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Japanese Knotweed

Fallopia japonica

Summary 7

Reynoutria japonica, synonyms Fallopia japonica and Polygonum cuspidatum, is a species of herbaceous perennial plant in the knotweed and buckwheat family Polygonaceae. Common names include Japanese knotweed and Asian knotweed. It is native to East Asia in Japan, China and Korea. In North America and Europe, the species has successfully established itself in numerous habitats, and is classified as a pest and invasive species in several countries.

Short Description 8

Stout, shrub-like herbaceous perennial plant that forms large dense clumps that measure between 1 to 2.7 m (3-9 ft) high. The stems are reddish, ridged, jointed and hollow. The leaves are alternate on the stem, 3 to 7cm (2-3 in) wide; the leaf veins are often reddish, and the petioles are 2.5 cm ( 1 in) long and ridged. The flowers bloom in late summer and are small, creamy white flowers that are arranged in spikes near the end of the plant's arching stems.

Source: EwA Invasive Pocket Fieldguide | © Earthwise Aware

Description 9

The flowers are small, cream or white, produced in erect racemes 6–15 cm (2+1⁄2–6 in) long in late summer and early autumn. Japanese knotweed has hollow stems with distinct raised nodes that give it the appearance of bamboo, though it is not related. While stems may reach a maximum height of 3–4 m (10–13 ft) each growing season, it is typical to see much smaller plants in places where they sprout through cracks in the pavement or are repeatedly cut down. The leaves are broad oval with a truncated base, 7–14 cm (3–5+1⁄2 in) long and 5–12 cm (2–4+1⁄2 in) broad, with an entire margin. New leaves of Reynoutria japonica are dark red and 1 to 4 cm (1⁄2 to 1+1⁄2 in) long; young leaves are green and rolled back with dark red veins; leaves are green and shaped like a heart flattened at the base, or a shield, and are usually around 12 cm (5 in) long.

Mature R. japonica forms 2-to-3-metre-tall (6+1⁄2 to 10 ft), dense thickets. Leaves shoot from the stem nodes alternately in a zigzag pattern. Plants that are immature or affected by mowing and other restrictions have much thinner and shorter stems than mature stands, and are not hollow.

Identification 9

Identification of Japanese knotweed may be confused with other plants suspected of being knotweed, due often to the similar appearance of leaves and stems. Dogwood, lilac, houttuynia (Houttuynia cordata), ornamental bistorts such as red bistort (Persicaria amplexicaulis), lesser knotweed (Koenigia campanulata), Himalayan balsam (Impatiens glandulifera), broad-leaved dock (Rumex obtusifolius), bindweed (Convolvulus arvensis), bamboo, Himalayan honeysuckle (Leycesteria formosa), and Russian vine (Fallopia baldschuanica) have been suspected of being Reynoutria japonica.

Invasive species 9

This species is listed by the World Conservation Union as one of the world's worst invasive species.

It is a frequent colonizer of temperateriparianecosystems, roadsides, and waste places. It forms thick, dense colonies that completely crowd out any other herbaceous species and is now considered one of the worst invasive exotics in parts of the eastern United States. The success of the species has been partially attributed to its tolerance of a very wide range of conditions; including drought, different soil types, variable soil pH, and high salinity. Its rhizomes can survive temperatures of −35 °C (−31 °F) and can extend 7 metres (23 ft) horizontally and 3 metres (10 ft) deep, making removal by excavation extremely difficult. The plant is also resilient in response to cutting, vigorously resprouting from the roots.

The invasive root system and strong growth can damage concrete foundations, buildings, flood defences, roads, paving, retaining walls and architectural sites. It can also reduce the capacity of channels in flood defences to carry water.

Recent reporting suggests that Japanese knotweed is not nearly as destructive to structures as once thought. Damage appears to only occur at or near areas that were already compromised.

Japanese knotweed shades out other vegetation, grows over buildings and other structures, encourages fire, and damages paved surfaces.

⭕ Control Methods (EwA Content) 10

To control Japanese knotweed effectively requires attacking the rhizome system (b). Relying solely on mechanical methods is often insufficient and more than often worsens the situation (a).

Eradicating Japanese knotweed is extremely difficult (e, f, g), and there is currently no consensus on the best control method (g). Prevention and Early Detection and Rapid Response (EDRR) are the most effective strategies, employing containment or eradication efforts as a last resort (g).

Before using control methods to manage a Japanese knotweed infestation, carefully analyze site-specific factors and goals, ensuring the chosen approach suits each unique situation. In some cases, experts suggest not taking immediate action against established populations if they pose no immediate threat but rather focusing on monitoring and EDRR for newly dispersed individuals to prevent further spread (g).

Chemical Treatment⚠

Some approaches can help control Japanese knotweed. When using herbicides as the sole management method, understanding the mode of action, dosage, timing, and coverage is critical (e).

Spraying glyphosate to the foliage twice a year (summer and fall) for multiple years effectively has proven to be an effective control method (e). Evidence suggests that spraying glyphosate is also effective at the plant's peak height (~3 m/10 ft). Repeating the treatment for several years is crucial for effective control (d). Glyphosate is widely used due to its accessibility, supposed low soil residue, and the possibility of applying it close to water. However, imazapyr may be more effective (h). If you want to control other species simultaneously, you can mix glyphosate with imazapyr or triclopyr; this mixture does not reduce the effectiveness against knotweed (b).

Stem herbicide injection is labor-intensive and slower than spraying. Glyphosate injection is only recommended for environmentally sensitive areas, where preserving surrounding plants matters most, or when spot applications are prohibited (d, e). In such cases, inject it in the second node above the ground level during fall (e). Note that untreated stems will survive. Each type of herbicide has a maximum amount that can be safely applied per acre/year. In the case of a large site, note that it is likely to reach this threshold amount before treating all the stems (a).

Imazapyr is another herbicide for knotweed control that can be used in the fall or when the plant has reached peak height (d). It is more effective than glyphosate as it disrupts the plant's phloem ability (c). Also, note that the herbicide is non-selective and persists longer in soil; it may also harm non-target plants by root contact for at least a year before planting on sites where imazapyr has been applied (a).

Read the herbicide label and follow the instructions when applying the herbicide. Be aware of possible impacts that may be caused to the surrounding fauna and flora.

Mechanical Paired with Chemical Treatment

Small seedlings can be hand-pulled on sites where seed reproduction occurs (a). Cutting or mowing Japanese knotweed is usually firmly discouraged (d, e). Cutting followed by herbicide application yields similar results to herbicide alone (d, e). However, cutting can be helpful in specific cases, such as when it is necessary to improve the visibility of the managed area and safety on site (g). Be cautious not to exacerbate the dispersion of the species (d, g) by inadvertently leaving any plant material behind or moving cuts or pieces around and far. Repeated cutting on a strict schedule can also be employed to decrease the vigor of knotweeds, thereby promoting the restoration of a competitive cover of native plants as part of the management strategy (g). It is also possible to use cutting to simplify herbicide application by lowering stem height and enabling the area to become navigable (d).

Disposal

When disposing of the invasive waste, make sure that the plant cannot regenerate. Plant parts can be disposed of in incinerators or landfills using appropriate bags. Solid waste facilities designed to inactivate potential pathogens in biosolids, capable of maintaining temperatures above 55°C (131ºF) for at least three consecutive days, can safely destroy plant parts. Plant parts can be completely dried above ground and burned on-site in permitted areas. During the drying period, preventing direct contact with the soil is crucial to avoid regeneration. Do not compost (a). Check state laws for proper disposal.

⚠ Disclaimers

Disclaimer #1: Harmless chemicals do not exist. Any chemical use is likely to harm non-target species and affect the soil. However, in some circumstances, chemicals might be the only way to succeed in removing an invasive species permanently. Researching and understanding the latest scientific literature before using a specific chemical or a mix of several is critical to reducing negative impact.

Disclaimer #2: Independently, several European countries recently announced future bans or massive restrictions on the use of glyphosate (e.g., Austria, Germany, France). The EU, at large, is expected to decide on the renewal of the approval of glyphosate Ultimo 2023 (More here).

Sources

[a] - Japanese knotweed - Invasive Species—Best Control Practices. 2012; Available in: https://mnfi.anr.msu.edu/invasive-species/JapaneseKnotweedBCP.pdf
[b] - Japanese knotweed. 2020; Available in: https://extension.psu.edu/japanese-knotweed
[c] - Bashtanova, U.B., Beckett, K.P. and Flowers, T.J., 2009. Physiological approaches to the improvement of chemical control of Japanese knotweed (Fallopia japonica). Weed science, 57(6), pp.584-592.
[d] - Jollimore, T., 2022. Assessment of Management Strategies for Japanese knotweed (Reynoutria japonica) in Nova Scotia.
[e] - Jones, D., Bruce, G., Fowler, M.S., Law-Cooper, R., Graham, I., Abel, A., Street-Perrott, F.A. and Eastwood, D., 2018. Optimising physiochemical control of invasive Japanese knotweed. Biological Invasions, 20, pp.2091-2105.
[f] - Jones, D., Fowler, M.S., Hocking, S. and Eastwood, D., 2020. Please don’t mow the Japanese knotweed!. NeoBiota, 60, pp.19-23.
[g] - Martin, F.M., Dommanget, F. and Evette, A., 2020. Improving the management of Japanese knotweed sl: A response to Jones and colleagues. NeoBiota, 63, pp.147-153.
[h] - Todd, L. and Jennifer, G., 2016. Knotweed Management Strategies in North America with the Advent of Widespread Hybrid Bohemian Knotweed, Regional Differences, and the Potential for Biocontrol Via the Psyllid Aphalara itadori Shinji.

Sources and Credits

  1. (c) Claire O'Neill, some rights reserved (CC BY-NC-ND), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/165977220
  2. (c) Claire O'Neill, some rights reserved (CC BY-NC-ND), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/95587553
  3. (c) Claire O'Neill, some rights reserved (CC BY-NC-ND), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/184515842
  4. (c) Claire O'Neill, some rights reserved (CC BY-NC), uploaded by Claire O'Neill, https://www.inaturalist.org/observations/43471578
  5. (c) Jeanine Farley, some rights reserved (CC BY-NC), uploaded by Jeanine Farley, https://www.inaturalist.org/observations/32982635
  6. (c) JeannieK, some rights reserved (CC BY-NC), uploaded by JeannieK, https://www.inaturalist.org/observations/77355087
  7. (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Fallopia_japonica
  8. (c) Claire O'Neill, some rights reserved (CC BY-SA)
  9. Adapted by Claire O'Neill from a work by (c) Wikipedia, some rights reserved (CC BY-SA), https://en.wikipedia.org/wiki/Reynoutria_japonica
  10. Adapted by Esther Meirelles from a work by (c) Claire O'Neill, some rights reserved (CC BY-SA)

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