Invasive Knotweed Complex
Polygonaceae Family | Class B Noxious Weed
The invasive knotweed complex consists of four species in the family Polygonaceae. Knotweeds are large, rhizomatous, herbaceous perennial plants that vary in height from 5 to 20 feet (Duncan, 2013; Parkinson & Mangold, 2017). The round, hollow, and thick stems with swollen nodes resemble bamboo. The stems are green to reddish in color, often red-speckled. The multiple high erect stems are often arched near the top (Parkinson & Mangold, 2017; King County BMP, 2015; WA NWCB). The large sized leaves are alternately arranged, bright green with smooth edges. In general leaves are ovate or broadly elliptic with a pointed tip, however leaf shape varies between species (Bobis et al., 2019; King County BMP, 2015). The rhizomatous root system of knotweed forms a deep, dense mat. Underground rhizomes are thick and extensive, storing large quantities of carbohydrates and spread aggressively (WA NWCB; King County BMP, 2015).
Rhizomes can grow 23 feet from parent plant, 50 to 65 feet laterally and penetrate 7 feet deep into the soil (King County BMP, 2015; Duncan, 2013). Rhizomes are root-like subterranean stems commonly horizontal in position that usually produce roots below and send shoots up towards the surface. Flowers are small and white to green in color with the exception of Himalayan flowers which are pink to white in color. Flowers grow in showy plume-like clusters and emerge from where leaves meet the stem; flowers have 5 petals (technically tepals), rarely 4, all of which are upright (Parkinson & Mangold, 2017; King County BMP, 2015). In the Pacific Northwest the invasive knotweed complex consists of four species including Bohemian, Giant, Japanese and Himalayan. All four species are Class B state listed noxious weeds in Washington State.
The World Conservation Union lists Japanese knotweed and congeners within the world’s worst 100 invasive alien species (Claeson & Bisson, 2013). Knotweed species are invasive to North America and spread aggressively along rivers establishing dense monotypic stands, thereby reducing native riparian plant diversity, structure, and function (Claeson & Bisson, 2013). Knotweed is capable of growing in a wide variety of soil types, and can alter soil chemistry and release allelochemicals that suppress the growth of competing plant species (Claeson & Bisson, 2013). Combined with its early emergence, and fast aggressive growth; knotweed forms a dense stand that shades and crowds out all other vegetation, displacing native flora and fauna (Claeson & Bisson, 2013; King County BMP, 2015).
The formation of extensive, monotypic stands lowers the quality of riparian habitat for fish and wildlife by displacing native plant communities, and associated changes in water quality, nutrient cycling and aquatic food webs that may impact fisheries (Parkinson & Mangold, 2017). Despite knotweed’s large rhizome mass, it provides poor erosion control (King County BMP, 2015). Knotweeds die back with the first hard frost in the fall, around the same time rain dominated streams begin to rise. Knotweed leaves streambanks vulnerable to erosion with little to no vegetative cover or woody root structure to hold the bank intact against the erosive actions of flooding streams (Parkinson & Mangold, 2017). Knotweed rhizomes in the soil are dislodged and transported downstream where they take root and re-sprout forming new clones further downstream where it repeats the invasion cycle: establish, outcompete, displace, dominate, alter, spread.
Knotweed displaces regenerating trees that, as adults, are key components of the structure and functioning of terrestrial and aquatic systems. (Urgenson, Reichard, & Halpern, 2009). Coniferous trees provide durable long lasting large woody debris (LWD) to the active stream channel and floodplain. LWD plays an important role in routing of water and sediments and in shaping channel morphology, which influence habitat quality for aquatic organisms (Urgenson, Reichard, & Halpern, 2009). Red alder fixes atmospheric nitrogen providing an important source of nitrogen in terrestrial soils and aquatic food-webs (Urgenson, Reichard, & Halpern, 2009). Knotweed is able to resorb a large amount of its foliar nitrogen before losing leaves in the fall, resulting in a lower proportion of foliar nitrogen to riparian soils and aquatic environments (Urgenson, Reichard, & Halpern, 2009). By displacing native plant communities, and replacing them with leaf litter of lower nutritional quality, knotweed invasion could affect the productivity of macroinvertebrate communities and in turn, the fish that use these invertebrates as a primary food source (Urgenson, Reichard, & Halpern, 2009).
Knotweeds damages public infrastructure and roadways. Plants can push through concrete, displacing foundations, walls, pavements, and drainage works (Parkinson & Mangold, 2017). Knotweed established along roadways reduces sight distances, blocks roads signs and damages the pavement (Parkinson & Mangold, 2017).
Many beekeepers are sympathetic towards knotweed because the showy flowers are attractive to pollinators, particularly honey bees. Honey obtained from the nectar of knotweed is dark in color indicating high bio-active properties and a high nutritional value (Bobis et al., 2019).
Integrated Weed Management
Preventing knotweed establishment is the highest priority; once established, eradication is extremely difficult (Duncan, 2013). If knotweed is already established in the management area, preventing further spread is your next highest priority (Parkinson & Mangold, 2017). Identifying and surveying known knotweed locations and recording them with a GIS/GPS device will allow the land manager to develop a control area around the infestation and determine what patches are the highest priority for control. Do not dig or spread soil within at least a 65ft radius of the knotweed patch, underground rhizomes can grow 50 to 65 feet laterally and the soil could contain root fragments that can easily regenerate into new plants (Duncan, 2013; Parkinson & Mangold, 2017). Prevent plants from spreading away from existing populations by washing vehicles, machinery, and equipment that have been in infested areas (King County BMP, 2015). Knotweed crowns, rhizomes, and stems with seeds, should be collected and discarded into a landfill for disposal. Composting crowns and rhizomes is not recommended as they are slow to dry out and remain viable for a long time (King County BMP, 2015). Stems with no seeds can be composted onsite as long as they are not left on moist soil or near water and completely dried out. Never dispose of knotweed plants or plant parts into waterways, wetlands, or other wet sites (King County BMP, 2015).
The Four Species of Knotweed
Giant knotweed is native to northern Japan specifically the Sakhalin Islands; the species name translates “from Sakhalin Island”. As its common name suggests giant knotweed is the largest species of knotweed in the Pacific Northwest. Giant knotweed is able to produce a small amount of viable seed and viable pollen, which could potentially hybridize with Japanese knotweed. Giant knotweed is insect pollinated, and not capable of self-pollination. Research indicates that giant knotweed produces allelochemicals from the roots, which aids in the aggressiveness and rapid colonization strategy (Parkinson & Mangold, 2017; WA NWCB).
- Height – 9 to 20 feet tall
- Leaves – to 20 inches long and 2/3 as wide. Huge oblong heart shaped “elephant-ear” leaves. Leaf base is deeply heart shaped due to a deep basal cleft.
- Flowers – Greenish-white to creamy-white in a compact, drooping arrangement. Flower clusters are close to half the length of the leaves.
(source: Duncan, 2013; WA NWCB; Parkinson & Mangold, 2017 )
- Bobis, O., Dezmirean, D. S., Bonta, V., Moise, A., Pasca, C., Domokos, T. E., & Urcan, A. C. (2019). JAPANESE KNOTWEED (FALLOPIA JAPONICA): LANDSCAPE INVASIVE PLANT VERSUS HIGH QUALITY HONEY SOURCE. Scientific Papers, LXII(1).
- Byers, J. E., Reichard, S., Hayes, D., Chornesky, E., Williamson, M., Seastedt, T., . . . Randall, J. (2001). Directing Research to Reduce the Impacts of Nonindigenous Species. Conservation Biology, 16(3), 630-640.
- Claeson, S. M., & Bisson, P. A. (2013). Passive Reestablishment of Riparian Vegetation Following Removal of Invasive Knotweed (Polygonum). Weed Science Society of America, 6, 208-218. Retrieved November 12, 2020.
- Davenport R. 2006. Control of knotweed and other invasive species and experiences restoring native species in the Pacific Northwest US. Native Plants Journal 7(1):20-26.
- Duncan, C. (2013). Identification and Management of Invasive Knotweed. Retrieved November 17, 2020, from https://www.techlinenews.com/articles/2013/identification-and-management-of-invasive-knotweeds
- King County Noxious Weed Control Program Best Management Practices: Invasive Knotweeds. (2015, July). Retrieved November 12, 2020, from https://www.kingcounty.gov/services/environment/animals-and-plants/noxious-weeds/weed-control-practices/bmp.aspx
- Parkinson, H., & Mangold, J. (2017). Biology, Ecology and Management of the Knotweed Complex. Montana State University Extension. Retrieved November 12, 2020, from https://www.montana.edu/plantinvasions/publications/pdfs/EB0196_knotweed.pdf
- Reardon, R. Japanese Knotweed Biological Control. Biological Control & Biopesticides. USDA Forest Service. Retrieved November 12, 2020, from https://www.fs.fed.us/foresthealth/technology/pdfs/FS_jaknotweed.pdf
- Urgenson, L. S., Reichard, S. H., & Halpern, C. B. (2009). Community and ecosystem consequences of giant knotweed (Polygonum sachalinense) invasion into riparian forests of western Washington, USA. Elsevier, Biological Conservation.
- Written Findings of the Washington State Noxious Weed Control Board. (2004). Retrieved November 12, 2020, from https://www.nwcb.wa.gov/images/weeds/Polygonum-bohimicum-2004.pdf