Persicaria wallichii
(Himalayan knotweed)

Pictures

Picture	Title	Caption	Copyright
Title Habit Caption Persicaria wallichii (Himalayan knotweed); habit. Marcourt, Belgium. October 2011. Copyright ©Gilles San Martin-2010/via wikipedia - CC BY-SA 3.0	Habit	Persicaria wallichii (Himalayan knotweed); habit. Marcourt, Belgium. October 2011.	©Gilles San Martin-2010/via wikipedia - CC BY-SA 3.0
Title Flowers Caption Persicaria wallichii (Himalayan knotweed); flowers and leaves. Marcourt, Belgium. October 2011. Copyright ©Gilles San Martin-2010/via wikipedia - CC BY-SA 3.0	Flowers	Persicaria wallichii (Himalayan knotweed); flowers and leaves. Marcourt, Belgium. October 2011.	©Gilles San Martin-2010/via wikipedia - CC BY-SA 3.0
Title Flowers Caption Persicaria wallichii (Himalayan knotweed); flowers and leaves. Marcourt, Belgium. October 2011. Copyright ©Gilles San Martin-2010/via wikipedia - CC BY-SA 3.0	Flowers	Persicaria wallichii (Himalayan knotweed); flowers and leaves. Marcourt, Belgium. October 2011.	©Gilles San Martin-2010/via wikipedia - CC BY-SA 3.0
Title Flowers Caption Persicaria wallichii (Himalayan knotweed); close view of flowers. Copyright ©Frank Vincentz-2007/via wikipedia - CC BY-SA 3.0	Flowers	Persicaria wallichii (Himalayan knotweed); close view of flowers.	©Frank Vincentz-2007/via wikipedia - CC BY-SA 3.0

Identity

Preferred Scientific Name

• Persicaria wallichii Greuter & Burdet

Preferred Common Name

• Himalayan knotweed

Other Scientific Names

• Aconogonon polystachyum (Wall. ex Meisn.) M.Král
• Persicaria wallichii var. wallichii
• Peutalis polystachya (Wall. ex Meisn.) Raf.
• Polygonum polystachyum Wall. ex Meisn.
• Reynoutria polystachya (Wall. ex Meisn.) Moldenke
• Rubrivena polystachya (Wall. ex Meisn.) M. Král

International Common Names

• English: cultivated knotweed; garden smartweed; Himalayan knotweed; Kashmir plume
• Chinese: duo sui shen xue ning

Local Common Names

• France: renouée à nombreux épis
• Germany: Vielähriger-Knöterich (Himalaya-Knöterich)
• India: saran
• Netherlands: Afghaanse duizendknoop

Summary of Invasiveness

Persicaria wallichii is a shrubby perennial herb up to 180 cm tall that originates from the temperate, western regions of Asia and the Indian subcontinent. It is naturalized in Europe, Canada and the United States, where it was introduced as a garden ornamental. It grows vigorously and creates large and dense stands that exclude native vegetation and prevent tree seedlings from growing. P. wallichii can greatly alter natural ecosystems and promotes the erosion of river banks. It is reported as invasive in its native range in northern India (Kala and Shrivastava, 2004), as well as in its non-native range in Belgium and the UK (Rich and Woodruff, 1996; Branquart et al., 2007). In the western USA it is a declared noxious weed in the states of Montana, California, Washington and Oregon (USDA-NRCS, 2015).

Taxonomic Tree

• Domain: Eukaryota
•     Kingdom: Plantae
•         Phylum: Spermatophyta
•             Subphylum: Angiospermae
•                 Class: Dicotyledonae
•                     Order: Polygonales
•                         Family: Polygonaceae
•                             Genus: Persicaria
•                                 Species: Persicaria wallichii

Notes on Taxonomy and Nomenclature

Persicaria is a genus of herbaceous plants belonging to the family Polygonaceae, which currently comprises 150 species worldwide (Simpson, 2010). It is a cosmopolitan genus, occurring mainly in temperate regions, but with some species in tropical and subtropical regions, from sea level to high altitude (Heywood et al., 2007).

The genus Persicaria was formerly included in the genus Polygonum. In 1754, Persicaria was established by Miller, but without a type specimen, and was later lectotypified by Britton and Brown (1913) based on Polygonumpersicaria L. The nomenclature of Polygonum has been interpreted differently by various authors and it seems that the definition of Persicaria and Polygonum is still open to debate (Decraene and Akeroyd, 1988; Heywood et al., 2007). Many authors treat Persicaria as a section of Polygonum s.l. (Dammer, 1892; Steward, 1930; Li et al., 2003). However, studies of Persicaria based on morphological characters, such as pollen (Hedberg, 1946), anatomy (Haraldson, 1978), flowers (Decraene and Akeroyd, 1988) and fruits (Decraene et al., 2000), suggest that Persicaria should be recognized as a separate genus (Manju and Vibhasa, 2013). The redefinition of Persicaria has also been supported by molecular studies (Sanchez et al., 2009; Sanchez et al., 2011; Schuster et al., 2011; Schuster et al., 2015). Nonetheless, it is still commonly referred to in the literature as Polygonumpersicaria which should be noted when searching and using published records.

Description

Following Hong (1993) and Alaska Natural Heritage Program (2011):

P. wallichii is a shrubby perennial herb with plant height at maturity of 40-120 cm, rarely up to 180 cm. The stem is unarmed, ascending to erect and branched, usually reddish-brown, often flexuous above, smooth to densely pubescent.The shape of the leaves are lanceolate to elliptic-lanceolate, (7.5-) 9-22 (-27) × 2.8-7.8 cm, smooth to merely densely pubescent above, sparsely to densely pubescent or sometimes with brownish woolly covering of fine, soft hairs below.

The inflorescence in P. wallichii is mostly a richly branched panicle. Panicles are wide, spreading, 4-11 cm long, and 1-5.5 cm wide. The flowers are 3-5 mm long, usually creamy-white or sometimes pink-ish. The lobes of the flower vary in number from three to five, but five is the most common number. Seeds are brown, 2.1-2.5 mm long, and 1.3-1.8 mm wide. The flowers of P. wallichii are heterostylous (distyly), usually with scattered, numerous reddish glands, slightly fragrant.

Plant Type

Distribution

P. wallichii occurs in the Himalayan subalpine region from Pakistan and northwestern  India through Nepal, Bhutan, northeastern India, southern Tibet and southwestern China to northern Myanmar. It has been introduced to several European countries (Tutin et al., 1964), to the UK in particular (Conolly, 1977; Lousley and Kent, 1981), as well as to western North America (Hitchcock et al., 1964), where it is now semi-naturalized (Hong, 1993). P. wallichii has been introduced to New Zealand, but there are few records regarding its introduction or current status. According to some botanists (Hooker, 1886; Hara, 1982), P. wallichii is also known from Afghanistan. However, Hong (1993) reports a dearth of material collected from the region.

Distribution Table

The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

History of Introduction and Spread

P. wallichii was introduced to the UK just before 1900, but was first recorded in the wild in 1917, in North Devon (Ison, 2011). Its distribution in the UK has expanded considerably since 1962. In Belgium, P. wallichii was introduced in 1898 (Branquart et al., 2007).

Introductions

Risk of Introduction

P. wallichii is commonly used as an ornamental and is planted in gardens and public green spaces, so the risk of introduction in some regions may be high. However, in the USA it is a declared noxious weed in four states, and this may reduce the risk of intentional introduction to neighbouring states.

In those countries where the plant is established, the risk of accidental introduction remains high. The rhizome of P. wallichii fragments easily and can be carried along river systems, with even small fragments able to form new plants (Branquart et al., 2007).

Habitat

P. wallichii is very common in both the moist and dry zones. It grows on rocks and slopes, in grassy meadows, bushy ground, wet forest, marshes and riparian zones. P. wallichii grows best on moist nutrient-rich soils, and tolerates various light conditions. Dense colonies of P. wallichii are mainly found in disturbed habitat types such as eroded, avalanche-prone, fragmented treeline and bouldery (Kala, 2004). Anthropogenic habitats like roadside ditches, irrigation canals, and other water drainage systems can be colonized by P. wallichii.

There are variable reports on the altitudinal range of P. wallichii. According to Hong (1993), the known altitudinal range is 1200-4600 m, while Osmastonj (1922) reports an altitudinal range of 2500-3500 m. However, because the plant is (semi)naturalized in Europe, it can be estimated that the altitudinal range of P. wallichii is 0-4600 m.

Habitat List

Biology and Ecology

Genetics

The cell study on P. wallichii (as Polygonum polystachyum) by Jaretzky (1928) suggested that the chromosome number for this species is 11 (2n = 22). This was confirmed by Hong (1993).

Reproductive Biology

P. wallichii has bisexual, insect-pollinated flowers and reproduces sexually by seeds, as well as vegetatively from extensive rhizomes. WSDA (2008) reports that the germination requirements of P. wallichii seeds are largely unknown, but that seedlings may not survive in shaded areas.

Several sources (DiTomaso and Healy, 2007; Ison, 2011) report that seed production is rare in California, British Columbia and the UK. Instead, in these introduced regions, P. wallichii tends to reproduce vegetatively from its rhizomes, benefitting from seasonal high water events and floods, which aid fragmentation and dispersal. Cut or broken stems and roots will sprout if left on moist soil or put directly into water (Soll, 2004).

The flowering period of P. wallichii is August to September, and it sets fruit from September until October (Missouri Botanical Garden, 2015). P. wallichii dies back in the winter leaving brittle brown stems. The amount of time seeds remain viable in the soil is unknown.

Associations

According to Kala (2004)P. wallichii is associated with Impatiens sulcata in Western Himalaya, India. Hong (1993) mentions Abies sp., Quercus sp., and Rhododendron as species that are associated with P. wallichii.

Climate

Latitude/Altitude Ranges

Air Temperature

Soil Tolerances

Soil drainage

• free
• seasonally waterlogged

Soil reaction

• acid
• alkaline
• neutral

Soil texture

• heavy
• light
• medium

Means of Movement and Dispersal

Hill et al. (2009) have rated the dispersal potential of P. wallichii as ‘high risk’. The species is highly fecund, can easily disperse by active or passive means over distances of more than 1 km per year and can initiate new populations. Means of dispersal include wind, water, animal movements, translocation by humans or accidental transport by human agency.

Natural Dispersal

Seeds of P. wallichii are dispersed by wind and water, while rhizome and stem fragments are dispersed in waterways or by flooding (DiTomaso and Healy, 2007). Seasonal high water events and floods spread plants into rivers, creeks, roadside ditches, irrigation canals, and other water drainage systems. Root and stem fragments as small as 1 cm can form new plant colonies.

Accidental Introduction

Stem or root fragments can be spread in contaminated fill material (Soll, 2004).

Intentional Introduction

The horticultural trade still acts as a pathway for new introductions of P. wallichii, both locally and internationally.

Pathway Causes

Pathway Vectors

Impact Summary

Environmental Impact

Impacts on Habitats

In the UK, Hill et al. (2009) rated the environmental impact of P. wallichii, using various parameters: dispersal potential, colonization of natural and semi-natural habitats, alteration of ecosystem function and adverse impacts on native species. The total scoring for invasiveness of P. wallichii based on these parameters was “potential threat”.

According to Hill et al. (2009), P. wallichii in the UK poses a ‘medium risk’ to natural and semi-natural habitats, and may occasionally colonize these areas. However, populations of this species are usually confined to habitats with low or medium conservation value. P. wallichii also brings a ‘medium risk’ of altering ecosystem function, including nutrient cycling, physical alteration, successions and food webs. The impact on ecosystem processes and structures was considered moderate and reversible (Hill et al., 2009).

In the USA, P. wallichii is known to reduce the quality of fish and wildlife habitat in riparian areas. Infestations may reduce insect populations that provide food sources to salmon. P. wallichii can also reduce the availability of nutrients in the soil. It can compete with trees and reduce shade along rivers and streams by displacing native, woody species (WSDA, 2008). Stands of P. wallichii produce dense mats of leaf litter that prevent the germination of native species (Wilson, 2007). Areas that are covered with stands of P. wallichii can be vulnerable to erosion when it dies back in the winter.

In its native range of India, P. wallichii is described as an aggressive, colonizing species within the Valley of Flowers National Park in Uttarakhand in the western Himalayas (Kala and Shrivastava, 2004). Kala and Shrivastava (2004) state that manual removal of P. wallichii can be counterproductive, because it is usually replaced by other invasive colonizers. Eradication of P. wallichii is also not recommended because it can initiate land instability.

Impact on Biodiversity

According to Hill et al. (2009), the adverse impacts of P. wallichii on native British species in terms of competition carries a ‘high risk’. It can cause local severe (> 80%) population declines of valued or rare species, and may reduce local species richness irreversibly. At a regional scale, it may cause species decline.   

P. wallichii emerges early in the growing season and can outshade and displace native vegetation. It also forms dense stands that exclude native species (DiTomaso and Healy, 2007; Wilson, 2007).

Risk and Impact Factors

• Invasive in its native range
• Proved invasive outside its native range
• Abundant in its native range
• Is a habitat generalist
• Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
• Pioneering in disturbed areas
• Highly mobile locally
• Reproduces asexually

• Damaged ecosystem services
• Ecosystem change/ habitat alteration
• Modification of nutrient regime
• Monoculture formation
• Negatively impacts aquaculture/fisheries
• Reduced native biodiversity
• Threat to/ loss of native species

• Competition - monopolizing resources
• Competition - shading
• Rapid growth

• Highly likely to be transported internationally deliberately
• Difficult to identify/detect as a commodity contaminant
• Difficult/costly to control

Uses

Social Benefit

In Punjab, India, there are records of the tender young shoots and leaves of P. wallichii being eaten as a vegetable (Bamber, 1916).

Uses List

Environmental

• Amenity

Human food and beverage

• Vegetable

Ornamental

• garden plant

Similarities to Other Species/Conditions

P. wallichii is similar to P. pinetorum in leaf shape, ranging from lanceolate to elliptic-lanceolate with acuminate apex and an auriculate or sometimes subcordate base, and any differences between the species in leaf size and shape are small and hard to distinguish (Hong, 1993).

The Alaska Natural Heritage Program (2011) reports on three other species that are similar in appearance to P. wallichii. These are giant knotweed (Fallopia sachalinensis), Japanese knotweed (F. japonica) and Bohemian knotweed (F. x bohemica). The Fallopia species can be distinguished from P. wallichii by the presence of wings or keels on the tepals, green-white to white flowers, ovate leaves with tapered or abrupt tips, and mottled, purple-brown stems (Wilson, 2007; Flora of North America Editorial Committee, 2015). F. sachalinensis has leaves that are often 20-40 cm long. F. japonica and F. x bohemica have few or no hairs along the leaf margins or on the veins on the leaf undersides (Wilson, 2007).

P. wallichii can also be confused with Alaska wild-rhubarb (Aconogonon alaskanum), which is native to Alaska. A. alaskanum has petioles that are 0.8-3.5 mm long, inflorescences that are 0-4 cm long, and green-white to white flowers (Flora of North America Editorial Committee, 2015).

Prevention and Control

Physical/Mechanical Control

Several physical control methods for invasive knotweeds (including P. wallichii) which are used in the Pacific Northwest (USA) are described by Soll (2004). These include manual cutting using a machete, loppers or pruning shears to cut the stems down to the ground surface as often as possible, but at least every 2-3 weeks from April (or as soon as the plant appears) until August. Sprouting slows after August, so cutting frequency can be reduced, but plants should be prevented from ever exceeding 15 cm in height. This practice should be continued for at least two or three years if the patches are well established. All cut material should be collected and disposed of according to regional legislation.

Mowing can also be carried out using a weed-eater or mower to cut as low as possible and as often as possible, but at least every 2-3 weeks through August. If the knotweed is growing in soft soil, the plant can be pulled out by the root crown, with removal of as much of the root system as possible. This will not kill the plant immediately, but it will reduce its root mass. New sprouts can appear at distances far from the original plant and should be searched for at least 6 m away and uprooted immediately.

Tiling alone will not provide control, but instead will create many resprouts. However, tilling may be useful as part of an integrated strategy, since it will increase the shoot to root ratio and potentially increase the efficacy of chemical control.

There are no reports of successful long-term control using covering alone. However, covering is likely to work better if applied on isolated and smaller patches in open terrain. Stems should be cut down to ground level and the area covered with thick black plastic or multiple layers of cardboard. The area covered should extend beyond the plant base for at least 2 m (preferably 7 m). This should be carried out at the beginning of the year or after cutting the plant a couple of times in the spring. The covering material should be left in place throughout the growing season and well into the next. The site should be checked until at least September the following year and again the year after that.

Biological Control

Goats have been reported to eat P. wallichii, and in some circumstances controlled goat grazing may be an option similar to intensive mowing. The disadvantage of this approach is that the goats will graze on desirable vegetation as well as P. wallichii (Soll, 2004).

Chemical Control

To successfully control P. wallichii using herbicide, the active ingredient in a herbicide product must have a mode of action designed to move the chemical from the leaves into the root system at sufficient concentrations to kill the root tissue. Herbicides with an active ingredient of glyphosate, triclopyr, 2,4-D, picloram and imazapyr have been shown to be variably effective in controlling knotweeds, either separately or in combinations (Soll, 2004).

Soll (2004) recommends management of P. wallichii at a landscape level due to its extensive rhizome and sprouting ability. It resprouts vigorously following cutting, mowing, digging and herbicide treatments, especially early in the growing season, until at least August. Successful eradication of just one patch is likely to take more than one year, and multiple treatment in most cases. Landscape level projects and large sites will almost certainly require integrating herbicide use into the control strategy.

Gaps in Knowledge/Research Needs

Gaps in Knowledge/Research Needs

According to Kala (2004), there is a lack of ecological studies on the proliferation of P. wallichii and its impact on other native species, especially in the Himalayan region.

References

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Bamber CJ, 1916. Plants of Punjab. Punjab, India: Superintendent Government Printing.

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Sanchez A; Schuster TM; Kron KA, 2009. A large-scale phylogeny of Polygonaceae based on molecular data. International Journal of Plant Sciences, 170(8):1044-1055. http://www.journals.uchicago.edu/doi/abs/10.1086/605121

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WSDA, 2008. IPM Plant Profile: Japanese Knotweed, Giant Knotweed, Bohemian Knotweed, Himalayan Knotweed. Integrated Pest Management. Washington, USA: Washington State Department of Agriculture. http://agr.wa.gov/PlantsInsects/Weeds/Knotweed/Knotweed.aspx

Contributors

14/5/15 Original text by:

Ymkje van de Witte, Consultant, Wageningen, The Netherlands

Distribution Maps