Knotweed spp.

Description

These perennial forb species are originally from eastern Asia. They were introduced to Europe and North America as ornamentals in the early 1800s and are listed as one of the worst 100 invasive plant species by the World Conservation Union. Bohemian knotweed is an aggressive and adaptable hybrid between Japanese and giant knotweeds and is probably the most common of the invasive knotweeds in British Columbia, although the other three species are also causing problems.

All four species have a similar appearance and are difficult to distinguish in the field. Himalayan knotweed has pinkish-white flowers while the other three species have greenish-white flowers. All four species have hollow reddish bamboo-like stems, large broad leaves, and rhizomatous roots. Giant knotweed can grow to 6 m, Japanese knotweed to 4 m, and Himalayan to 3 m tall.

Type

Perennial forb

Habitat & Impacts

These species grow in a wide range of open sites but do not tolerate shade. It is most common in moist low-lying sites but can also tolerate extreme heat and drought. The knotweeds form dense stands and may virtually eliminate other vegetation on a site.

Method of Spread

The main form of reproduction is vegetative with new shoots forming from underground rhizomes or root fragments.

Location

Central Coast, one site at 150 Mile House along Highway 97 and within residential gardens.

Mechanical

Cutting mowing or grazing can limit stem size and density but are not effective at eliminating an infestation unless repeated several times per year for several years. Digging or pulling is not effective and the large roots aggressively resprout following this treatment. Shading the plants by covering with landscape fabric and mulching overtop is currently being tested on the Queen Charlotte Islands.

Chemical

Glyphosate, Imazapyr, triclopyr may be required to control large infestations or heavily infested landscapes. Stem injections are effective in late summer or fall. Foliar applications at late bud. Salt water has also been proven to be effective in salt-tolerant ecosystems.

Biological

Biocontrol is not currently available. Screening research started in 2007.

CCCIPC Priority & Treatment Strategy

Priority 1 (new invader). All sites should be treated with the management goal of elimination.

Local Level

Repeated cutting, shading, herbicide. Avoid using in landscaping.

Landscape Level

Herbicide or Salt treatments where appropriate. Otherwise mechanical treatments.