Priority: - Contain
Tags: Aquatic | Terrestrial
There are four species of knotweeds: Japanese knotweed (Fallopia japonica), Giant knotweed (Fallopia sachalinese), Bohemian knotweed (Fallopia x bohemica), and Himalayan knotweed (Polygonum polystachyum). All of these occur in the Fraser Valley, though Japanese and Bohemian are most common, and are difficult to tell apart.
All have the following characteristics:
Large, woody, bamboo-like shrub that grows 1-5 m tall. Stems or canes are hollow, upright, and bamboo-like with reddish-brown speckles and think, papery sheaths. Grows in large, dense thickets.
Leaves are heart to triangular-shaped on all species except Himalayan knotweed, which has lance-shaped, elongated leaves. Leaves are 8-10 cm wide and 15 cm in length, except for giant knotweed, which has leave double the size of the other species. Japanese knotweed leaves appear zig-zagged along the stems.
Small, white-green flowers grow in plume-like, branched clusters along the stem and leaf joints.
Knotweeds are perennials that spread primarily vegetatively. The rhizome system may extend from a parent plant up to 20 meters laterally and to 3 meters deep. Root and stem fragments as small as 1 cm can form new plant colonies. Fresh stems produce shoots and roots when buried in a soil medium or floated in water. Stems submerged in water can produce viable plants within 6 days. Dispersal occurs through root (rhizome) and stem fragments by human activities or by water. Reproduction also occurs by seed in Bohemian knotweed.
Habitat & Ecology
Thrive on freshly disturbed soil in roadside ditches, low-lying areas, irrigation canals, and other water drainage systems. Also found in riparian areas, along stream banks, and in other areas with high soil moisture. Able to grow in partial shade or full sun. Can grow through cracks in concrete.
Dominates stream banks, increases erosion, degrades wildlife and fish habitat, including salmon habitat; blocks access for recreation.
Manual/mechanical methods of knotweed control is not recommended. Mowing spreads vegetative fragments that can root and start new infestations. Chemical control is the best, most effective method of controlling knotweed.
Herbicides containing the active ingredient Glyphosate are effective, as are herbicides containing the active ingredient aminopyralid.
Treatment should occur when plants are 1-2 m tall. Although shorter plants may not have adequate leaf surface to absorb, and translocate, enough chemical to be effective, young, rapidly growing plants do have a more efficient biological process to translocate chemicals. A spring herbicide application or cutting will set the plant back so that it can be sprayed at an effective height and growth stage later in the year. Ideally treat plants twice in a season – first in June or July and again in September or October (before the first frost)
Established patches (hundreds or thousands of stems) will almost certainly require foliar treatments over two or more years. Be sure to search for new shoots up to 20 feet or more away from the central patch after herbicide treatment begins.
Download the Invasive Species Council of BC's TIPS brochure on invasive knotweeds here.
King County has some excellent resources on knotweed identification and management.