Forestry Focus

Invasive Species

Invasive non-native species introduced into Ireland are of increasing concern to ecologists, land managers and fishery biologists. Introduced species and genetic material can have a major impact on biodiversity.  When non-native species become invasive they can transform ecosystems, and threaten native and endangered species.

The number of alien species that currently affect Irish forests are relatively few but their impact can be locally very significant.



Rhododendron (Rhododendron ponticum) is perhaps the most troublesome of the invasive plant species that affect our forests. It is native to the western and eastern Mediterranean i.e. Spain, Portugal and Turkey and also occurs eastwards through Asia into China. It was first introduced in the late 18th Century and became popular on country estates in Victorian times for its ornamental value, as well as cover for game birds.


Rhododendron invading native oak woodland – Co. Wicklow (photo A. Pfeifer)

Rhododendron thrives in mild, moist climatic conditions, where the soils are poor and acidic. It invades areas both vegetatively and via seed. Established plants spread by lateral horizontal growth of the branches. A single plant may eventually end up covering many metres of ground with thickly interlaced, impenetrable branches. Where the horizontal branches touch the ground, they will root, continually extending the area of the species. Rhododendron seeds are tiny and hence wind dispersed.  Each flower head can produce between three and seven thousand seeds, so that a large bush can produce several million seeds per year.

Rhododendron is particularly troublesome in forests on the Old Red Sandstone soils of west Waterford and south Tipperary, also on the peats in Cork and Kerry and up the western seaboard into Donegal. The invasion of Rhododendron into forests in these areas has serious consequences for their management.

  • Harvesting costs can be increased depending on the density of the shrub.
  • Clearing Rhododendron is expensive and site preparation for restocking sites following clearfelling is greatly increased.
  • The regrowth of Rhododendron from cut stumps and regenerating seedlings is a major problem on restock sites as it competes with the planted trees for growing space, light and nutrients. It requires several years of chemical weed control before the planted trees are tall and robust enough keep ahead of the Rhododendron.
  • Heavy invasion into mature forest stands with light crowned species such as Scots pine on very poor sites can significantly reduce the growth of the trees.
  • Invasion into native woodland can kill off or prevent natural regeneration occurring; and also eliminate the ground flora and associated wildlife. This is particularly serious in conservation areas such as Special Areas of Conservation (SACs) as the cost of maintaining these sites free of Rhododendron can be prohibitive.
  • Streams can become completely overgrown and shaded out by Rhododendron, severely affecting the insect life and fish numbers in the stream.

Control measures

As Rhododendron is confined to acidic soils it is a problem in conifer plantations and the few remaining native oak woodlands that occur on these soil types. Control measures consists of both mechanical and manual clearing and the use of herbicides.


In conifer plantations spraying regenerating cut stumps and seedlings of Rhododendron with a herbicide is the main method of control. The plant is notoriously difficult and expensive to actually kill. The leaves are waxy and herbicide treatment must include a chemical additive to help break this surface down. Even then, where Rhododendron is well established with a large root system, herbicide treatment usually has to be repeated over several years as the herbicides do not translocate well through the plant. Complete elimination of Rhododendron is difficult to achieve, but if it can be controlled until the planted trees close canopy, then shading will arrest its development.

It has been observed that the dense evergreen crowns of some conifer species such as Western hemlock (Tsuga heterophylla) throw a deep heavy shade which prevents the Rhododendron regenerating within a stand. The possibility of using these tree species to control Rhododendron has been considered but is likely to have only a local effect as it is impractical and undesirable to have large areas of forest under the one species.  Also, the amount of seed produced annually by Rhododendron and its dispersal by wind will ensure that it will still spread to areas where germination is favourable.

Native woodlands

The invasion of Rhododendron into native oak woodlands is particularly serious. Once Rhododendron has invaded an area few native plants survive. In woodlands only those trees which manage to grow above the level of the Rhododendron canopy will persist. These of course have a finite life span and on their death, there is no replacement because seedlings cannot become established under the lightless canopy. At this point, the Rhododendron completely dominates the area. This is particularly noticeable in Silver Birch woodlands as this species is short lived.

Once the native plants have disappeared, the animals which rely upon them either directly or indirectly for food cannot survive. Thus Rhododendron areas are essentially barren. Even where trees exist above the Rhododendron canopy, species such as woodland butterflies disappear. This is because the caterpillars of most woodland butterflies can only feed on the wildflowers and grasses which are found in the glades and rides of well managed woodland.

The general toxicity of Rhododendron to herbivores means that it cannot be controlled by grazing. In native woodlands chemical control is undesirable but must be one of the tools available to control the species along with mechanical removal and pulling seedlings by hand.

In restoring habitats there are a variety of techniques for the mechanical removal of Rhododendron. These usually employ a tractor or tracked swing shovel with a rotary flail mounted on a moving hydraulic arm. In sensitive conservation areas such techniques may well not be appropriate. Such mechanical devices often leave a thick layer of smashed Rhododendron on the ground which may have to be removed using expensive manual labour.

In Ireland, the Killarney and Glengarriff native oak woodlands have been particularly affected by Rhododendron invasion. The National Parks and Wildlife Service has pursued a costly programme of clearance and control of reinfestation for many years, and have had some success in controlling the species within these important conservation areas. However, reinfestation is a constant threat coming from seed emanating from surrounding areas outside the parks.  Control at landscape level, where all landowners participate, is the best option of ensuring that clearance and restoration efforts are sustainable in the long term.

Further information at: Rhododendron ponticum: a guide to management on nature conservation sites


Currently research into the control of Rhododendron is being carried out by the Waterford Institute of Technology under the PLANFORBIO programme funded by COFORD.  Investigations are underway to find a fungus that is capable of acting as a bioherbicide that will kill young Rhododendron plants. If successful it would be a very useful tool for the effective non-chemical control of the species.  Click Rhodo for further information on this project.

Cherry Laurel

Cherry laurel (Laurus laurocrasus) is a large evergreen shrub or small tree often used for landscaping, usually for hedging. Related to cherry trees, cherry laurel gets its common name from its resemblance to the true laurel tree (Laurus nobilis) or bay tree. Native to southwestern Asia and southeastern Europe, this species has escaped cultivation and has become widely naturalized and has become an invasive species in some habitats. Its rapid growth, coupled with its evergreen habit and its tolerance of drought and shade, often allow it to out-compete and kill off native species. It is spread by birds, through the seeds in their droppings.

Laurel Photo H Zell

Laurel (photo H. Zell)

In Irish forests cherry laurel poses similar problems to Rhododendron but being mostly confined to the more fertile old woodland sites is less of a threat. However, in these areas it can form a shrub or small tree 5-10 metres in height that can cause serious logistical problems and increased costs for harvesting and restocking operations.

Control measures

Control measures for cherry laurel are similar to Rhododendron i.e. chemical spraying of regrowth from cut stumps and mechanical or manual control. Uprooting of stumps is also sometimes practiced and while effective is very costly. 

Further information on both Rhododendron and cherry laurel is available at:

 Best management practice for the control of Rhododendron and cherry laurel



Deer can cause severe damage to plantations through the browsing of shoots and thrashing stems of young trees when cleaning the velvet from their horns; also through bark stripping in older trees.  They can cause problems for farmers by eating cereals and root crops and in certain areas are a hazard for motorists when crossing roads.   Of the four species of deer in Ireland – red, fallow, sika and the recently introduced muntjac, the latter two pose the greatest threat to our forests.  They are introduced species but are well adapted to conditions in this country and reproduce very quickly.  Their ranges are expanding due to deliberate illegal introductions in areas where they are absent; and also due to the increased habitat provided by the maturing forest plantations that have been established by farmers and private owners over the past two decades.

Sika deer

Sika_Deer by D Macauley - Wikimedia

Sika deer (photo D. Macauley)

Sika deer, which are native to Japan, were first introduced to Ireland in 1860 by Viscount Lord Powerscourt of Co. Wicklow. Through escapees and deliberate releases they have become established in many parts of the country, with the main epicentres in the northwest, the east and southwest regions of Ireland. The high numbers of sika deer are of particular concern to foresters in Wicklow as they increase the cost of establishment (deer fencing) and also prohibit the planting of Douglas fir, a valuable timber species for which the county is particularly suited.

Muntjac deer 

The recent deliberate introduction of Reeves’ Muntjac into Ireland poses a serious threat to woodland management, eating almost any plant material that grows within their browse line, which in turn impacts upon the natural habitat of many species of plants, insects and small birds. However, agricultural and forestry damage is less than with the other much larger species of antlered deer.

Muntjac deer by LHG Creative Photography

Muntjac deer – a recent introduction (photo LHG creative photography)

Owing to their subtropical origin, muntjacs are not seasonal breeders.  They produce single fawns every seven months and populations can reach very high densities and cause significant impact on ground flora.

Their preferred foods are ivy, bramble, coppice shoots, flowers and seeds of many plants, also fruit, nuts, dead leaves, and fungi. They are of particular concern to nature conservationists as they can seriously reduce the ground flora and natural regeneration of native shrub and tree species in high conservation areas.

Control Measures for Deer

In the absence of any natural predators, deer numbers can increase to a stage where they become a nuisance and cause serious economic damage. Reducing the number of deer through shooting is the most effective method of avoiding damage to plantations and woodlands. Deer densities of approximately 1-2 per square kilometer can be tolerated as they will not cause any significant damage to the forest at these levels.

Hunting is one of the non-timber benefits from forests and as private plantations mature it is also likely to become a source of income to forest owners. Coillte issues licenses to recreational hunters to shoot deer on selected forest properties and they also carry out culls by professional hunters to keep the numbers of deer down to recommended levels.

Deer fencing is another measure to prevent damage and is essential when susceptible species are being planted, such as Doulas fir and broadleaves. It is, however, expensive and requires on-going maintenance to ensure that there are no breaches in the fence line.  The introduction of deer lawns (open unplanted spaces in the forest) is sometimes used by foresters to encourage deer to graze on grasses in preference to young trees.

Deer management in Ireland

Wild deer association of Ireland

Invasive species Ireland – Muntjac deer

Distribution and range expansion of deer in Ireland

Forest mammals –  management and control

Grey squirrel

Gery squirrel by A Bridge - Wikimedia

Grey squirrel – a pest of broadleaved woodlands (photo A. Bridge)

 The grey squirrel was introduced into Ireland at Castle Forbs in Co. Longford in 1911. Several pairs were given as a wedding present from the Duke of Buckingham to one of the daughters of the house. Originating in the Eastern North America, the squirrels have multiplied and have spread to nearly all counties east of the River Shannon and are spreading south. Their preferred habitat is mixed woodland of conifers and broadleaves. They are more aggressive than the native red squirrel and compete for food, but more importantly, they carry the squirrel pox virus for which red squirrel does not have immunity. Today they are threatening the survival of the red squirrel which has been designated as near threatened on Ireland’s Red List No. 3 of Terrestrial Mammals.

The grey squirrel causes severe damage to commercial broadleaved plantations through stripping bark on branches and main stems as the crops pass from thicket to pole stage. Multiple forking of the main stem can occur as a result of the tree top dying back. Damage can be so severe that an entire plantation can be ruined for commercial timber production leaving it only fit for low value firewood. Sycamore and beech are particularly susceptible to squirrel damage, oak less so, while wild cherry is not affected. High densities of greys are implicated in the loss of wild bird populations, because they take eggs from their nests.

Control measures

Grey squirrels have few natural predators in Ireland but it has been observed that where the pine marten occurs that the grey squirrel is absent. Pine martens are known to prey on grey squirrels in America but this has still to be confirmed in this country.

The spread of the grey squirrel is of major concern to both foresters and nature conservationists alike and research is ongoing to monitor the spread of the species to help in developing strategies for its control.

Control measures currently consist of shooting, trapping and using hoppers baited with cereals treated with the chemical pesticide warfarin.

Controlling Grey Squirrel Damage in Irish Broadleaved Woodlands

Invasive Species Ireland – grey squirrel