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Persicaria mitis (Schrank) Opiz ex Assenov (= Polygonum mite
Schrank = Persicaria laxiflora (Weihe) Opiz), Tasteless Water-pepper

Account Summary

Native or possibly a quite recent neophyte; rare, but readily subject to mis-identification. European temperate.

1988; NI Lakes Survey; Derrykerrib Lough, Upper Lough Erne.

July to August.

Growth form, identification and preferred habitats

A summer annual very similar to P. hydropiper (Water-pepper), but leaves not wavy margined and without the sharp (but often slowly developing) taste of that species, and the inflorescence nearly erect or slightly nodding (not drooping), usually redder (pink to purplish-pink) than P. hydropiper rarely greenish white. Glands on the flower perianth and the peduncle (ie the inflorescence stalk) are smaller, flatter and rather sparse (only c twelve per flower) in comparison with P. hydropiper. Other distinctive features that separate the two species are bristles at the tip of the stipules >3mm (not <3 mm), and the achenes are 3-4.5 mm, biconvex, ± shiny (not matt). Ripe fruit are almost essential in order to successfully separate out the riparian Persicaria species (Clapham et al. 1987; Parnell & Simpson 1988; Rich & Jermy 1998; Stace 2010; Sell & Murrell 2018).

In genetic terms, P. mitis (2n=40) is an autopolyploid derivative of P. hydropiper (2n=20), and therefore unsurprisingly they are very similar in their ecology. P. mitis is a plant of wet or damp, nutrient-rich ground near lakes, ponds, streams and ditches and in wet hollows in pastures (Parnell & Simpson 1988). It also appears as a pioneer colonist of nutrient-rich or manure-polluted mud or fen peat left exposed in dry summers when water levels in ditches, ponds and lakeshores is drawn down. It does not appear to display any particular preference in terms of soil reaction (J.O. Mountford, in: Stewart et al. 1994).

By themselves, the habitat conditions do not account for the rarity of this species, which seems to be more concerned with correct determination of critical plant characters. At the present time (April 2018), P. mitis remains rather poorly and unreliably recorded, due to its close similarity to P. hydropiper.

Fermanagh occurrence

All 20 records for this annual species in the Fermanagh Flora Database are from the NI Lakes Survey (1988-91), but since P. mitis (= P. laxiflora (Weihe) Opiz = P. mite Schrank) is regarded as a very rare species in Ireland, and it is readily confused with other Polygonum/Persicaria species, the identifications of all of the records really do still require expert determination. The Fermanagh records for P. mitis lie in a total of 15 tetrads, the majority of which are from the shores of both parts of Lough Erne. The remaining five lake sites are Keenaghan Lough, Parabaun (or Finnauan) Lough, Ross Lough near Carr Bridge, Lough Bresk near Lisnarrick and Watsons Lough.

It appears that only two voucher specimens were deposited in BEL to support the Fermanagh records of P. mitis, and Paul Hackney, when herbarium curator, reckoned one of them was incorrectly identified and required re-determination. Although considerable doubt is thus cast on all the NI Lakes Survey identifications of this taxon, in a case like this the neglect is tempered by the fact that the study was carried out by competent scientists who would (or should) have been aware of the relevant and important work carried out in NI towards distinguishing this species from its relatives. The relevant studies were published by Webb (1984) and by Parnell & Simpson (1988).

Irish occurrence

In his review of all the available Irish herbarium material, Webb (1984) defined several distinguishing characters and examined how it was that mistakes had been made by the experienced botanists concerned. Webb concluded with regret that all the pre-1969 Irish records were unreliable, and that most of them were wrong. The plants in question instead proved to be P. minus, P. hydropiper or P. persicaria.

The reasons for the mis-identifications were undoubtedly associated with poor choice of reputedly diagnostic characters by taxonomists of a pervious era, plus inadequate description in many or all of the published Floras of the time, together with the usual taxonomic problems and confusions surrounding difficult or critical plant groupings. P. mitis, P. minus and P. hydropiper all show a wide range of phenotypic variation in response to environmental factors, and P. maculosa (Redshank) and P. lapathifolia (Pale Persicaria) can also be difficult to separate from the others, particularly from P. mitis. Webb (1984) suggested that P. mitis was most probably introduced to Ireland fairly recently, perhaps during World War II (although this could hardly be proven). He concluded that the earliest reliable records for the species in Ireland were John Harron's post-1968 finds in the Lough Neagh basin (Kertland & Lambert 1972; Harron 1986).

In their Lough Neagh study, Parnell & Simpson (1988) found that P. maculosa and P. lapathifolia were in fact readily distinguishable from the three other species; the denser spike and small black patches on the leaves of P. maculosa, and the possession of both these features plus pedicel glands in P. lapathifolia, were perfectly distinctive. Parnell & Simpson (1988) made a detailed numerical analysis of 17 morphological characters to compare P. mitis, P. minus and P. hydropiper from Lough Neagh, and they devised a table of the nine characters which best separate the three species.

Previous work by Webb (1984) and by Lousley & Kent (1981) had indicated that the possession of glands on the perianth of P. hydropiper is one of the key characters allowing its discrimination from both P. mitis and P. minus. However almost all the P. mitis plants Parnell & Simpson examined also possessed perianth glands, which on close examination proved to be smaller, fewer in number and almost flat in comparison with the more numerous and more prominent raised glands on P. hydropiper.

The same situation was found with respect to perianth glands when British and European P. mitis plants were examined, a feature ignored in standard works on the genus. As is often the case, a combination of characters needs to be compared when attempting to separate these three similar species (Parnell & Simpson 1988).

Hybrids

To add to identification difficulties there are intermediate hybrids formed with P. maculosa (Redshank), P. hydropiper and P. minor (Small Water-pepper), although they all are either rare, very rare, absent or in need of confirmation in Britain and Ireland (Stace et al. 2015; Sell & Murrell 2018).

British occurrence

Stace (1997, 2010) and Sell & Murrell (2018) regard P. mitis as native in Britain, where the plant is described as being, "rare and very scattered over England, Wales and NE Ireland". Stace (1997, 2010) is of the opinion that it is over-recorded. The New Atlas treatment adds little or nothing to the above, but recognises that it is a scarce species.

European and world occurrence

P. mitis is a native of temperate areas of W & C Europe, and from the published map in Jalas & Suominen (1979), Map 403, appears to be mainly confined between 40° and 55°N, the distribution thinning markedly both towards the Mediterranean and the Baltic. P. mitis does not feature at all in Jonsell et al. (2000). P. mitis is shown as an Eurasiatic species by Hultén & Fries (1986, Map 648), although they describe their map as tentative because of difficulties in identification of the species. Clapham et al. (1987) also mention P. mitis occurring in W Asia.

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 'mitis' means 'mild', 'mellow' or 'bland' (Gilbert-Carter 1964), and clearly refers to the absence of the peppery taste of P. hydropiper, the more common species with which it is most likely to be confused.

The English common name 'Tasteless Water-pepper' is an excellent example of an invented, so-called 'book name', given to a plant with absolutely no folk associations attached to it.

Threats

None.