Mentha pulegium

Taxonomy 

At the species level, Mentha pulegium L., Pennyroyal, is an uncontroversial taxon. It is highly distinctive (especially by smell) and is easily recognised.

Syn: Pulegium vulgare Miller.

British botanists have long considered there to be two varieties - an upright form (var. erecta) and a prostrate one (var. decumbens). These are not recognised in any major taxonomic work such as Flora Europaea (Tutin et al. 1972) or the New Flora (Stace 1997).

Chromosome No.: 2n = 20 (Stace 2010). 

Mentha pulegium

Photography: A.J. Lockton

Distribution 

Pennyroyal is considered native to the Mediterranean basin and Asia Minor (Fl. Europaea), but it is widely established as an agricultural weed in America, Australia and Africa. Britain is at the northern edge of its range, and populations here tend to be short-lived. It has been recorded as far north as Scotland but it is very rare away from the south coast.

BSBI Hectad Map 

Click on the map to view full-size on the BSBI Maps Scheme website.

Status 
  • Origin: it is on the edge of its range in Britain, and it has been cultivated in this country for centuries. It is possible that it also occurs as a native plant.
  • Rarity: Nationally Scarce in Britain; rare in Ireland; probably absent from Scotland.
  • Threat: it is currently (Cheffings & Farrell 2005) considered Endangered in Britain, having been downgraded from Vulnerable in 1999 (Wigginton).
  • Conservation: It is a Priority Species on the UK Biodiversity Action Plan and it is protected on several SSSIs (e.g. Bray Pennyroyal Field, Berkshire). In much of the rest of the world it is considered a problematic weed of cultivation, and research is undertaken into its control and eradication. On the BSBI Maps Scheme it is shown to be one of the 100 fastest spreading species. The apparent contradiction between its proven abundance and its rarity status is explained by the careful distinguishing of native and non-native populations, as shown in the New Atlas (Preston et al. 2002). This is traditionally done by considering M. pulegium var. decumbens to be native, while var. erecta is introduced. Cowan et al. have identified genetic markers which they consider to distinguish native from non-native plants.
Ecology 

Wrench (2002) studied the communities in which this species occurs, and failed to find a characteristic semi-natural native vegetation type for it. It is found mostly along tracks and roads, in disturbed and periodically inundated soils. Sometimes it occurs in similar habitat along river valleys and, occasionally, around the margins of fluctuating lakes and reservoirs. It also sometimes turns up in newly-sown grasslands, leading to the suggestion that it is introduced as a seed contaminant (Briggs 1996; Leach 1996).

Further Work 

It would be very helpful to have a clear description of the two varieties of M. pulegium, and for it to be demonstrated that these characters are retained in cultivation. Once such demonstration was made, the putatively native and non-native populations should be surveyed to see which variety they contain. Ideally, genetic studies to back this up need to be extended to Continental Europe if native status is to be demonstrated. No semi-natural habitat has yet been described for it, and for the plant to be effectively conserved, it would be useful to have a better understanding of its ecological niche.

References 
  • Briggs, M. 1996. Non-native Mentha pulegium (Pennyroyal). BSBI News 74:50.
  • Cowan, R.S., Devey, D. & Fay, M.F. (n.d.) Genetic fingerprinting studies of Mentha pulegium. Royal Botanic Gardens, Kew (unpublished).
  • Leach, S.J. 1996. Contaminants in grass seed. BSBI News 73:23-25.
  • Wrench, D.H. 2002.
Citation 
Lockton, A.J. (date accessed). Species account: Mentha pulegium. Botanical Society of the British Isles, www.bsbi.org.uk.

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