Persicaria hydropiper (L.) Delarbre, Water-pepper

Account Summary

Growth form and preferred habitats

The very hot, peppery taste of the 3-8 cm long lanceolate leaves is a very distinctive feature of this strictly summer annual. P. hydropiper is a hairless (ie glabrous) plant up to 75 cm tall, with green stems, often with a red ring beneath each node. As the plant ages the stems become increasingly red. It produces long, slender, nodding inflorescences of greenish-white flowers from July to September. The perianth flower parts are covered with yellow glands, and the number of styles is usually two, more rarely three. The peppery taste of the leaves arises from glandular dots on their surfaces. There is no odour off the plant (Timson 1966; Clapham et al. 1987).

P. hydropiper is confined to a quite limited range of habitats and phenotypic variation in this species is slight. Nevertheless, it grows in a variety of wet, often muddy, open, waterside, but always terrestrial habitats. Indeed, it colonises any bare or open muddy ground where shallow water tends to stand in winter and dry out in summer. Occasionally this includes the plant appearing in ± temporary puddles, created for instance by vehicle tracks, hoof-prints or simply the lie of the land in damp, patchy meadows, on woodland rides, or along roadside verges.

P. hydropiper colonises a wide range of soil textures from clay to peat, but while Praeger (1934) regarded it as a strongly calcifuge (lime avoiding) species in Ireland, and in their Flora of Connemara and the Burren Webb & Scannell (1983) more or less concur ("distinctly calcifuge in its general distribution"), the latter authors allow that P. hydropiper does occur in abundance in some calcareous habitats, especially in the eastern inland portion of the Burren, Co Clare (H9). Lousley & Kent (1981) hedged their bets and described the species as, "usually calcifuge". In Fermanagh, plants are most often growing in moderately acid to near-neutral, base- or lime-poor to rich soil conditions. As it occupies such a wide range of soils, it is not a useful species indicator of marked substrate acidity, or of lime- or base-avoiding nature.

Being essentially a pioneer colonist of open, bare, muddy ground, Water-pepper is probably only weakly competitive, but it has most of the properties of a successful ruderal and it behaves in a similar manner, being tolerant of stress in the form of trampling and winter flooding. It cannot withstand either drought or frost and therefore is mainly confined to lowland sites, typically near water (Timson 1966).

Reproduction

Like other small-flowered Knotweeds which have long, lax, terminal inflorescences and small subsidiary inflorescences in the axils of leaves, P. hydropiper is rarely visited by insects and is almost entirely inbreeding (ie it is autogamous and automatically self-fertilises). Furthermore, the flowers in the small inflorescences formed in the axils of leaves on lower stem nodes are cleistogamous, meaning they self-fertilize while still in the unopened bud stage of development (Timson 1966). The fruit of each individual flower is a single seeded nut-like achene. The achenes are c 3 × 2 mm, biconvex or trigonous (three-sided), depending on whether the flowers that formed them had two or three stigmas. The achenes produced by cleistogamous flowers are irregularly shaped, and all achenes are dark brown to black in colour and have a matt surface.

P. hydropiper flowers in all its many habitats, but it produces fewer seeds in drier or poorer soils. The typical plant produces around 300-400 seeds, but there is great variation in quantity, in response to habitat conditions (Timson 1966). Like other members of the genus, the single-seeded nut-like, achene fruits are capable of long-term survival in the soil seed bank (more than five years), although the record on this is quite variable, several European studies suggesting they are ephemeral (Thompson et al. 1997).

In view of the degree of selfing shown, the small amount of Persicaria pollen produced and its sticky nature, Timson (1965b, 1966) felt that the virtual absence of cross-pollination ruled out any likelihood of previously claimed hybrids involving P. hydropiper, or else at least it rendered them extremely rare reproductive anomalies. Also, the putative parents all being annual species curtails the time available for the genetic readjustment necessary for some of the possible hybrids to survive. The absence of any form of asexual reproduction or apomixis certainly prevents the survival of triploids or any other genetically unbalanced forms that might arise (Timson 1965b).

Hybrids

A study at Lough Neagh of P. hydropiper and its close relatives, P. mitis (= P. laxiflora Weihe) Opiz.) (Tasteless Water-pepper) and P. minor (Small Water-pepper), showed that while the latter two species did cross with one another, P. hydropiper did not hybridise with any other species (Parnell & Simpson 1988). Later study appears to have accepted that P. hydropiper can rarely form hybrids with other annual species P. maculosa, P. lapathifolia, P. minor and P. mitis (Jonsell et al. 2000; Anonymous 2002, p. 230). Stace et al. (2015) examined hybrid claims in Britain and Ireland involving crosses between P. hydropiper and P. lapathifolia, P. mitis and P. minor. They concluded that all of the hybrid claims made in both islands still require confirmation.

Dispersal

As one would expect, the strong acrid flavour of the plant sap is extremely effective in protecting the herb from grazing animals, which soon learn to avoid it. However there is a downside to this protective feature, since it rules out an efficient means of dispersal – ie the internal animal transport of ingested achenes (Timson 1966). However, despite lacking any obvious means of species dispersal, apart that is from accidental transport in mud, transient flotation in water, or as a contaminant of agricultural crop seed (especially amongst clover seed), P. hydropiper is a common and widespread species in Britain, Ireland and beyond. Inefficient seed dispersal also results in Water-pepper tending to occur in dense patches, either large or small (Timson 1966).

Fossil history

Fruits have been recorded from the Pastorian interglacial and the late Flandrian, and also from two intervening glacial stages – the late Anglian and tentatively, from the Middle Weichselian. From the late Flandrian the fossil record stretches through the Bronze Age more or less continuously to the Mediaeval Period. Many of the finds are from archaeological sites, but the record certainly represents that of a native species (Godwin 1975).

Fermanagh occurrence

Water-pepper is the most frequently recorded member of the genus Persicaria in Fermanagh. It beats P. amphibia (Amphibious Bistort) into second place by nearly 100 records. While it is considerably more common than the third ranking species P. maculosa (Redshank), however, it is not as widely distributed as the latter, being recorded in 221 tetrads, 41.9% of those in the VC, compared with Redshank's 53.8% tetrad presence. This situation probably reflects Water-pepper's lack of animal transport. A small minority of people cannot detect the acrid taste, however, and this may also happen with some herbivores!

Nevertheless, P. hydropiper is the most common and the second most widespread member of the genus in our survey, and as the tetrad map shows, it is widely scattered throughout Fermanagh, although it appears to be most heavily represented around the shores and water-meadows of Upper Lough Erne. This distribution is undoubtedly achieved and maintained by achene flotation. However, as mentioned previously, it is possible that the extremely detailed botanical survey of the Upper Lough carried out by DOENI field survey staff in the mid-1980s (the forerunners of the EHS Habitat Survey Team), may skew our species distribution, over-representing the presence of the plant in comparative terms with the rest of the VC, which has not received the same level of recorder man-hours.

British and Irish occurrence

The New Atlas describes and shows that P. hydropiper is widespread in both Britain and Ireland, although it is rather rare in C Ireland (previously the main area for raised bogs, and with soil underlain by a rock skeleton of Carboniferous limestone). In Britain, the species has a distinctly western trend in its distribution as one goes northwards from the English Midlands and on up into Scotland, where it gradually thins out and disappears to the east and then the north (Preston et al. 2002).

European and world occurrence

A native of Eurasia, P. hydropiper is widely distributed in middle latitudes across Europe, thinning noticeably towards both the north and the Mediterranean. It is not uncommon in southern parts of Norway, Sweden and Finland and it penetrates to a couple of sites within the Arctic Circle (Timson 1966; Jalas & Suominen 1979, Map 404). The distribution extends east into C Asia and it is thinly scattered as a naturalised alien through many areas of China, Japan, SE Asia, S Australia and New Zealand, presumably introduced as an impurity with crop seed (especially with clover) (Hultén & Fries 1986, Map 649).

The status of the species in N America is uncertain (Timson 1966). Hultén (1974, Map 209) discussed the question and decided it needed a monograph study to resolve it. Hultén & Fries (1986) suggest there is a mixture of native and introduced forms of the species spread across N America, but the very widespread alien occurrence of P. hydropiper elsewhere in the world, renders this idea unconvincing.

Uses

As its peppery taste indicates, P. hydropiper contains a sharply acrid, acidic sap that causes irritation and inflammation to the skin, and to the gut if it is ingested. However, there are no recent reports of poisoning from this source (Cooper & Johnson 1998). In the past, P. hydropiper was a valued plant used in herbal medicine (Grieve 1931), and recently published biochemical studies, mainly from the Far East, supports the notion that it has useful healing properties.

European herbalists regarded it as a stimulant, a diuretic (induces increased urine), diaphoretic (induces sweating), emmenagogue (induces menstral flow) and it arrests bleeding. In Scotland, it was also used to treat dysentery, flatulence and piles as well as gravel in the kidneys or bladder. It was used for treating minor cuts and bruises and ailments such as colds, coughs, toothache and herpes ulcers, as well as for more serious complaints such as epilepsy, gout and gangrene (Launert 1981; Darwin 1996). In Bangladesh, leaves of the plant are traditionally used in the treatment of rheumatic pain, gout, and for skin diseases such as ringworm, scabies, boils, abscesses, carbuncles and the bites of snakes, dogs and insects.

The juice of the plant is a common remedy against body lice in cattle and sheep, and it is also used as an insect repellent. Yellow and gold-coloured dye can also be obtained from the chopped plant (Darwin 1996).

Names

The genus name 'Persicaria' is from the Latin 'persicum' meaning peach, and translates as either 'peach-leaved' (Gilbert-Carter 1964), or 'peach-like' (Gledhill 1985). The Latin specific epithet 'hydropiper' is a combination of the Greek 'hydor', 'water', and Latin 'piper' meaning 'pepper' and hence the most commonly used English common name, 'Water-pepper', is a straight translation of the botanical species name.

Alternative common names include 'Marsh-pepper', 'Pepper Plant', 'Biting Persicaria', 'Bity Tongue', 'Smart-weed', 'Smartass', 'Arcmart', 'Hot Arsmart', 'Arsesmart', 'Red Knees', 'Bloodwort', 'Ciderage' and 'Culrage' (Grieve 1931, p. 743; Darwin 1996). As Grigson (1987) quotes John Minsheu (1617) Ductor in Linguas, it is 'Arsesmart' "because if it touch the taile or other bare skinne, it maketh it smart, as often it doth, being laid unto the bed greene to kill fleas".

Threats

None.

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