Giant knotweed

Giant knotweed

Fact Sheet


Latin Name: Reynoutria sachalinensis

Origin: Asia

Physical Description: it is a perennial herb from a rhizome. The bamboo-like stems are erect, numerous, branching, hollow, reddish-brown in colour and grow up to 4m tall. The stems have membranous sheaths at nodes that appear in regular intervals along the stem. These nodes connect alternating leaf stalks to the stem and are the site at which flowers bloom late in the summer.

Its leaves are 20 to 40 cm long and have a deep heart-shaped base that tapers off to a tip. The leaves also have tiny wispy hairs along the leaf edges and along the veins on their undersides (note: Himalayan knotweed typically has hairs on their leaves as well but are stiffer and less easily visible with the bare eye. Japanese and Bohemian knotweed, on the other hand, usually appear hairless).

Its flowers are small, greenish white to creamy-white in colour, and grow in branched, compact clusters that can appear to be upright or drooping. While most knotweed species have similar appearing flowers, the flowers of giant knotweed commonly produces seeds (unlike the others which produce them less frequently).

Habitat: it prefers moist environments and are commonly be found growing in fields, riparian areas, parks, gardens/yards, waterways, waste areas, and roadsides.

Impacts: it releases allelopathic chemicals from their roots that inhibit the growth of competing species in their proximity. As a result, it threatens biodiversity by displacing native plants and the wildlife that rely on them. Also, its roots lack true root hairs necessary to bind to soil, resulting in erosion and stream sedimentation along banks of creeks and rivers where it has established. It can also grow though concrete and asphalt, damaging infrastructure and homes.

Reproduction: it’s primarily achieved vegetatively with rhizomes, which are horizontal underground stems that form new shoots and roots for the plant. These rhizomes can extend laterally beyond the plant (up to about 20 m), allowing it to rapidly extend its range and extensive network. Reproduction can occur from as little as 0.7 grams of stem or root tissue, and buried rhizomes can regenerate from depths up to 1m. Reproduction via viable seed is also possible, but less common. The flowers contain both male and female parts, meaning they can self-fertilize.

Management Options

Mechanical: hand pulling, mowing or digging is not recommended since these activities increase the risk of breaking up rhizomes which can establish into new plants. For these methods to be effective, they must be done carefully, repeatedly, and in combination with herbicide use. Cutting small infestations may be effective if carried out repeatedly when new growth appears to continue, depleting the plants energy reserves. All cuttings should be bagged to dry out and disposed of in the landfill. Inspect and clean tools after use to ensure there are no plant parts attached.

Chemical: Several herbicides with the following active ingredients can control Giant knotweed: 2, 4- D, glyphosate, aminopyralid, triclopyr, picloram, dicamba, imazapyr.  For available products, contact your local agri-supply store. Prior to any herbicide application, read and follow the label instructions.

Biological: None available

Additional Resources:

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