Japanese Knotweed

Description

Knotweeds, including Himalayan and Japanese Knotweed, are invasive perennials that were introduced to British Columbia and other parts of North America from eastern Asia for their rapid growth and ornamental qualities. These species have since become established across the country, adversely affecting riparian areas and man-made structures with their aggressive growth. Knotweeds can grow through concrete and asphalt, causing significant damage to infrastructure[1].

  • Scientific name: Persicaria wallichii, Reynoutria japonica

  • Common name(s): Himalayan Knotweed, Japanese Knotweed

  • French name: Renouée du Japon

  • Regulatory status: Designated as a Provincial Noxious Weed in British Columbia and listed as restricted invasive species under the Invasive Species Act, 2015, in Ontario, due to their detrimental effects on infrastructure, ecosystems, and biodiversity.

Japanese Knotweed bush

© thoscarter via iNaturalist.org, used under CC BY NC

Habitat

Knotweeds prefer moist soil and can thrive in full or partial sun. They are commonly found in riparian zones, along roadsides, railways, and in gardens. These plants are especially problematic in areas with disturbed soils where they can quickly establish and outcompete native vegetation [2].

Identifying Features

  • Growth Pattern: Resembles bamboo with tall, straight, and dense stems, growing up to 5 meters in height.

  • Stems: Hollow and green, sometimes with reddish-brown speckles. May persist through winter as bare stalks.

  • Leaves: Unlike other knotweed species with heart-to-triangular-shaped leaves. Himalayan Knotweed features elongated, tapered leaves, 8-10 cm wide and up to 15 cm in length, Flowers: Small, attractive plumes of white to green flowers emerge in spring

Feature Japanese Knotweed Himalayan Knotweed
Plant Size 1.5 to 3 m tall 2 to 3 m tall
Leaf Size (L/W) 10 to 17 cm, 2/3 as wide 10 to 20 cm, 1/2 as wide
Reproduction Female/Perfect (rare) Perfect and fertile
​Flower Colour Green-white to cream-white Pinkish-white to pink
Flower Arrangement Loose, drooping Loose, spreading

Impacts

Knotweeds:

  • Crowd out native vegetation, leading to decreased biodiversity.
  • Provide poor habitat and food for wildlife.
  • Cause erosion and destabilizates riverbanks.
  • Damage infrastructure by growing through concrete and asphalt.
  • Easily spreads by seed, root fragments, and stem fragments which makes control efforts more complicated.

Management and Control

Prevention is crucial for managing the spread of knotweeds. It is recommended to:

  • Consult local government or regional invasive species organizations for advice on control in your area.
  • Utilize non-invasive plant alternatives in gardens and landscaping.
  • Clean equipment and vehicles that have been in infested areas to prevent spread.
  • Follow legal restrictions on the transport, sale, and planting of invasive knotweeds.
  • Employ mechanical, chemical, or combined methods for eradication, considering the age and size of the infestation.

What You Can Do

  • Learn to identify invasive knotweeds and differentiate them from native species.
  • Plant Wise – Do not plant or propagate knotweeds. Guides like Grow Me Instead can recommend non-invasive alternatives such as Black elderberry, False solomon’s seal, Goat’s beard, Red-osier dogwood, or Saskatoon berry.
  • Play Clean Go – Stay on designated trails and clean shoes, pets, and vehicles after visiting infested areas.
  • Report sightings to local authorities or through invasive species reporting platforms.

For more information on identifying and managing invasive knotweeds and protecting your local ecosystems, please visit the following resources:

1. Fennell, Mark & Wade, Max & Bacon, Karen. (2018). Japanese knotweed (Fallopia japonica): An analysis of capacity to cause structural damage (compared to other plants) and typical rhizome extension. PeerJ. 6. e5246. 10.7717/peerj.5246.
2. Jacob N. Barney, Nishanth Tharayil, Antonio DiTommaso, and Prasanta C. Bhowmik. 2006. The Biology of Invasive Alien Plants in Canada. 5. Polygonum cuspidatum Sieb. & Zucc. [= Fallopia japonica (Houtt.) Ronse Decr.]. Canadian Journal of Plant Science. 86(3): 887-906. https://doi.org/10.4141/P05-170

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