Northern Ireland's Priority Species

Mentha pulegium – pennyroyal


Distribution map

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Mentha pulegium L.
Family: Lamiaceae

Pennyroyal is a mint with prostrate shoots and upright flowering stems with clusters of small lilac-coloured flowers, now confined in Northern Ireland to the shores of Lough Beg, north of Lough Neagh.

In brief

  • In Northern Ireland, confined to Lough Beg, north of Lough Neagh

  • Found on the winter-inundated sandy and marshy pastures around the lake and also on the shore

  • Best seen when in flower which is from August onwards

  • Listed as a UK Priority Species

  • Subject to rapid decline in Ireland generally

  • Also subject to decline in the UK generally, with many sites lost before 1930

  • Losses have been caused by lowering of the Lough Neagh and Lough Beg lake levels.

Species description
A perennial, but short-lived, species of mint, with the usual minty smell when crushed. The main shoots lie prostrate but the flowering shoots are upright, rising to about 20cm in height. The small lilac-coloured flowers are borne in globular clusters (whorls) along the shoots. Leaves are oval, stalked and bluntly toothed, often drooping downwards. The whole plant is downy-hairy.

Life cycle
Pennyroyal is a short-lived perennial that persists only where trampling enables stems to take root in the soil. Flowers are insect-pollinated, but, although seeds are produced, it is likely that propagation is largely vegetative.

Similar species
The plant is not likely to be confused with any other mint; its combination of spreading habit, whorls of flowers and comparatively small leaves distinguish it from commoner mints like corn mint or water mint.

How to see this species
The main population is found on the west side of Lough Beg, County Londonderry. A well-known site is that on the shore of Church Island, which is no longer an island proper since the lowering of the lake water level, although it does become isolated during winter floods. It is best seen when it is in flower which is from August onwards. Relevant access permissions should always be sought prior to visiting any sites.

Current status
The western side of Lough Beg is now the principal area of occurrence in Northern Ireland, and one of only two principal areas within the UK as a whole (the other being the New Forest in Hampshire). It is protected under the Wildlife (NI) Order 1985. It has been lost from its former sites in Counties Armagh and Tyrone and the Creagh Bog in County Antrim. In the Republic of Ireland it is mainly a south-western species, where, however, it was lost from the great majority of its sites before 1930.

Why is this species a priority in Northern Ireland?

  • It is a UK Priority Species

  • It is an Irish Red Data Book Species classed as vulnerable.

Threats/Causes of decline

  • The loss of seasonally wet habitats, either through drainage or excavation to create permanently wet conditions

  • Abandonment or infilling of disturbed habitats favoured by pennyroyal, such as unmade tracks and gateways

  • Cessation of traditional grazing management and subsequent successional changes

  • Habitat destruction by agricultural intensification (such as fertilising or ploughing) and by development

  • In its Lough Neagh / Lough Beg sites, losses are probably also associated with the lowering of the water level of these lakes at various times in the past 150 years, partly resulting in a reduction of winter flooding of lakeside pasture.

Conservation of this species

Current action
There is a UK Species Action Plan which was published in 1998.

  • All known remaining sites in Northern Ireland are within the Lough Beg ASSI

  • The sites are also within the Lough Neagh and Lough Beg SPA, and part of the west shore of Lough Beg forms the Lough Beg National Nature Reserve, including Church Island – one of the main sites for pennyroyal

  • Implementation of the Northern Ireland habitat action plan for Floodplain Grazing Marsh.

Proposed objectives/actions

  • Maintain viable populations at all extant sites

  • Establish an ex-situ programme to protect genetic diversity, create a reserve population and provide experimental material

  • Seek protection of all extant sites from damaging activities including infilling with hardcore, metalling of trackway sites, drainage and application of agro-chemicals

  • Ensure the long-term protection and appropriate management of all extant native sites

  • Collect seed and deposit in the Millennium Seed Bank at Wakehurst Place (Kew)

  • Ensure that landowners and managers are aware of the presence and importance of this species and are advised on appropriate management

  • Ensure that all relevant agri-environment project officers are advised of locations of this species, its importance and management needed for its conservation

  • Undertake detailed monitoring and ecological research with a view to improving understanding of its requirements and refining management techniques for its conservation.

What you can do
If you visit any of the sites, you can assist by recording the size and health of the Lough Beg population and any damage to the site. Report your findings to either Botanic Society for the British Isles (BSBI), c/o Botany Department, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU or to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Co. Down, BT18 0EU, Tel: 028 9039 5256, [at] Report any damage to the site or the plants to Environment and Heritage Service, Tel: 028 9054 6595.

Further information

Flora of Northern Ireland

Northern Ireland Habitat Action Plans

Harron, J. (1986). Flora of Lough Neagh. Irish Naturalists’ Journal/University of Ulster, Coleraine.

Hackney, P. (1992). Stewart & Corry’s Flora of the North-east of Ireland, 3rd edn. Institute of Irish Studies, Belfast.

Text written by:
Paul Hackney