Short description of Persicaria wallichii, Himalayan Knotweed

A robust rhizomatous perennial to 1.8m, with pinkish or white flowers. The leaves are lanceolate, up to 20cm long, and with sheaths surrounding the stem at the base of their stalks as in other knotweeds.

Impact summary: Persicaria wallichii, Himalayan Knotweed

It is capable of colonising roadsides and riverbanks to the detriment of the native flora.

Habitat summary: Persicaria wallichii, Himalayan Knotweed

Damp grassland and streamsides.

Overview table

Environment	Terrestrial
Species status	Non-Native
Native range	Himalayas
Functional type	Land plant
Status in England	Non-Native
Status in Scotland	Non-Native
Status in Wales	Non-Native
Location of first record	v.c.54
Date of first record	1917

Origin

Native to the Himalayas.

First Record

In cultivation in GB by 1900. First recorded in the wild 1917 in North Devon.

Pathway and Method

Introduced as an ornamental garden plant. Very persistent in abandoned gardens and on roadsides, etc. where garden waste has been dumped. The plant is still available from some suppliers.

Species Status

Locally naturalised, especially in north and west GB. Although it was recorded in 374 10km squares between 1987 and 1999 as compared with 205 from 1970 to 1986, it is not clear whether its range is expanding, because of differences in recording practice. In all date classes it has been recorded in 608 10km squares up to 2010.

Dispersal Mechanisms

Plant fragments moved by cutting machinery or flowing water; deliberate dumping of garden waste.

Reproduction

Himalayan Knotweed has bisexual insect-pollinated flowers, but apparently rarely sets seed in GB. Reproduction is almost entirely vegetative, from fragments of rhizome, or pieces of aerial stem containing nodes. 

Known Predators/Herbivores

None known.

Resistant Stages

Overwinters as rhizomes after the aerial stems have died back.

Habitat Occupied in GB

Rough grassland, including railway banks, roadside verges and streamsides.

Scattered records throughout lowland GB, but with most records concentrated in the southwest and west Wales.

Environmental Impact

Forms very dense spreading stands that exclude other vegetation.

Health and Social Impact

None known.

Economic Impact

Slight at present, but it could become costly to control if there is significant spread from its established sites.

Identification

Stace, C.A. (2010) New flora of the British Isles, Third Edition, Cambridge University Press, Cambridge.

Biology, ecology, spread, vectors

Botanical Society of the British Isles. Vascular Plant Atlas Update Project http://www.bsbimaps.org.uk/atlas/ [March 2011].

Conolly, A.P. (1977) The distribution and history in the British Isles of some alien species of Polygonum and Reynoutria. Watsonia, 11, 291-311.

Management and impact

Child, L. & Wade, M. (2000) The Japanese Knotweed Manual. Chichester: Packard.

General

Ecological Flora of the British Isles http://www.ecoflora.co.uk/search_species2.php?plant_no=470020360 [March 2011].

Lousley, J.E. & Kent, D.H. (1981) Docks and Knotweeds of the British Isles. Botanical Society of the British Isles, London.

https://www.cabi.org/isc/datasheet/120210

https://secure.fera.defra.gov.uk/nonnativespecies/downloadDocument.cfm?id=1413