Invasive Species Week 15th – 21st May 2023
Did you know that invasive non-native species are one of the top five drivers of global biodiversity loss and cost the Irish economy €2.6 million a year, and can even harm our health?
You can help us to protect our environment and wildlife by taking part in Invasive Species Week which kicks off today!
For Invasive Species Week we want to share with you some information on the invasive non-native species that we have here in Wild Nephin National Park, particularly Rhododendron ponticum, but first it’s important to know what is an Invasive Non-Native Species (INNS).
INNS, sometimes referred to as invasive alien species, are those that have been introduced to an area outside of their natural geographic range and cause environmental, social and/or economic impacts. INNS drive biodiversity loss by negatively impacting native species through competition, predation, altering habitats and introducing diseases.
Not all non-native species are invasive, with only 10-15% being considered invasive. However, those that do become invasive can be very damaging & hard to manage.
See the short video on INNS by our colleagues at the National Biodiversity Data Centre
For Invasive Non-Native Species Week we are focusing our attention on Rhododendron ponticum, also known as Rhododendron. A shrub that is native to SW Europe and SW Asia, genetic studies show that Irish populations are derived from plants brought here from the Iberian peninsula.
Why was it introduced?
Rhododendron was probably introduced in the late 1700s, having been brought to Ireland as an ornamental plant by the gardeners in the big estate houses and associated parklands. It was also commonly used as a cover for game by the hunting lodges. Unfortunately, Rhododendron is still being used today, but with awareness this practise is changing.
How can you identify it?
It is easily identified at this time of year by its attractive pink flowers. It has leathery, evergreen leaves and its solid stems can form trunks as the shrub matures. Another similar plant that it can be confused with is cherry laurel, which has similar leaves. Cherry laurel is also an invasive non-native species.
How does it spread?
Rhododendron can spread by seeds and suckers (i.e. basal shoots). Rhododendron usually starts flowering at between 10‐12 years and when conditions are good it can flower every year. Plants can spread rapidly across the landscape thereafter, with thousands of seeds being produced by each flower head annually and the small seeds being carried over long distances by wind. Suckers, emerging from root systems, form new plants. When Rhododendron is cut back it regrows vigorously and the regrowth from cut sumps creates multi-stemmed regrowth with flowers being produced within 3-4 years. This means that cutting back branches, stems and even whole plants alone is not effective for long term control.
Why is Rhododendron ponticum so problematic?
Since its introduction, Rhododendron has become naturalised and is thriving in Ireland’s temperate mild climate. It establishes on well drained, acid soils and has invaded woodland, forestry plantations and peatlands. Mature plants and stands grow in dense thickets, replacing the native shrub layer and ground flora and prevent natural tree regeneration. As well as causing habitat loss, Rhododendron is not palatable to herbivores such as deer and livestock. It also plays a role as a host to Phytopthora fungal pathogens that can spread and kill tree species.
How are they so good at taking over?
The thick, waxy, evergreen leaves absorb and reflect light, and the mature plants cast a deep shade making it impossible for any plants to grow underneath. As well as growing in full sunlight, Rhododendron can grow in the shade of other trees, apart from the heavy shade of dense conifers. It will however grow well beneath a canopy of deciduous trees, making it a serious threat to our native woodlands. As mentioned earlier, single plants can produce a vast amount of seed with a plant in Killarney estimated to produce 1 million seeds. These tiny seeds are dispersed by wind mostly, which can germinate under very low light levels. Seedlings readily establish on peatlands too, although the invasion process is slower. Rhododendron is well adapted to growing in the dominant acidic soils of peatlands. Seedlings in these open landscapes are often seen on vertical peat banks that are moist and in heathlands on sloping hillsides. Invading plants can also be found along stream and river sides.
What are the other impacts?
Rhododendron can also invade public spaces and beauty spots. See the linked article from 2014 where a couple had to be rescued in the Knockmealdown Mountains after being trapped in dense Rhododendron for several hours: https://www.rte.ie/news/2014/0617/624367-rhododendron-rescue/
How should it be managed?
For general information about dealing with invasive species see the report by Invasives.ie @biodatacentre: https://invasives.ie/what-can-i-do/management/
Specific guidelines for managing Rhododendron are outlined here: https://invasivespeciesireland.com/wp-content/uploads/2012/01/Rhododendron-BPM.pdf with a more detailed report by NPWS here: https://www.npws.ie/sites/default/files/publications/pdf/IWM33.pdf
There are six steps in the process to ensure successful management of #Rhododendron:
- Mapping presence to determine the extent of the infestation
- Recording maturity of stands/plants and noting any previous treatments
- Prioritising areas for control taking into account a number of factors such as experiences from past works, targeting less heavily infested areas initially or places where seed production is not happening yet, working with prevailing wind direction that may help mitigate seed dispersal into cleared areas
- Creating favourable conditions to allow native ground flora to re-establish e.g. excluding grazers. This will reduce the likelihood of Rhododendron recolonising.
- Setting out a clear strategy to guide work, including timelines for clearances and repeating treatments.
- Taking into account the ongoing nature required for management, including the need to remove small plants and seedlings that will inevitably have been missed.
How are we tackling the problem in Wild Nephin National Park?
Here in Wild Nephin National Park, Rhododendron ponticum has infested quite extensive areas. These include areas mainly on the edges of the Park and throughout the Nephin Forest.
Some Rhododendron control has been undertaken in the past few years including sites to the west of the National Park and in the Nephin Forest. In the Nephin Forest some of this work was associated with native tree planting within fenced areas to exclude grazers.
The majority of infestations have now been mapped by Conservation Rangers and funding for works is expected over the coming months. To conserve areas of important peatlands, grasslands and regenerating woodlands it’s imperative to control Rhododendron by cutting and treating with herbicide in a controlled manner. This is extremely labour intensive work which has been undertaken by Park staff and contractors.
Some great work has been undertaken in our sister National Park in Connemara with recovery of native ground flora appearing very soon after clearance works. It is hoped that similar positive results from clearance works will be seen in Wild Nephin National Park.