Knotweeds (Salt Spring Island Conservancy)


By Jean Wilkinson, Stewardship Committee, Salt Spring Island Conservancy

Native to Asia, knotweeds were once popular garden ornamentals, and are still sold in some B. C. nurseries. They are increasingly recognized as a serious problem due to their extensive roots, dense growth, and rapid spread. They threaten local biodiversity, disrupt food chains, destroy foundations and structures, increase erosion and degrade habitat, including that of salmon.

Listed by the World Conservation Union as one of the 100 worst invasive species on the planet, knotweed has recently been designated a noxious weed in B. C., meaning land-owners and managers have a legal responsibility to control it. Increasing numbers of plants are being found on Salt Spring, and it’s time to stop these invaders in their tracks!

Sometimes mistakenly thought to be a type of bamboo, knotweeds are among the most difficult plants to eradicate due to their huge, deep root systems. But with persistence, small patches can be controlled without using herbicide, and eliminated before they’re an overwhelming problem. Any time in the growing season is a great time to start getting rid of knotweeds, and late summer/early autumn when the plants are putting most of their reserves into flowering, is ideal.


Identification –(Polygonum aka Fallopia species include P. cuspidatum, F. japonica , F. sachalinensis P. polystachum and F. x bohemica) Japanese, Giant, Himalayan and Bohemian Knotweeds are all large lush plants, growing 2 to 5 m (up to 15 feet) tall, with hollow green or reddish stems, alternating smooth-edged leaves, and small spikes of feathery white flowers in late summer. Plants are perennial, dying back in fall and re-sprouting in spring. Reddish brown stems may remain in winter.

Impacts – Knotweeds out-compete and permanently displace native plant species. Large dense stands often develop along streams, blocking access for recreation and degrading fish and wildlife habitat. Increased erosion can occur because the roots do not hold soil well, and the leaves drop in late fall, exposing streambanks to winter rains. Extensive root system can damage foundations, roads, retaining walls, dykes, pavement, etc.

Found –in moist soils with partial shade or full sun; along rivers, streams, ditches, road-sides and disturbed areas.

Spreads – vegetatively, with underground rhizomes that that can extend up to 20 m. Root and stem fragments as small as 1 cm can form new plant colonies when dispersed by people, flooding, soil removal or construction equipment. Bohemian knotweed also produces viable seeds that spread downstream along riparian areas.

Control – Prevent new infestations by ensuring soils, fill materials, construction equipment and vehicles are not contaminated, and take special care when removing knotweed to prevent the dispersal of plant parts. Cut plants close to the ground twice a month between April and August, then monthly until first frost for at least 5 years to exhaust root reserves. Do not let plants grow taller than 15 cm. Do not disturb roots as this encourages rhizome lateral growth. Monitor areas up to 10 m from parent to ensure new plants don’t sprout.

Disposal – Plant parts should be incinerated or put on tarps to dry thoroughly in the sun. Alternately, place in bags, label to indicate invasive nature of contents, and take to landfill or transfer station. Ensure plant fragments are not left on soils or near streams and are not spread by machinery or tools. Do not compost any plant parts.

Alternatives –(Grows in *Sun, +Part Shade or #Full Shade. M = Moist and D = Dry Conditions, DT is Drought Tolerant)

Native plants: Red Osier Dogwood (*+ M,DT); Willows (*M,DT); Pacific Ninebark (# M); Oceanspray (*+ D, DT); Goat’s beard (+# M); False Solomon’s Seal (+# M); Red Elderberry (+ M, DT); Thimbleberry ( # M,DT)

Ornamentals: Virginia Sweetspire (*#M, D); Dappled willow (*+ M); Clumping Bamboo (+ M,D); Tatarian Dogwood (*+ M,D); Summersweet (*+ M,D); Bugbane (+# M); Joe Pye Weed (* M); Fothergilla (*+ M )

More Info: Salt Spring Island Conservancy Stewardship Committee 250-537-4877, Coastal Invasive Species Committee, Invasive Species Council of B.C.