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Himalayan knotweed

Himalayan knotweed

Himalayan knotweed

(Polygonum polystachyum)

Priority: -  Contain

Tags: Aquatic | Terrestrial

Identification and Reproduction

Identification:

  • Himalayan knotweed is a perennial, herbaceous plant with a woody root. 
  • Stems are 2-3 m tall and are branched in the upper half. Canes are hollow and have joined nodes. 

  • New shoots start to arise during the early spring. They are very fast growers and can reach full height by the end of June. 
  • Mature leaves are lance shaped and can reach 20 cm in length. They have stiff hairs on the leaf edges and are tapered at the end. 
  • Flowers are found in branched clusters, ranging from white to pink. 

Reproduction: 

  • Knotweeds are perennials that spread primarily vegetatively.
  • It is aggressive much like other knotweed species. 
  • They can also reproduce through rhizomes. 
  • Rhizome pieces as small as 2cm can re-sprout. 

Habitat & Ecology

  • Thrive on freshly disturbed soil in roadside ditches, low-lying areas, irrigation canals, and other water drainage systems. 
  • Can grow in loamy, silty or sandy soils and prefers moist soil. 

Impacts

Social: 

  • Create dangerous situations by obstructing sightlines along roadsides.
  • Damage infrastructure like buildings, sidewalks/roads, or drainage system

Ecological: 

  • Form dense monocultures. 
  • Increase erosion on streambanks.
  • Destroy natural habitat for native species.

Management

Mechanical/Manual Control: 

  • If its a small patch digging can be effective. Be cautious and ensure that rhizomes and fragments are all removed off site. 
  • A combination of cutting and covering may also be effective for small areas. Cut plants to the base and then cover with black fabric/geo-textiles or layers of cardboard. 
  • Repeated treatment will be necessary for 3-5 years for successful eradication. 

Chemical Control: 

  • Effective herbicides include: imazapyr, glyphosate, triclopyr, and aminopyralid. 
  • Herbicides can be applied to plant through hand spraying, backpack spraying, wipe-on, painting and stem-injection. 
  • If land owners are not comfortable or able to conduct chemical control on knotweed on their property, there are some local invasive plant management companies that can be hired to complete the work - See list here.

Always read and follow the chemical product label!

Resources

Download the Alberta Invasive Species Council's Factsheet on Himalayan Knotweed here

Download the Invasive Species Council of BC's Factsheet on invasive knotweeds here.

Downlad the Metro Vancouver's Best Management Practices for the Knotweed Species here. 

Header photo (Vinayaraj).