Persicaria campanulata (Hook. f.) Ronse Decr., Lesser Knotweed

Account Summary

Growth form and preferred habitats

This is a vigorous, colonising, rhizomatous garden perennial, native of the Himalaya and W China. P. campanulata is valued by the gardener for its long decorative display of many, small, clustered, dense panicles of usually pink, rarely red or white bell-flowers from midsummer until well into October. Leaves are 2-4 cm wide and have a dense, pinkish-brown tomentum of hairs beneath. Lesser Knotweed likes moist or cool soil, and in the garden it looks best when grown in part-shade (Thomas 2004).

P. campanulata was introduced to gardens in Britain and Ireland around 1909 and was first reported in the wild in 1933, since which time it has slowly but continually expanded across these isles. The species can tolerate a wide variety of growing conditions however when found discarded on roadsides in W Ireland. It appears to thrive in the suitably wet, cool soils provided by our mild oceanic climate.

Much of the increase of P. campanulata in Britain and Ireland, which has been very well described by Conolly (1977), has been in scattered W & SW coastal areas of both islands, rather than in inland or east coast regions. This distribution suggests the species rather surprisingly might not be all that frost hardy. The New Atlas hectad map supports this notion to some extent, but the true position is not clear since a minority of inland records do exist both in the English Midlands, and in the climatically more continental SE region of the country. It is possible that the represented stations lie within urban areas that provide higher night-time temperatures, but we do not know if this is the case or not.

Reproduction

The small, pink or red, bell-shaped (campanulate) flowers are heterostylous, with long- and short-styled flowers borne on separate plants. This feature may help account for the surprising lack of reports of seed-set in these islands (Lousley & Kent 1981). Lesser Knotweed, which despite its English common name is often tall, some annual stems reaching up to 60-90 cm in height. Plants spread and maintain themselves by means of extensive prostrate stems and both above-ground stolons and buried rhizomes. In winter they die down to just a few basal leaves.

Taxonomy and nomenclature

Classification of this family is such that many members have changed genus and species names time after time. Synonyms for this particular species include: Aconogonum campanulatum (Hook. f.) H. Hara; A. lichiangense (W. Smith) Soják; Polygonum campanulatum Hook. f. and Polygonum campanulatum var. lichiangense (W. Smith) Stewart.

There are several garden cultivars of this plant in use. The darker rose-pink flowered form is called 'Rosenrot' or 'Roseum', and there are white-flowered forms called 'Album' and 'Southcombe White' (Griffiths 1994).

Fermanagh occurrence

P. campanulata is rather rare in Fermanagh, there being just eleven records, all post-dating 1995. It was first discovered, dumped with garden rubbish, in a small roadside quarry in 1996 as listed above, and since then ten additional finds, all involving RHN, have been made in seven further tetrads, scattered mainly in the northern half of the county. The plant is generally found in damp ground on roadsides, rarely far from houses.

Apart from the first record above, the local record details are: roadside at Gubbakip Td, 13 August 1996, RHN & RSF; opposite abandoned cottage, Mountdrum Td, 19 October 1997, RHN; Clonelly, NW of Kesh, 17 March 1999, RHN & HJN; gateway at Lough Skale, 22 September 2000, RHN; roadside, E end of Boa Island, 6 September 2001, RHN; roadside, Castle Caldwell, 7 September 2002, RHN & HJN; roadside Lackboy, 7 September 2002, RHN & HJN; Coolbuck Td, 11 September 2002, RHN & HJN; gateway, 100 m N of Slisgarrow Td, 16 August 2006, RHN; between road and shore, Bleenalung Bay, Lower Lough Erne, 4 October 2010, RHN & HJN.

British and Irish occurrence

In England & Wales, Lousley & Kent (1981) regarded P. campanulata as only semi-naturalised in wet places at or near the point of planted introduction. They contrasted this with the behaviour of the species in Scotland & Ireland, where they considered it fully naturalised in remote areas of both countries. The New Atlas hectad map shows the species is only thinly and very widely scattered across both Britain and Ireland, although it covers the whole range of latitude from Jersey to Shetland. The New Atlas map also indicates the quite marked westerly trend in distribution found in both Britain and Ireland.

Apart from the expected 'garden escapes', in Ireland, P. campanulata is sometimes recorded as a relic of previous cultivation in landed demesnes and parklands, eg Castleward in Co Down (H38) and Castle Dobbs in Co Antrim (H39) (Cat Alien Pl Ir). Some of the stands in ground remote from habitation may have arisen as the result of fly-tipping of garden refuse and outcasts, a reprehensible and all too common practice nowadays throughout these islands. The most extensive naturalised stand of the plant, reported by Conolly (1977), was along the bank of the River Camp in south Co Kerry (H1), where it stretched for at least 1 km. It appears unusual for P. campanulata to spread greatly from the area where it is either planted or discarded, but damp, linear habitats, such as riverbanks, probably represent sites where the species is best able to colonise and spread, assisted by water flow.

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 'campanulata' is a diminutive of 'campana', meaning 'bell', and thus means 'small bell', in this case a very good description of the individual flower (Gilbert-Carter 1964).

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