Mentha aquatica L., Water Mint

Account Summary

Growth form and preferred habitats

M. aquatica is a very variable, creeping, rhizomatous, aromatic perennial of wetland habitats. Stems grow 15-90 cm tall, 4-angled, ± erect, simple or branched, hairless to densely tomentose. The stem hairs can vary from rather soft, long and spreading, to short, downwardly pointing, white.

The leaves release a powerful minty smell when pressed, which is generally perceived by the majority of people as pleasant and refreshing. It is sometimes alternatively described as 'orange-like' (Genders 1971). Perception of odours by the human nose is very variable, however, and finding words to describe smells can be extremely difficult and subjective. Other descriptions of M. aquatica suggest the smell of the plant varies from a fetid pungent odour to a pleasant camphor-like perfume (Melderis & Bangerter 1955).

Water Mint most typically occurs in permanently wet ground around the margins of open water in marshy ground, or in streams and ditches where it is regularly submerged. It is also very common, however, on temporarily flooded grasslands and, to a much lesser extent, in moderate broadleaf shade in wet, muddy woods and scrub.

M. aquatica forms extensive clonal patches by means of its creeping rhizome, but it is restricted to situations where competition from taller wetland dominants is reduced. The outcome of this competition may involve some degree of either soil infertility, or disturbance, limiting the growth of the dominant species, eg grazing, trampling or active management involving the cutting of the taller vegetation. Having said that, M. aquatica tends to be absent from unproductive soils of really low fertility (ie markedly acidic ones) and also from heavily disturbed sites (Grime et al. 1988, 2007).

Differences between aerial and submerged stems

Emergent plants of M. aquatica are well furnished with white hairs, but when the stem and foliage are submerged they are quite hairless. Submerged leaves are generally thinner and limper than aerial ones; the amount of chlorophyll is often decreased, but non-photosynthetic pigments, especially water-soluble anthocyanins and oil-based anthoxanthins, often appear in the vacuolar cell-sap of the outer tissues of the epidermis, turning the plant a dark crimson-purple colour (Sculthorpe 1967). Under these circumstances, the production of the anthocyanin pigment is a reaction to nutrient stress, most likely the result of an imbalance in the C/N ratio in the plant's amino-acid (R.O. Mackender, pers. comm., June 2002). Phosphate deficiency might also sometimes be involved (Hopkins 1995).

Anthoxanthins are plastid-based pigments involved in the dissipation of excess light energy absorbed during photosynthesis, diverting and breaking down excess excitation energy and converting it to heat. The anthoxanthins impart a yellowish-purple colour to tissues (Taiz & Zeiger 1998).

Anaerobic stress

As a semi-aquatic species, M. aquatica has only a limited ability to tolerate anaerobic stress, although in experimental measurements on detached rhizomes, Barclay & Crawford (1982) found it survived for four days immersion without any loss of regenerative power, a result similar to other marsh emergents such as Carex rostrata (Bottle Sedge), Juncus conglomeratus (Compact Rush) and J. effusus (Soft-rush). Anaerobic tolerance can vary with season and the state of the plant tissues. The two common Juncus species mentioned are both killed by as little as five days anoxia, as they possess a relatively shallow root system. These two Rush species really are oxygen stress avoiders, rather than tolerators and the same may well apply to M. aquatica. The roots of both Mentha and Juncus species produce relatively open aerenchyma tissue in their roots in wet or flooded soil. This provides large, continuous air spaces within the tissue that allows oxygen to diffuse down to roots penetrating below the water-table. Normally anoxia tolerance is greatest in spring and becomes lowered in summer when carbohydrate reserves are reduced (Crawford 1989).

Fermanagh occurrence

The most common and abundant mint in Fermanagh as elsewhere in B & I (present in every VC), M. aquatica frequents a wide range of wetland habitats in both sun and shade and occurs in 333 tetrads, 63.1% of the VC total. Water Mint is the 16th most frequently recorded plant in the Fermanagh Flora Database, a reflection of the extent of wetland in the survey area. As the tetrad map shows, M. aquatica is common on wet ground everywhere in Fermanagh and it is absent only from the most acidic bogs and the better drained lowland farmland in the east of the county.

Flowering reproduction

Flowering takes place from July to September, the small lilac flowers being clustered in a dense terminal head on the tip of the unbranched, erect stem, plus in axillary whorls around the next 1-3 lower nodes on the stem. The protandrous flowers have a short tube and protruding stamens and the stigma is of the 'brush type' (Faegri & van der Pijl 1971, p.241). The flowers attract flies, including those of the crane fly and mosquito families, as pollinators, plus occasional butterflies, such as the Small Skipper, Thymelicus sylvestris (Proctor et al. 1996). If insects fail to visit, the self-compatible flowers set seed regardless (Harley & Brighton 1977). It would be entirely normal and expected for the number of selfed seed set to be considerably fewer than those following cross-pollination. Seed is set from August to October, after which the aerial shoots die down. Seed dispersal is mainly by flotation, the nutlets having a thin corky outer coat with rather large air-filled intercellular spaces in the pericarp, providing prolonged buoyancy (Ridley 1930, p. 222; Sculthorpe 1967, p. 328).

Dormancy is broken after chilling, seeds either germinating in the spring in vegetation gaps, or becoming added to the persistent soil seed bank.

Toxicity and herbivory

Mint species contain irritant oils that can cause inflammation of the skin, cramps and other unpleasant symptoms if eaten in large quantity (Cooper & Johnson 1998, p. 126). However, M. aquatica is suppressed to some extent in grazed marshes, which suggests that it is browsed by stock animals.

Vegetative reproduction

Against the above mentioned suppression of the species populations by browsing stock, fragmentation of the rhizome due to trampling of wet, waterside ground by heavy animals enables dispersion of plant parts by flotation, increasing existing populations and extending distribution at least downstream.

In common with most non-weed plant species, no statistics are available on the frequency of M. aquatica seed regeneration. Seedlings are rarely observed, however, which suggests establishment and spread probably quite frequently involves vegetative reproduction.

Fossil record

As with all Mentha species, the fossil record for M. aquatica is very much based on the hard fruit nutlets and they are regularly inseparable from M. arvensis. They occur in the Cromer Forest Bed series and in every interglacial since. Godwin (1975) reckons M. aquatica has been present in B & I at least from the opening of the current Flandrian interglacial (in Ireland known as the 'Littletonian'), if perhaps not during the previous glacial period.

British and Irish occurrence

M. aquatica is common, widespread and often locally abundant throughout lowland areas of Britain and Ireland and there has been no obvious change in its presence, measured at the hectad level of mapping, in the two BSBI survey atlases (Perring & Walters 1962; Preston et al. 2002).

European and world occurrence

M. aquatica s.l. belongs to the European temperate element and occurs throughout Europe except the extreme north, although it is present in Iceland. It is also present and considered native in SW Asia, N Africa, The Azores and Madeira. It is introduced (somehow) in N & S America, S Africa and New Zealand (Hultén & Fries 1986, Map 1615).

Medicinal uses

The dried Water Mint herb yields about 4% essential oil that has an odour of Pennyroyal (M. pulegium). Russian Spearmint oil is also derived from a form of M. aquatica (Grieve 1931). The medicinal action of the plant is said to be emetic, stimulant and astringent. In herbal medicine, it is used to treat diarrhoea and as an emmenagogue (ie a substance that stimulates or increases menstrual flow). A mint tea made with this plant is recommended for colds and influenza or in any complaint where setting up perspiration is necessary and in all inflammatory cases, internal or external, it is regarded as a helpful treatment. A strong infusion is inclined to be emetic. A decoction of the plant prepared with vinegar is recommended to stop blood vomiting (Grieve 1931). A secondary focus of M. aquatica medicinal use is in treating indigestion, constipation, stomach ache and other digestive system ailments (Allen & Hatfield 2004).

Names

The genus name 'Mentha', in Latin was first used by Pliny, the myth associated with it appearing in Ovid, but it goes back further than this suggests. In Greek, the name is 'Minthe', the name of a beautiful nymph, and it is said to have been associated with the mint plant for up to 4,000 years, making it one of the oldest plant names still in use (Gilbert-Carter 1964; Stearn 1992). The Latin specific epithet 'aquatica' means 'growing in water'.

Threats

Probably under pressure from increasing eutrophication of wetland sites.

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