Mentha pulegium (pennyroyal)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Biology and Ecology
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Impact Summary
- Economic Impact
- Environmental Impact
- Risk and Impact Factors
- Uses List
- Prevention and Control
- Gaps in Knowledge/Research Needs
- Links to Websites
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Mentha pulegium
Preferred Common Name
Local Common Names
- Brazil: hortelã-miúda; menta-miúda; poejo-das-hotas; poejo-real
- China: chun e bo he
- France: menthe pouliot; pouliot
- Germany: Poleiminze
- Italy: Menta puleglio; pulegio
- Portugal: poejo
- Russian Federation: mjata blošnica
- Spain: menta; poleo
- Sweden: pole jmynta
- UK/England and Wales: English pennyroyal; European pennyroyal; grows-in-the-ditch; mint pennyroyal; mosquito plant; penny royal; pennymint; pennyroyal; penny-royal; pennyroyal mint; peppermint; pudding grass; squaw mint
Summary of InvasivenessTop of page
M. pulegium is a perennial herb native to the Middle East, Europe and North Africa. It has been introduced to the Americas, parts of Asia, Mozambique, Australia and New Zealand. It is considered an environmental weed in Victoria and Western Australia, and as a minor or potential environmental weed in South Australia and New South Wales (Weeds of Australia, 2013).
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Lamiales
- Family: Lamiaceae
- Genus: Mentha
- Species: Mentha pulegium
Notes on Taxonomy and NomenclatureTop of page
According to Lockton (2013), British botanists have long considered there to be two varieties of M. pulegium - an upright form (var. erecta) and a prostrate one (var. decumbens). However, these are not recognised in some major taxonomic work such as Flora Europaea (Tutin et al., 1972, cited in Lockton, 2013) or the New Flora of the British Isles (Stace, 2010).
DescriptionTop of page
Modified from Bossard (2000):
Perennial herb with prostrate creeping and erect stems. Stems hairy when aerial, hairless when submerged, branched, 4-angled, 10-90 cm high. Leaves about 1-2.5 cm long, smaller further up the stem, lower leaves petiolate, higher ones subsessile. Leaf narrowly ovate to elliptic, base tapered to obtuse, tip rounded, margin entire to finely serrate, lower surface short-hairy, hairless when submerged. Inflorescence of false whorls (technically verticillasters) of densely packed, short-stalked flowers subtended by reflexed leaves or smaller, leaf-like bracts.
Flower calyx 2-4.5 mm long, with 5 lobes, purple, hairy, dotted with glands on the outside, with a prominent white hairy tuft in the throat. Corolla 4-8 mm long, mauve, often paler with age, upper lip notched, lower three-lobed, 4 stamens, subequal, exceeding corolla, usually exserted at anthesis. Ovary superior, 4-lobed, 2 chambered, each with 2 ovules; 1 style arising from the junction of the ovary lobes. 2 unequal stigmas. Fruits of 4 nutlets, 0.5-0.75 mm long, pale brown, ovoid to oblong.
Plant TypeTop of page
DistributionTop of page
M. pulegium is native to a large area covering Central Asia, the Middle East, Europe and North Africa. It has been introduced to the Americas, parts of Asia, Mozambique, Australia and New Zealand.
In Australia the species is widely distributed in the temperate south (southern and eastern New South Wales, Victoria, Tasmania, southern South Australia and south-western Western Australia), where it has invaded grasslands, alluvial plains and wetter habitats such as riparian area and freshwater wetlands.
In California, USA, M. pulegium is considered uncommon in much of the state but occurs in the Sierra foothills, Central Valley and most coastal counties from the Mexican border to Oregon, being a wetland indicator species in seasonally inundated soils of valley bottoms, usually below 500 m elevation (Bossard, 2000). It flourishes, like many other weedy species, on frequently disturbed sites like heavily grazed pastures, or on such sites as those that suffer from seasonal depositions of silt or organic debris.
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.Last updated: 10 Feb 2022
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|China||Present||Introduced||Beijing Shi, Nanjing Shi, and other cities|
|Indonesia||Present||Present based on regional distribution.|
|Pakistan||Present, Only in captivity/cultivation||Introduced|
|Serbia and Montenegro||Present||Native|
|U.S. Virgin Islands||Present||Introduced|
|-New South Wales||Present, Widespread||Introduced||Invasive||Southern & Eastern parts|
|-South Australia||Present, Widespread||Introduced||Invasive||Southern parts|
|New Caledonia||Present, Only in captivity/cultivation||Introduced|
History of Introduction and SpreadTop of page
As a culinary herb also valued for its reputed medical properties, M. pulegium has been introduced around the world by European migrants, notably to North America and Australasia. It is now found naturalized in wildlands around the world (Grieve, 1976, cited in Bossard, 2000). Duke (1985, cited in Bossard, 2000) added that humans have contributed to its dispersal and establishment in non-cultivated areas by propagating and growing it for its purported medicinal or herbal values.
The first record of its introduction into Australia, according the Australia’s Virtual Herbarium (2013), was 1874 in Victoria, but Parsons and Cuthbertson (1992) reported that it was recorded from an Adelaide (South Australia) garden in 1841, possibly introduced there as an ornamental or culinary herb.
In New Zealand it was first reported as naturalised in 1882 from Whangarei and several places about Auckland (Thomson, 1922). After 1882 it showed continuous spread and aggression in the North Island. In 1901 it was included in the Second Schedule of the Noxious Weeds Act. By 1926, Hilgendorf had already described M. pulegium as one of the worst weeds of the North Island of New Zealand (Webb et al., 1988). Guthrie-Smith (1953) claimed that this species ‘is of missionary origin’ in New Zealand.
IntroductionsTop of page
|Introduced to||Introduced from||Year||Reason||Introduced by||Established in wild through||References||Notes|
|Natural reproduction||Continuous restocking|
|Australia||1874||Yes||Australia’s Virtual Herbarium (2013); Royal Botanic Gardens Sydney (2004); Royal Botanic Gardens Sydney (2013)||Victoria|
|New Zealand||1882||Yes||THOMSON (1922)||Whangarei|
Risk of IntroductionTop of page
Although continued spread in countries where it is already established is likely, the spread of M. pulegium to new countries is less likely, but still possible because the seeds may become attached to the fur or fleeces of animals moving to new countries across land barriers. In addition, it is grown as a crop in some countries (such as India, Indonesia, Canada, Pakistan and Cuba) and could spread from these agricultural areas to nearby areas with suitable conditions for its growth. Its seeds are also freely and commonly available on the internet, which could allow M. pulegium to be introduced to new locations or countries.
HabitatTop of page
In general, M. pulegium grows in grasslands, wet habitats and disturbed sites.
In its native Tunisia, M. pulegium grows spontaneously in the north and centre of the country, where rainfall varies from 300 to 900 mm per year. It grows best in fertile, moist sandy and clayey soils with a pH of 5 to 8.5 (Fadhel and Boussaid, 2004). However, populations there have been overexploited by massive eradication of plants in order to extract the essential oils. Furthermore, other human disturbances like farming, hard grazing, deforestation and loss of seasonally wet sites have also reduced native habitats. Surviving populations are found along roadsides liable to flooding in winter, reforested and margin Pinus and oak (Quercus spp.) forests, ephemeral watercourses and abandoned fields.
In the UK, the species seems to have declined. This is probably partly due to the draining of wet areas and also due to the loss of traditional village greens. Chatters (2004) stated that, in Britain, M. pulegium occurs in seasonally inundated grassland, usually in and around ephemeral pools and runnels. The grasslands supporting M. pulegium are very short turf overlying clay and silt and are subjected to intense all-year-round grazing, trampling, dunging or disturbance by livestock or vehicles, causing poaching and ruts. This habitat is found within traditionally managed lowland village greens, settlement-edge lawns adjacent to open heath and the verges of unmetalled trackways.
In California, Bossard (2000) reported that M. pulegium grows in vernally flooded or seasonally wet areas like seeps, streamsides, vernal pools, swales, marshes, valley bottoms and ditches, especially on heavy clay or silty soils. It flourishes, like many other weedy species, on frequently disturbed sites like heavily grazed pastures, or on such sites as those that suffer from seasonal depositions of silt or organic debris. Optimal development appears to occur where other vegetation shades stems and rhizomes and contributes to sustained moderation of soil moisture and temperature.
Weeds of Australia (2013) described the species as having escaped cultivation and invaded grasslands, alluvial plains and wetter habitats such as riparian areas and freshwater wetlands. Dean (1990) described its incidence in Western Australia in summer-moist pastures where annual rainfall exceeded 1000 mm.
Habitat ListTop of page
|Terrestrial||Managed||Managed grasslands (grazing systems)||Present, no further details|
|Terrestrial||Managed||Disturbed areas||Present, no further details|
|Terrestrial||Managed||Urban / peri-urban areas||Present, no further details|
|Terrestrial||Natural / Semi-natural||Riverbanks||Present, no further details|
|Terrestrial||Natural / Semi-natural||Wetlands||Present, no further details|
Hosts/Species AffectedTop of page
Biology and EcologyTop of page
Harley and Brighton (1977) counted chromosomes in species of Mentha and found that M. pulegium plants from England, Greece, Bulgaria, Turkey and Morocco were diploid (2n=20) with a range of morphological variation. The only tetraploid was from Portugal. Fadhel and Boussaid (2004) examined the genetic diversity in wild Tunisian populations of M. pulegium and found that it showed a high level of genetic variation within populations, indicating a predominantly outcrossing mating system and the recruitment of new genotypes through seed dispersal.
Seeds are produced abundantly from the numerous dense inflorescences from late spring through to autumn (Bossard, 2000). Seedlings appear in heavy, silty clays once pools of water formed in winter have almost dried (Bossard, 2000).
In Australia, Parsons and Cuthbertson (1992) reported that, although seeds can germinate at any time of the year if moisture is available, germination is usually restricted to the moist April (autumn) to December (early summer) period, with peaks of emergence in May and October. As the seedlings establish, prostrate shoots or stolons spread in all directions, often rooting at the nodes. Autumn-germinated and established plants produce flowering stems in late September or October, when the interconnecting stolons die, separating daughter plants from the parent and from each other. Panetta (1985b) estimated seed production by counting the numbers of main and axillary stem nodes, the number of florets per node and the proportion of viable seed and obtained figures of between 73,000 and 289,000 seeds m-2. The sizes of seed banks under the different populations of flowering plants were not positively related to current seed production.
Physiology and phenology
In California, USA, M. pulegium seeds germinate after exposure to alternating temperatures and need light to germinate (Bossard, 2000).
Panetta (1985a) investigated the germination and establishment of M. pulegium in Western Australia and also found that light was needed for germination, but that seeds germinated readily at a range of constant temperatures from 10 to 30oC, although percentage germination of some populations was higher with alternating temperatures. Panetta (1985a) also studied the emergence and survival of seedlings in the field and found that most seedlings emerged between April and September but some as late as late December. Mortality rate of seedlings varied greatly between sites but at all sites few seedlings which had appeared during spring or later in the year survived to flower. The author deduced that the capacity of seedlings to emerge and establish under water improved the ability of the species to invade and persist in pasture.
Bossard (2000) described the growth of M. pulegium in California, where it is inconspicuous during vegetative growth in spring but becomes very obvious when it flowers in summer. He suggested that populations on warmer, drier sites flower earlier than those on wetter shaded sites. By late autumn the flowering stems have dried but seed heads remain intact through the winter. The herbaceous above-ground tissues regenerate in spring and die back in autumn. Established plants overwinter as rhizomes just below the soil surface, with roots and flowering stems growing from conspicuous nodes in mid-spring.
Seed banks of M. pulegium can reach very high densities of up to 176,000 m-2, especially under pastures that are frequently disturbed by trampling from domestic stock (Panetta, 1985b). Seed banks fell in pasture sites from which dairy cattle had been excluded, and were very much lower in non-pasture sites. Although little is known about the length of viability of viable dormant seed, the author quoted Cheam (1983) as reporting that seeds can survive for at least 12 months and surmised that the large seed bank populations found under pastures could survive for many years.
There seems to be little information as to how long individual plants survive, although with its ability to produce daughter colonies vegetatively, there is in theory little limit to its lifespan.
Population size and structure
M. pulegium can form very large dense stands under appropriate conditions, and individual plants cannot be distinguished because of the formation of daughter plants from stolons. Panetta (1985b) measured inflorescence density at three different populations in Western Australia at between 233 and 637 m-2.
Thomson (1922) wrote that, in 1919, in the north of Auckland the flowers were visited by immense numbers of the common blue butterfly Lycaena labradus. This butterfly is native to Australia and was probably carried by wind to New Zealand.
There is little published information is available for M. pulegium, apart from its preference for wet or moist soils, or sometimes heavy, silty clays (Bossard, 2000). PFAF (2013) quoted several gardening references in suggesting it prefers a ‘stiff moist soil that is slightly acidic.’
Means of Movement and DispersalTop of page
Natural dispersal (non-biotic)
The seeds are well adapted for dispersal by water (Parsons and Cuthbertson, 1992). Local spread of colonies occurs through both seed production and development of daughter plants along the stolons.
Vector transmission (biotic)
The persistent hairy calyces of the flowers, still containing nutlets, stick to wool, fur and other materials and so disperse the seed (Parsons and Cuthbertson, 1992). The same authors wrote that fruit or nutlets contained therein can also be spread as contaminants in hay or other produce, in mud on the hooves or pelts of animals and on agricultural machinery.
Nutlets may be ingested by animals and deposited elsewhere (Bossard, 2000). In California, Quinn et al. (2008) successfully germinated M. pulegium seeds from horse manure.
The presence of M. pulegium in newly-sown grasslands in Britain suggests that it may be introduced as a seed contaminant (Lockton, 2013). The Online Atlas of the British and Irish Flora (2013) claimed that a robust variety has been introduced with North American seed mixtures to a number of sites (Briggs, 1997).
Humans have contributed to its dispersal and establishment in non-cultivated areas by propagating and growing it for its purported medicinal or herbal values (Duke, 1985, cited in Bossard, 2000). Intentional introduction is the most likely source of new infestations, because the species is currently cultivated in several countries in which it has not, as yet, naturalised.
Pathway CausesTop of page
Impact SummaryTop of page
|Cultural/amenity||Positive and negative|
Economic ImpactTop of page
In 1926, Hilgendorf described M. pulegium as one of the worst weeds of the North Island of New Zealand. It is sometimes so abundant in pastures in New Zealand that they appear completely purple from a distance when it is flowering (Webb et al., 1988). The same author reported that M. pulegium is disliked by some New Zealand farmers because of its strong aroma which can taint milk. Parsons and Cuthbertson (1992) reinforced this view, writing ‘this pungently aromatic plant is unattractive to stock’. They added that it replaces higher quality feed, and that livestock avoid infested areas, so pasture area available to them is reduced. Dean (1990) added that farmers, agronomists and the Agricultural Protection Board (Western Australia) were of the opinion that the species has a high potential to cause grazing loss.
Environmental ImpactTop of page
Impact on habitats
Cal-IPC (2013) stated that the effects of M. pulegium are generally unknown, but that it could affect evapotranspiration from vernal pools and other wetland habitats.
Impact on biodiversity
M. pulegium has the potential to hybridise with M. arvensis, which is native to North America (Cal-IPC, 2013). Bossard (2000) stressed that the environmental impacts of M. pulegium are not well documented, but suggested that it may have displaced some native species in California.
Risk and Impact FactorsTop of page
- Negatively impacts agriculture
- Reduced native biodiversity
- Highly likely to be transported internationally deliberately
UsesTop of page
M. pulegium has been used in herbal remedies for very many years, even nearly being eradicated in Tunisia due to demand (Fadhel and Boussaid, 2004). It is now grown as a crop in several countries. Anon (2008) described its cultivation:
‘Cultivated in Europe and America (Canada, USA, Mexico, Cuba, Chile, Brazil) for obtaining the essential oil. Area and extent of cultivation is declining. Cultivated as a condiment in the Mediterranean, Georgia, occasionally also in India, Java and elsewhere. This mode of use is somewhat increasing. Also cultivated as a honey plant. Known as a medicinal plant since antiquity. Officinal drug in Middle Europe during the 16th C., now used only as homoeopathical remedy. The essential oil is rich in pulegon and is used against insects and for the preparation of perfume for soaps as well as for synthetic menthol.’
M. pulegium has been used in herbal medicine for centuries, its main value being as a digestive tonic, where it increases the secretion of digestive juices and relieves flatulence and colic (Chevallier, 1996; PFAF, 2013). M. pulegium also powerfully stimulates the uterine muscles and encourages menstruation; therefore it should not be prescribed for pregnant women since it can procure abortions, especially if the essential oil is used (Bown, 1995; Chevallier, 1996). The herb is reported to be antiseptic, antispasmodic, carminative, diaphoretic, emmenagogue (menstrual flow stimulant), sedative and stimulant (Grieve, 1976; Hedrick, 1972; Uphof, 1968; Launert, 1981; Mills, 1995). A tea made from the leaves has traditionally been used in the treatment of fevers, headaches, minor respiratory infections, digestive disorders, menstrual complaints and various minor ailments (Foster and Duke, 1990; Bown, 1995; Chevallier, 1996). It is occasionally used as a treatment for intestinal worms. Externally, an infusion is used to treat itchiness and formication, inflamed skin disorders such as eczema and rheumatic conditions such as gout (Chevallier, 1996). The essential oil in the leaves is antiseptic, though it is toxic in large doses (Foster and Duke, 1990).
The leaves are edible, either raw or cooked, and are used as a flavouring in salads or cooked foods (PFAF, 2013). It has a rather coarse spearmint-like flavour, and is no longer used very often in Britain. A herbal tea is made from the fresh or dried leaves.
The leaves of M. pulegium are sometimes used as a condiment. Mawe, in England in 1778, calls it a fine aromatic; it was among American potherbs in 1806. It was highly regarded in ancient times and had numerous virtues ascribed to it by both Dioscorides and Pliny. From the frequent references to it in Anglo Saxon and Welsh works on medicine, we may infer that it was esteemed in Britain (Hedrick, 1972).
Since rats and mice intensely dislike the smell of mint, M. pulegium was used in homes as a strewing herb and has also been spread in granaries to keep the rodents off the grain. (Phillips and Foy, 1990, cited in PFAF, 2013)
M. pulegium was used by sailors to purify drinking water on long voyages. It has also been used as an insect repellent (Parsons and Cuthbertson, 1992).
Uses ListTop of page
Drugs, stimulants, social uses
Human food and beverage
- Source of medicine/pharmaceutical
Prevention and ControlTop of page
Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.
Parsons and Cuthbertson (1992) suggested grubbing single plants or small infestations before flowering, taking care to remove as many roots and stolons as possible, and then burning the plants. This should be followed up by checking the treated area regularly and killing any seedlings or surviving plants. They also suggest cultivation of larger colonies on dry soils and sowing appropriate perennial pasture grasses. In wetter areas cultivation would probably be less effective, and herbicides could be used to control seedling weeds in the developing pasture. Bossard (2000) suggested that late spring or summer mowing, repeated over several years, may weaken plants by depleting photosynthetic reserves, but this lacks any practical confirmation, and the author asked for ‘definitive research data.’
Care should be taken in moving livestock from infested areas to new, susceptible (wetter) areas, because seed may be carried in wool or hair or in the digestive systems of livestock.
No research has been conducted on biological control agents of M. pulegium (Bossard, 2000). Support for biological control is unlikely, given the high value of this and other species of Mentha.
Seedlings are susceptible to 2,4-D, but established plants are resistant (Parsons and Cuthbertson, 1992). Older plants are best treated with glyphosate, metsulfuron-methyl, triclopyr or a triclopyr/picloran mix.
Many of the herbicides suggested for control of M. pulegium may pose hazards to non-target species in wetlands. This is true in California, where the species has invaded seasonally wet areas (Bossard, 2000). In addition, any control achieved by herbicides has only lasted for short periods, as reinvasion rapidly occurs from regrowth and germination from the seedbank (Dean, 1990).
In 1917, Deem, cited in Hilgendorf (1926), advised draining, subsoiling and liming, followed by fallowing for a season, followed by a quick-growing crop for one or two years, before growing grass again. On unploughable land, he suggested top-dressing with lime or basic slag, closing the field in early spring and not to admit stock until the New Year. In general this advice would probably be not too much different today, except that better knowledge of appropriate grass species to sow could improve the competition offered to newly emerging seedlings of M. pulegium.
Control by utilization
M. pulegium is reportedly avoided by livestock, so control by utilization is probably impractical and also carries the risk of transporting the seeds to unaffected areas.
Gaps in Knowledge/Research NeedsTop of page
Bossard (2000) highlighted the shortage of scientific literature about M. pulegium, especially concerning its management and control. Not much information has been published on the biology or control of this species either. Dean (2000) reported that in Australia, too, farmers in susceptible areas are ‘demanding the initiation of field experimentation to find a solution to the problem.’ Lockton (2013) commented that a clear description of the two varieties of the species found in Britain would be useful before determining whether populations are of putatively native or non-native provenance.
ReferencesTop of page
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: Original text by:
Ian Popay, consultant, New Zealand, with the support of Landcare Research.
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