Persicaria wallichii
(Himalayan knotweed)

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).

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.

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).

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.

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.

Environmental

• Amenity

Human food and beverage

• Vegetable

Ornamental

• garden plant

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).

Due to the variable regulations around (de)registration of pesticides, your national list of registered pesticides or relevant authority should be consulted to determine which products are legally allowed for use in your country when considering chemical control. Pesticides should always be used in a lawful manner, consistent with the product's label.

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

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.

14/5/15 Original text by:

Ymkje van de Witte, Consultant, Wageningen, The Netherlands