponticum rootstocks but most of these eventually reverted back to R.ponticum plants by producing shoots around the graft.

Rhododendron can spread through a site by a process called stem layering which is where rhododendron branches can put out roots where they touch the ground. Although this is a slow way of spreading, it is one of the main causes of rhododendron expansion in woodlands, particularly because woodland soils are soft and moist and thus provide an excellent growing medium to root into.

Each rhododendron bush can produce thousands of tiny, viable seeds which can then populate new areas of ground – in fact, around 5000 seeds per flower head can be produced! Rhododendrons have thick, evergreen leaves which cast dense shade and can shade out native plants, particularly when the rhododendron canopy closes over. Rhododendrons also send out toxins into the ground which kill off other plants including native woodland flora and these toxins can often linger for many years in the soil which inhibit the regeneration of native plants. Few browsing animals attempt to graze rhododendron bushes, possibly because it is unpalatable and toxic to some of them. At some sites, areas of rhododendron become so thick that they can end up being impenetrable to people which then cause problems with woodland management because the woodlands are difficult to access.

More recently, it has been known that rhododendron bushes can be a host to the Phytophthora ramorum disease which affects and kills larch trees in Britain. Both rhododendron and P.ramorum disease have flourished in south west England and western Britain because they seem to thrive in the warm, moist Atlantic climate.

Where rhododendron has spread through woodlands on Exmoor, it has caused many of these problems which have resulted in the quality of the woodlands being seriously damaged.

Grant-aided work on Exmoor

In 2006-8, Exmoor National Park Authority undertook a survey of Exmoor's woodlands to try and gauge the extent of the rhododendron problem and discovered that 2000ha, or about 23%, were affected by rhododendron. Of these, 500ha were severely affected with 33-100% cover. Soon afterwards, the National Park Authority was able to offer some conservation grants to help landowners to clear rhododendron in small targeted areas.However, because the rhododendron cover was so extensive, these grants only had a limited impact in some areas of Exmoor National Park.

Since 2007, the Forestry Commission's Woodland Improvement Grants targeted to ancient woodlands were very successful in helping woodland owners to clear rhododendron from their woodlands. In August 2013, Exmoor National Park Authority estimated that woodland owners had cut the equivalent of 331ha of rhododendron during the previous fifteen years, which is almost the size of Minehead. About double this area needed a follow-up herbicide re-spray to tackle any rhododendron re-growth and seedlings. This is a fantastic achievement to have taken place in the National Park and shows the commitment of woodland owners in tackling this issue and trying to improve the health of their woodlands.

The main driver for this work has been the link between rhododendron and Phytophthora ramorum disease which has in turn helped reduce the impact of the P. ramorum disease across the region. However, it would have been impossible to carry out the clearance without the realistic grant rates from the Forestry Commission which reflected both the size of rhododendron and the difficult terrain to work on in the south west of England.

There is still more work to do however. From calculations of rhododendron cover across the National Park, there is estimated to be approximately 100ha of rhododendron remaining in Exmoor's woodlands. Exmoor National Park Authority and the Forestry Commission will be working closely with woodland owners on Exmoor in the coming years to continue to manage the rhododendron issue and try and improve the health of Exmoor's woodlands.