Project Background
Hottentot Fig (Carpobrotus edulis) is a popular garden plant from South Africa. It is also an aggressive invader of coastal habitats, which forms vast mats to the exclusion of all other plants. On the Gower peninsula of Wales and along the Cornish and Devon coasts of Great Britain it has formed extensive colonies smothering many kilometres of coastal cliffs. On the drier eastern coasts of Ireland, especially on Howth Head it poses a serious ecological threat. The first record for Carpobrotus edulis in the wild in Ireland is from Howth Head with an Atlas record for 1962 (Reynolds 2002). A further 11 or so records occur in Ireland in counties Cork, Waterford, Wexford, Wicklow and Down.
Project Aims
The project aimed to control and ultimately eradicate Hottentot fig from Howth Head. This plant has been present at the Needles on Howth since 1962 and in the intervening 47 years it has grown into patches up to 40 metres across – a steady 1 a metre a year. It has often been noted in the literature that after an initial settling in period an invasive plant species reproduces at an exponential rate. In addition numerous smaller patches, as well as its presence on each of the southern headlands (Sutton Dinghy Club, Drumleck Point, Lion’s Head and Baily Lighthouse), demonstrated that it was actively spreading and establishing new colonies on Howth.
Chemical control of Hottentot Fig on Howth Head Target Control Site, 2010
The six sites at Howth were chemically treated between the 8th and the 20th of September 2010 during dry calm days. A wheelbarrow power sprayer (KS, 120 litre tank, petrol motor) which was easily wheeled to access points along the cliff path and the 30m hose extension to the tank meant that operators could access the Hottentot fig without having to wear a cumbersome knapsack sprayer. Knapsack sprayers (10 litre) were used at the sites near Sutton Sailing Club, Sea cottage and Lion’s Head where it was safe for operators. Volunteer labour was used and the chemical (3 g/l glyphosate and 0.3g/l diquat) was mixed on site. Water was filled at Howth pier or brought along in drums and transported by wheelbarrow once on site.
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- Andy Booth of Conservation Services treats Hottentot fig with Resolva with Blue Dye added to aid visualisation of treatment. Note also the numerous developing seed heads on the Hottentot fig in September.
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- Treated patches showing obvious discoloration and dying back on the 16th October 2010.
Survey work in 2011
A botanical survey carried out in August 2011 has revealed the remarkable success of the project to date. Treated sites demonstrated a 97% kill, and all surviving patches were retreated in September.
The most heartening observation, however, is the astonishingly rapid success of native plants in re-establishing themselves from amongst the dead Carpobrotus patches (Below). In separate areas, extensive populations of Inula crithmoides and Crithmum maritimum have already developed in the first year. Some 20% of the treated areas have been revegetated in this way.
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- Rock Samphire (Crithmum maritimum) regenerating through a mat of killed Carpobrotus at the Baily Lightouse. September 2011
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- Golden Samphire (Inula crithmoides), with Rock Samphire at top and Sea Beet (Beta maritima) below, at Lion’s Head. September 2011.
Acknowledgements
The project team wish to thank the Heritage Council for providing the funding to carry out this pilot control project at Howth Head. We would also like to thank Ann Murphy, Hans Visser, Deborah Tierney and Florence White of Fingal Co. Council, The members of the Howth Committee, Maurice Eakin and Niall Harmey of NPWS.