Persicaria wallichii Greuter & Burdet, Himalayan Knotweed
Account Summary
Introduced, neophyte, occasional.
1947; MCM&D; Mountdrum Td, N of Lisbellaw.
May to November.
Growth form and preferred habitats
This 2 m tall, large-leaved, vigorous rhizomatous, decorative garden perennial forms dense clumps, and is a native of the Himalaya, the Indian subcontinent and Afghanistan. It is similar to the more notorious Japanese Knotweed (Fallopia japonica), but the leaves are longer and more pointed. The exact year of introduction of this large, dense, spreading, dominant perennial to gardens in Britain and Ireland is unknown, but it must lie around the very end of the 19th century. The first report beyond the garden wall was in 1917 in N Devon (Conolly 1977).
Most records of this giant plant in Britain and Ireland are from roadsides, hedge banks, railway banks, streamsides, waste ground and old quarry settings. From the evidence of the associated species, they clearly derive from discarded garden rubbish, presumably containing pieces of the rhizome. Very few reports occur anywhere in Britain and Ireland of situations where the plant might have spread beyond the confines of a garden by natural vegetative extension, ie under its own power of growth, without human assistance to transport it.
Taxonomic synonymy and variation
P. wallichii has been previously classified in three other genera, Reynoutria, Aconogonum and Polygonum. Its most recent synonym is Polygonum polystachyum (Wallich ex Meissner). A more hairy stemmed variant form exists and has been around since 1917 (Conolly 1977). It has been given the name Polygonum polystachyum var. pubescens Meissner (Clement & Foster 1994). So far, this form has not been recorded in Ireland (Conolly 1977; Reynolds 2002).
Fermanagh occurrence
The Flora Database records suggest this garden escape or discard has been rapidly increasing in the VC in recent years. Previously there was just the one old record listed above from the Meikle era (1947-53), the plant having escaped (presumably to a roadside location) from a nearby cottage garden outside Lisbellaw. Nowadays, however, there are 30 additional records dating from 1988 onwards. As the Flora map indicates, these are scattered across a further 21 tetrads, mainly in the NE and E of the VC. Most of the Fermanagh records are from roadside, waste ground and old quarry settings, and from the evidence of the associated species, derive entirely from discarded garden material.
Reproduction
Himalayan Knotweed rarely if ever sets seed in Britain and Ireland, and its propagation and dispersal is achieved by rhizome fragments transported by man in soil, mud, or amongst garden rubbish. Very few reports occur of situations in Britain and Ireland where the plant has or might have spread beyond garden confines by natural vegetative extension, ie under its own power of growth, without human assistance.
In recent years, we have also noticed Himalayan Knotweed becoming quite a common roadside plant in W Donegal (H35), where it again forms well-established, often dense pure stands, and indeed where it was first reported outside a garden setting in Ireland as long ago as 1928 (Conolly 1977).
British & Irish occurrence
While the New Atlas map shows there is a continuing increase in records throughout these islands, it is in SW Britain and in NW Ireland, where the two first 'escapes' occurred (or were recorded), that P. wallichii still maintains its greatest, most invasive presence (Preston et al. 2002). The great English horticultural expert Graham Stuart Thomas (2004) regarded P. wallichii as, "a tremendous spreader and only fit for landscape planting in moist ground, where it will smother everything and provide a wonderful display of fragrant plumes in October".
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 'wallichii' is the genitive meaning 'of Wallich', the surname of a taxonomist botanist associated with the genus.
Threats
None as yet in Fermanagh, particularly since it does not seem to set any seed. If this situation should change, perhaps encouraged by rising temperatures, secondary dispersal from existing established clumps could then pose a major risk to semi-natural vegetation. This has already happened with other invasive alien species in Britain and Ireland. As it is, the plant produces very dense clumps with which native species cannot compete. It has no natural enemies, and once established will be practically impossible, or extremely expensive, to eradicate.