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).
Knotweeds are found in a wide variety of habitats, but prefer partial to full sun and moist soils. (Reardon, Japanese Knotweed Biological Control; Claeson & Bisson, 2013). Knotweeds are rarely limited by soil types or nutrient levels; however knotweed established on nutrient poor sites will have reduced growth rates. (Parkinson & Mangold, 2017). Escaped plants are often found growing along waterways (streambanks, canals, lakeshores, wetlands, coastlands, estuaries), roadways, ditches, railroad rights-of ways, urban areas, waste places, neglected gardens, utility pathways, gravel pits and mining areas.(King County BMP, 2015; WA NWCB; Parkinson & Mangold, 2017).
Reproduction is primarily vegetative by rhizome fragments but plants can also reproduce by seed (Duncan, 2013). Virtually all parts of the plant can generate new roots if separated from the parent plant including the rhizomes, root crowns, and stems; dislodged stem sections need a node present to generate roots (Davenport R., 2006). Very small sections of the root crown and rhizomes are capable of regenerating; fragments as small as 0.02 lbs (7 grams) and only 0.5 inches long can produced new plant colonies (King County BMP, 2015; Parkinson & Mangold, 2017). If stem or root fragments contact moist soil or end up in a waterway they will root and establish new plant colonies. (King County BMP, 2015). Plant fragments are transported by both natural and human vectors. Hydrologic disturbances such as floods and erosion dislodge root fragments and wash them downstream; wildlife such as beaver are known to cut and move knotweed stems throughout the active floodplain. Human actions that spread knotweed include roadside clearing and mowing, moving contaminated fill dirt, moving heavy machinery from one site to another without proper washing, and landowners dumping plant materials and cuttings along roadsides and waterways (Davenport R., 2006; King County BMP, 2015).
Knotweeds are insect pollinated and seeds are wind dispersed, however seeds are not believed to be the primary means of reproduction (Reardon, Japanese Knotweed Biological Control). Seed can remain viable for as long as 15 years, however seeds in the upper 1 inch of the soil generally remain viable for 4 to 5 years (King County BMP, 2015).
New shoots emerge from rhizomes and crowns in mid-spring to late summer. Sprouts are fleshy, pointed at the tip, and slender resembling asparagus shoots (Parkinson & Mangold, 2017). New shoots may not be hollow until they mature. Following emergence, plants can grow 2 to 4 inches per day in the spring (Parkinson & Mangold, 2017). Flowering occurs from August to September, with fruit set beginning in September (Duncan, 2013). When seed is produced, it typically forms 2 to 3 weeks after flowering (Parkinson & Mangold, 2017). Above ground growth is not frost tolerant and dies with the first hard frost. Dead brown stems often remain upright throughout winter (Duncan, 2013; Parkinson & Mangold, 2017).
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).
Manual and Mechanical Control
Stem cutting, mowing and digging can be effective on small newly established patches (50 stems or less). Methods must be repeated at least three times during the growing season and continued for more than three years for successful control (Duncan, 2013) (King County BMP, 2015)
Cutting and Mowing
Cutting will not likely eradicate but will reduce the impact and deprive the rhizomes of stored energy. (King County BMP, 2015). Cutting is labor intensive but effective. Cut stems as close to the ground as possible at least three times during the growing season, for at least 3 to 5 consecutive years. This will significantly reduce rhizome reserves. For best results, cut stems between May and September, making sure the last cutting occurs before plants begin to lose their leaves with the onset of winter. King County recommends a more thorough cutting regime of cutting twice a month between April and August and then once a month until the first hard frost (Parkinson & Mangold, 2017; King County BMP, 2015).
Mowing can be effective if repeated for several years. Mower height should be as close to the ground as possible, and mowing should be repeated when plants reach a height of 6 inches. Continue mowing throughout growing season until a killing frost occurs. (Parkinson & Mangold, 2017)
Rake and pile up the cut stems where they will dry out. Dried stems can be crushed, composted or burned. Do not leave plant fragments on moist soil or where they can be transported into water, they will sprout nodes and re-infest the area. (King County BMP, 2015)
Combination of cutting/mowing followed by herbicide application can be effective. Repeated cuttings tend to produce numerous small stems that may make future treatment with stem injection more difficult, foliar spray is recommended for this method (King County BMP, 2015). Cut knotweed stems to the ground, single time or several times over the growing season, spray regrowth 6 weeks after the last cuttings during July through October. This method reduces the amount of herbicide use and is more labor efficient (King County BMP, 2015).
Digging
Digging can be effective if done consistently on new small patches when plants are young and the soil is moist. Dig up as much root as possible in August if soil remains moist, if the soil is dry and compact dig in early summer after rainfall. The patch should be treated twice every month after first digging, to remove new sprouts as they emerge from June to first hard frost. Repeat for at least three consecutive years (King County BMP, 2015; Parkinson & Mangold, 2017).
Biological Control
Surveys for existing natural enemies in North America found no native herbivores with any potential for control against knotweed (Reardon, Japanese Knotweed Biological Control).
Insects and Pathogens
Three insects and one pathogen have been a focus for potential biological control agents for release in North America: Aphalaris itadori (a sap sucking psyllid), Gallerucida bifasciata (a leaf feeding chrysomelid beetle), Ostrinia ovalipennis, (a leaf and stem feeding moth), Mycosphaerella sp., (a leaf spot pathogen). (Reardon, Japanese Knotweed Biological Control). In 2003-2005, release and control sites were identified in Washington State and data collected concerning knotweed growth patterns. In 2003, the technical advisory group (TAG) recommended release of Aphalaris itadori in the US (Reardon, Japanese Knotweed Biological Control).
Grazing
Grazing has been observed to reduce the establishment and growth of knotweed where grazing pressure is high. Young shoots are palatable to sheep, goats, cattle and horses. Grazing will not kill the plants, but repeated grazing can weaken them. (Parkinson & Mangold, 2017)
Chemical Control
The most effective herbicide for controlling knotweed is imazapyr, followed by glyphosate (King County BMP, 2015). Herbicides with the active ingredients imazapyr, glyphosate, and aminopyralid have shown to be variably effective in controlling or suppressing knotweed (King County BMP, 2015). Triclopyr and aminopyralid will provide short-term control but generally will not kill the plants. 2-4 D is not effective on knotweed (King County BMP, 2015). This list is not all encompassing and will likely change over time. When treating knotweed use a systemic herbicide that translocates from leaves and stems into the roots. Always read and follow the label, and do not apply herbicide near waterways or wetlands without a National Pollutant Discharge Elimination System (NPDES) permit and WSDA applicators license with an aquatic endorsement. Contact your local county noxious weed coordinator or local WSU extension office if you have any questions regarding the use of herbicide.
Foliar Treatment (Backpack Sprayers, Handheld Sprayers)
Easiest and fastest method, but risk of drift onto desirable vegetation or dripping of herbicide off leaves into water and soil (King County BMP, 2015). Apply herbicide to green foliage, from July to October before the first hard frost (when leaves begin to discolor and fall off). Fall application may be the most effective because plants will translocate more herbicides to rhizomes rather than above ground growth (Parkinson & Mangold, 2017; King County BMP, 2015).
Avoid spraying knotweed when bees and other pollinators are present on flowers whenever feasible (King County BMP, 2015. If knotweed patch is in an upland area, mow or cut knotweed in early summer to prevent or delay flowering during treatment window. Before spraying allow plants to regrow for 6 weeks or when regrowth is at least 3 feet tall (King County BMP, 2015).
Spraying the same patch twice once in the spring/summer and once in the fall sets plants back so they can be sprayed at the appropriate growth stage at the easiest height, but increases overall herbicide use (King County BMP, 2015).
Imazapyr (Foliar Treatment)
1% solution with 1% surfactant
Apply midsummer until first hard frost
Imazapyr has residual soil activity and may impact roots of non-target plants.
Glyphosate (Foliar Treatment)
2-8% solution with 1% surfactant
Apply from midsummer until first hard frost
Stem Injection
This method is highly effective and selective, but very time, labor and herbicide intensive compared to foliar application. Keep track of herbicide amount used, as it is easy to over exceed maximum usage rates, as a general rule you can only inject 2,375 canes per acre (King County BMP, 2015). Use the JK Injection Tool or another small hand-operated injection device to deliver 3 to 5 ml of concentrated glyphosate into each knotweed stem that is at least 0.5 inches in diameter. Inject the herbicide between the second and third node of the cane. Mark canes with spray paint or grease pen to keep track of what stems were injected (King County BMP, 2015).
Cut Stem Method
Practical in large restoration sites. This method uses less herbicide than stem injection and large areas can be cut and treated. In riparian areas it is risky to leave cut stems where they could be transported by the stream or contact moist soil and re-root (Davenport R., 2006). Stems less than 3 cm in diameter should not use this method. (Davenport R., 2006).
Cut/mow all knotweed stems greater than 3 cm in diameter. Use a 1:1 to 1:5 (glyphosate:water) dilution rate. Apply herbicide directly to the cut well above the second node, effective if the dilution rate supplies 2 ml of herbicide (Davenport R., 2006). The lower rate is effective provided the stem is large enough to hold sufficient herbicide mixture. (Davenport R., 2006).
Manual Bending of Stems
Bending of growing knotweed stems, then applying a foliar herbicide to the regrowth. Reduces height and vigor of plants while they are rapidly growing and produces a shorter stem that is treated later in the summer/fall when knotweed is more susceptible to herbicide. (Davenport R., 2006). Spray regrowth with a 5% glyphosate solution. (Davenport R., 2006). Bending will create smaller diameter regrowth stems that cannot be inject (Davenport R., 2006).
Covering
Recommended for very small infestations, 50 stems or less. (Parkinson & Mangold, 2017). Cut stems down to ground level, single or several times before covering to exhaust the root reserves. Cover with heavy duty geo-textile fabric or black plastic. Weight down cover with heavy rocks or cement blocks. Extend cover at least 7 feet beyond the outside stems. Leave in place for at least five growing seasons. (King County BMP, 2015). Monitor and stomp down any re-growth under covering material (King County BMP, 2015).
Revegetation
Often an overlooked component in noxious weed control projects, replanting knotweed control sites is a crucial step in the long term control and restoration of knotweed invaded ecosystems. In highly disturbed riparian areas, especially along large streams, active reintroduction of native plant species may help promote recovery and prevent exotic plant reinvasions through increased direct competition or propagule pressure (Claeson & Bisson, 2013). Planting larger native plants appropriate to floodplain riparian settings can provide shade, potentially weakening shade intolerant exotic seedlings while supporting shade tolerant native species. (Claeson & Bisson, 2013). Claeson and Bisson, found that overstory cover was positively associated with greater levels of native species and cover, and native tree cover was negatively correlated with initial knotweed stem count; while knotweed was able to become denser in areas where overhead tree cover was already low.
Both native and exotic plants colonize recently treated knotweed sites. Without successful native vegetation recovery, these ecosystems are prone to reinvasion by the same exotic or secondary exotic species (Claeson & Bisson, 2013). Eradicating knotweed removes a dominant competitor, thus providing resource and niche opportunities that favor the recruitment of rapidly-growing species and fluvial transport of propagules aids colonization by new species in recently disturbed riparian habitats. (Claeson & Bisson, 2013). Revegetation alone is unlikely to be an effective control method. Other methods should be taken to first control knotweed (combination of digging/grubbing, mowing, covering, biological control, and herbicides). (Parkinson & Mangold, 2017).
Once a patch appears to be eradicated, revegetation is strongly recommended to suppress reinvasion of knotweed or another opportunistic invasive species. (Parkinson & Mangold, 2017). Generally, sites should not be planted the first year after treatment because knotweed will often survive the first treatment and pose problems for establishing native species (Davenport R., 2006). If knotweed was treated with imazapyr, the herbicide may still be active in the soil and potentially cause damage to the planting 1-2 years after treatment. After control, replant bare areas with appropriate native vegetation or cover with mulch while desirable vegetation becomes established. Do not leave large areas of bare soil, or other opportunistic weeds will establish. (King County BMP, 2015).
Species recommended for revegetation (Species should be selected based on site conditions).
Conifer Trees – Sitka spruce, western redcedar, grand fir, Douglas fir, western hemlock, and shore pine.
Deciduous Hardwoods – Willow (Salix spp.), black cottonwood, red alder, big leaf maple, Oregon ash, western crabapple.
Shrubs – Pacific ninebark, black twinberry, Nootka rose, red-osier dogwood, douglas spirea.
Groundcover – Slough sedge, Western sword fern, dull-Oregon grape.
Tree protectors placed over installed plants can protect them from future herbicide treatment and animal damage. Post planting maintenance and weed management can damage native recruits, consider protecting natural recruitment plants, in addition to installed species, as they have proved to be an important component for the recovery of native plant communities (Davenport R., 2006). Choose native plant species that will ultimately create a diverse, competitive native community that is similar in species composition to nearby reference sites and can be maintained over a number of years until shade is established (Davenport R., 2006).
Monitoring and follow-up treatments will be necessary within a 60 foot radius of the original patch for several years, even after the patch appears to be eradicated (Parkinson & Mangold, 2017). Regrowth can often be very small and hard to find at first glance. Herbicide applications and other treatment methods will leave the area exposed to secondary invasive species; controlling these exotics while their abundance is low may be critical for the recovery of native plants (Claeson & Bisson, 2013). These areas need to be monitored and revegetated with appropriate species if desirable vegetation is not returning naturally (Parkinson & Mangold, 2017).
The Four Species of Knotweed
Polygonum sachalinense
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).
Identification
- 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 )
Polygonum x bohemicum
Bohemian knotweed is native to northeastern Asia and is the hybrid between Japanese and Giant knotweed. Bohemian knotweed either came vegetatively from European sources or developed from crosses between giant and Japanese knotweeds on American soils. Colonies rarely establish from back crossing with Japanese or Giant knotweed. Typically found in WA as a male clone. Believed to be the most widespread knotweed in the West. Reported to have better regeneration ability than both parents. May be more competitive and have a faster rate of spread than Japanese or giant knotweed. (WA NWCB; Duncan, 2013; Parkinson & Mangold, 2017).
Identification
- Height – 6 to 16 feet tall
- Leaves – 4 to 12 inches long and 2/3 as wide. Leaf shape is variable resembling either giant or Japanese knotweed. Leaves appear more ovate rather than triangular. Leaf base ranges from slightly to deeply cordate, basal cleft present in varying degrees.
- Flowers – Greenish-white to creamy white in an erect or loose, drooping arrangement. Flower clusters are close to the length of the leaves.
(source: Duncan, 2013; WA NWCB; Parkinson & Mangold, 2017 )
Polygonum cuspidatum
Japanese knotweed is native to Japan, China, Korea and Taiwan. It is an early successional species and is commonly the first plant to colonize volcanic slopes. Japanese knotweed was introduced to the United Kingdom from Japan as an ornamental in 1825 and from there to North America in the late 1800s. Typically found as a female clone, recognized by presence of fruit. The plant is dioecious, so male and female versions of the flowers are produced on separate plants. Like the other knotweeds Japanese spreads by seed and by long stout rhizomes; however, colonies rarely establish from seed. Japanese knotweed is the smallest of the knotweeds in terms of stem height and leaf size (Parkinson & Mangold, 2017; WA NWCB; Reardon, Japanese Knotweed Biological Control).
Identification
- Height – Plants range from 4 to 8 feet tall.
- Leaves – 4 to 6 inches long, and 2/3 as wide. Leaves are generally ovate with a blunt base and abrupt pointed tip. Leaf base is flat and forms a right angle with the leaf stem.
- Flowers – Greenish-white to creamy white in an erect or loose, drooping arrangement. Flower clusters are close to the length of the leaves.
(WA NWCB; Parkinson & Mangold, 2017; Duncan, 2013)
Persicaria wallichii
Himalayan knotweed is native to the Himalayan region of southern Asia. In its native range, it proliferates with disturbance, spreading in avalanche prone areas, along tree lines, and on eroded slopes. Most divergent in appearance and usually not confused with other knotweeds – recognized by its long slender lance shaped leaves and pink colored flowers (WA NWCB; Parkinson & Mangold, 2017).
Identification
- Height – 6 to 9 feet tall
- Leaves – 5 to 12 inches and less than half as wide, leaves are leathery, narrow, lance shaped with sharply pointed tips. Slightly heart shaped to tapered leaf bases.
- Flowers – Pinkish-white to pink, in a loose, spreading arrangement
(source: Duncan, 2013; WA NWCB; Parkinson & Mangold, 2017 )
Works Cited
- 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