It is not known what type of fire regime the 3 knotweeds are best adapted to. They are common in riparian areas in many parts of North America, and FIRE REGIMES in riparian areas are often related to the FIRE REGIMES of adjacent upland communities. Thus FIRE REGIMES for plant communities with the 3 knotweeds could be quite variable. See the Fire Regime Table for further information on FIRE REGIMES of vegetation communities in which the 3 knotweeds may occur.
Barcode of Life Data Systems (BOLD) Stats Specimen Records:403 Specimens with Sequences:555 Specimens with Barcodes:300 Species:128 Species With Barcodes:122 Public Records:230 Public Species:98 Public BINs:0
It is not clear whether occurrence of the 3 knotweeds changes the fuel characteristics of invaded plant communities. Fuel characteristics of plant communities could potentially be altered in areas where the 3 knotweeds exhibit fuels traits (e.g., flammability, fuel loading, plant architecture), densities, and/or growth patterns differing from those of native vegetation.
Laboratory analyses suggest that Japanese knotweed foliage and stems are not particularly flammable . Giant knotweed appears to exhibit similar characteristics. In the Czech Republic, the water content of spring giant knotweed stems reached values as high as 1,800% of dry mass. After 2 months, stem water content decreased and matched that of leaves, at 400% of dry mass . In Russia, giant knotweed stems and leaves reportedly had an 82% moisture content. Consequently, Russian forestry officials investigated the use of 7-foot-tall stands of giant knotweed (referred to as Sakhalin buckwheat) as a live fire break to protect tree plantations. The testing occurred on a hot, dry day with a mild south wind. "The reaction of Sakhalin buckwheat thicket to forest fire was tested by igniting a brush fire in the wind direction 7 feet from the thicket edge. The incendiary fuel was brushwood, pine branches, drenched copiously with kerosene. Set ablaze on the buckwheat thicket side of the plantation, the brush fire whipped by wind burned fiercely for 30 minutes. The test established that buckwheat thickets minimize the effect of fire and at 8-inch depth localize it. The fire does not spread across the thicket to the tree plantation growing on the other side of the protective green zone. The zone itself suffers little from the effect of fire. Only 10% of the foliage embraced in flames, was curled up from burning. The thicket continued to stand upright. The commission concluded that thickets of Sakhalin buckwheat are a completely effective barrier against low forest fires and can be recommended for planting in fireproof zones and banks" (Russian Agricultural Ministry cited in ).
As of this writing (2010) no studies have documented fuel characteristics of litter of the 3 knotweeds. Litter accumulations can be high in some stands. In old fields in south-central New York, litter depth was higher in Japanese knotweed stands compared to outside the stands, though the litter material was almost all large, fibrous stems (P<0.05) . In Pennsylvania it was noted that dense litter built up every autumn under riparian stands composed of all 3 knotweeds, though litter was generally washed away by summer . In riparian forests of northwestern Washington, total mass of autumn litterfall did not differ between plots with and without established giant knotweed, though litter composition varied .
Occurrence of the 3 knotweeds may result in changes in aboveground vegetative structure that could impact fuel characteristics. Changes in vegetative architecture were observed in old fields in south-central New York. Vegetation height increased abruptly at the edge of Japanese knotweed stands, and average height of the first leaf was higher in Japanese knotweed stands compared to outside of stands . In Belgium, plots with Japanese knotweed had higher aboveground biomass compared to plots with native vegetation (2.95 vs. 0.56 kg/m²) [29,30]. The 3 knotweeds often cover extensive areas [31,44,53,61,103,107,116,125] and may dominate local vegetation ([73,87], review by ) (see Plant growth).
The presence of giant [42,78,115], Japanese ([12,53,65,73,78,83,98,114,115], reviews by [11,88,102]), and Bohemian [115,153] knotweed in disturbed areas suggests a preference for early-successional sites. In their native ranges, giant  and Japanese (, review by ) knotweed are early-successional species, often among the first species to colonize recent lava flows. In the Pacific Northwest, all 3 knotweeds commonly established on freshly disturbed soils, like floodplains and cobble bars, following flooding . One review listed plant communities where Japanese knotweed could establish in mountainous ecoregions of the northwestern United States. In all cases, disturbance was listed as a requirement for establishment . Another review stated that Japanese knotweed is promoted by soil disturbance and high light environments in the southern United States; it does not appear to establish in undisturbed sites or forest interiors, though it may be able to establish in canopy gaps created by natural or human-made disturbance .
Some evidence suggests that giant  and Japanese ([19,81,103], reviews by [11,38,102]) knotweed grow better with more light. In Massachusetts, Japanese knotweed grew in full sun to full shade, but was considered "hardier" in full sun . In growth chamber experiments, Japanese knotweed plants in high light had greater leaf area (P<0.05) and belowground biomass (P<0.001) than those in low light . Two-year-old Japanese knotweed rhizomes planted on streambanks near Washington, DC, exhibited significantly higher survivorship (P<0.05) and greater stem growth (P<0.001), basal stem diameter (P<0.001), and stem height (P<0.01) than rhizomes planted in a forest understory. Streambank sites had significantly higher light levels than understory sites (P<0.01) . Managers treating Japanese knotweed in southwestern Washington found smaller stems and plants in deeply shaded areas compared to more open areas . Results from both field (full and 50% sun) and growth chamber (high and low light) experiments in North Carolina documented that giant knotweed plants grown in high-light conditions had higher photosynthetic rates than those grown in low-light conditions (P<0.05) .
Despite these findings, all 3 knotweeds have been documented growing in shady understories. One review reports giant knotweed growing well in semi-shaded places and along the edges of forests . In northwestern Washington giant knotweed was found in low terrace and floodplain forests with an average canopy cover of 47.2% (range 2.5% to 85.0%) . In Nova Scotia, Japanese knotweed did not appear to spread from shaded into open areas . Along the Hoh River in northwestern Washington, Bohemian knotweed was found to grow slowly but persistently in the complete shade of red alder or willow, as well as under a dense, coniferous canopy .
Successional role: Changes in native plant cover or species richness in areas where the 3 knotweeds have established [13,48,57,77,136] suggest they could alter the successional trajectories of native plant communities (see Impacts). Some sources identify Japanese knotweed as a nursery plant in early successional communities, particularly in its native range, because it facilitates soil development and the establishment of later successional species (review by ).
Impacts: Studies in both North America and Europe have documented a decrease in native plant cover or species richness in areas where giant [13,57,136], Japanese [13,48,57,77], or Bohemian [13,57] knotweed have established. Mechanisms suggested for native plant exclusion include the accumulation of leaf and stem litter (review by ), nutrient (review by ) and light (, reviews by [52,53]) limitation, and allelopathy [107,143].
Establishment of giant  and Japanese  knotweed may lead to changes in leaf litter dynamics. Japanese knotweed establishment may lead to high levels of some soil nutrients [29,30]. Studies in North America and Europe have documented changes in faunal communities, including a decrease in the diversity and abundance of invertebrates in areas with giant [66,128] and Japanese [48,72] knotweed, and a decrease in the abundance and fitness of green frogs in an area with Japanese knotweed . Changes in fungal assemblages were reported in areas with Japanese knotweed . In contrast, one study in Idaho found that instream leaf decomposition rates, microinvertebrate colonization, abundance, and species richness of some types of microinvertebrates did not differ in leaf litter containing Japanese knotweed compared to litter containing leaves of native gray alder and black cottonwood .
One study supported the assertion that giant knotweed displaces riparian species and has cascading effects on the structure and function of riparian systems. In northwestern Washington, riparian forests with higher giant knotweed stem density had lower juvenile conifer (P<0.01), juvenile red alder (P<0.001), juvenile broadleaved tree (P<0.001), and shrub (P<0.01) stem density; lower herb (P<0.01) and native herb (P<0.001) cover; and lower shrub (P=0.001), herb (P<0.001), and native herb (P=0.002) species richness compared to forests with lower giant knotweed stem density. Total mass of autumn litterfall did not differ between plots with and without established giant knotweed, but sites with giant knotweed had 70% loss in leaf litter mass of native species (P<0.001). Giant knotweed and native species differed greatly in C:N ratios in fresh and senescent leaves. Fresh giant knotweed leaves had 61% to 65% lower C:N ratios than red alder and willow leaves. In contrast, senescent giant knotweed leaves had 38% to 58% higher C:N ratios than leaves of native taxa. Estimates of nitrogen reabsorption prior to litterfall were high for giant knotweed (76%) and low for native species (red alder (5%) and willow (33%)). Consequently, areas dominated by giant knotweed had less nitrogen available for uptake by both terrestrial and aquatic organisms than areas dominated by native species. The authors suggested that changes in native species density and diversity and differences in litter quality resulting from giant knotweed establishment likely have cascading effects on the structure and function of riparian systems, though this hypothesis was not tested .
In old fields in south-central New York, one study examined how changes in plant diversity and stand structure resulting from Japanese knotweed establishment negatively impacted the foraging success of green frogs. Green frogs in areas with established Japanese knotweed failed to gain mass, while more than 50% of green frogs in areas without established Japanese knotweed gained mass. The authors suggested that the lower success of green frog foraging was due to declines in populations of invertebrate prey, though this hypothesis was not tested. The authors did document several changes in vegetative characteristics that could have influenced invertebrate populations and the ability of green frogs to forage. No native plants were found 33 feet (10 m) inside Japanese knotweed stands. Vegetation height increased abruptly at the edge of Japanese knotweed stands, from an average of 33 inches (84 cm) outside the stand to an average of 78 inches (198 cm) at the stand's edge. Average height to first leaves was higher in Japanese knotweed stands compared to vegetation outside of stands. Litter was deeper in Japanese knotweed stands, with most litter material consisting of large, fibrous stems .
Other ecological impacts of the 3 knotweeds include reduced recruitment of in-stream woody debris (review by ) and reduced habitat quality for wildlife (reviews by [52,87,111]). Establishment of these knotweeds may also increase the risk of streambank erosion  or flooding when decaying shoots are washed into rivers during high flows (reviews by [7,87]). Rhizomes and shoots may displace foundations, walls, pavement, and drainage works (review by ) or limit recreational access to riparian areas (, review by ). In the United Kingdom, lack of Japanese knotweed control in urban areas may lead to an increased risk of an area being "used as an illicit litter dump or as a refuge for vandals and muggers" .
Control: In all cases where invasive species are targeted for control, the potential for other invasive species to fill their void must be considered, no matter what control method is employed . Control of biotic invasions is most effective when it employs a long-term, ecosystem-wide strategy rather than a tactical approach focused on battling individual invaders . Information presented in the following sections may not be comprehensive and is not intended to be prescriptive in nature. It is intended to help managers understand the ecology and control of the 3 knotweeds in the context of fire management. For more detailed information on the control of giant, Japanese, or Bohemian knotweed, consult the references cited here or local extension services. For a review of control recommendations for the 3 knotweeds in the Pacific Northwest, including commentary on hand cutting, mowing, digging, covering, goat browsing, and many methods of herbicide application, see . For a review of control methods for Japanese knotweed in the United Kingdom plus information on preventing establishment, see .
Several sources suggest that the 3 knotweeds are difficult to eradicate [31,59,98,109,142] due to their extensive root and rhizome systems , the ability of multiple plant parts to regenerate vegetatively [31,115], sprouting immediately [115,116] or 1 to 3 years after treatment , and the large scale of stand establishment . Control of the 3 knotweeds may require multiple treatments within a single growing season [101,115,116] or several years of treatment ([103,109,115,116], review by ) to be effective. Careful disposal of removed plant parts is important to prevent downstream transport  or reestablishment. Control and eradication efforts always face the potential for floods or high water to expose and/or transport buried rhizomes or propagules from upstream populations .
Prevention: One way to minimize the establishment and spread of the 3 knotweeds is to avoid planting them. The Tennessee Exotic Pest Plant Council recommends the following native plant species as alternatives to Japanese knotweed in landscaping: goat's-beard (Aruncus dioicus), Culver's root (Veronicastrum virginicum), white snakeroot (Ageratina altissima), black cohosh (Actaea racemosa), coastal sweetpepperbush (Clethra alnifolia), and Virginia sweetspire (Itea virginica) . Native alternatives to Japanese knotweed in the mid-Atlantic states include Virginia sweetspire, coastal sweetpepperbush, maleberry (Lyonia ligustrina), silky dogwood (Cornus amomum), fragrant sumac (Rhus aromatica), or flameleaf sumac (R. copallinum) .
It is commonly argued that the most cost-efficient and effective method of managing invasive species is to prevent their establishment and spread by maintaining "healthy" natural communities [75,106] (e.g., avoid road building in wildlands ) and by monitoring several times each year . Managing to maintain the integrity of the native plant community and mitigate the factors enhancing ecosystem invasibility is likely to be more effective than managing solely to control the invader .
Weed prevention and control can be incorporated into many types of management plans, including those for logging and site preparation, grazing allotments, recreation management, research projects, road building and maintenance, and fire management . See the Guide to noxious weed prevention practices  for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.
Cultural control: No information was available as of this writing (2010).
Physical or mechanical control: For a review of mechanical control recommendations for the 3 knotweeds in the Pacific Northwest see . Physical or mechanical control of the 3 knotweeds is complicated by extensive rhizome systems that allow plants to regenerate after single or multiple mechanical control attempts [101,103,115]. In the Pacific Northwest, the 3 knotweeds sprouted following cutting, mowing, and digging, sometimes within a week of mechanical treatments . In the United Kingdom, cut Japanese knotweed stems produced new shoots from dormant buds on the "rootstock". Plants cut twice over 2 years had sufficient reserves in the rhizomes for "vigorous" regeneration . Also in the United Kingdom, cutting Japanese knotweed stands increased both the lateral spread of clumps and stem density (review by ).
Hand-pulling was considered an ineffective means of Japanese knotweed control in the United Kingdom (review by ). In the United Kingdom, 3 years were needed to eradicate 2 small Japanese knotweed plants by hand pulling. In established riverbank stands, 10 years of hand pulling reduced Japanese knotweed cover but did not eliminate it. Two years of biweekly mowing in the growing season were considered effective at controlling Japanese knotweed . In greenhouse studies near Washington, DC, grubbing was not an effective control strategy for Japanese knotweed; it did not remove all rhizome fragments and Japanese knotweed was able to regenerate. Grubbing also exposed large amounts of mineral soil, facilitating the establishment of other nonnative plants . In the Czech Republic, mowing twice in the growing season and both high- and low-intensity grazing by domestic sheep and goats were effective at reducing the survival of planted rhizomes of all 3 knotweeds (P<0.001) .
In greenhouse studies near Washington, DC, cutting of Japanese knotweed stems originating from rhizome fragments led to a decrease in belowground biomass when the plants were cut between 5 June and 28 August (P<0.05). Multiple cuttings led to greater decreases in belowground biomass. In this experiment, 5 cuttings were needed to cause a net depletion of belowground biomass. The authors suggested that cutting should be done at least 8 weeks prior to senescence; otherwise the treatment would have no impact on underground reserves .
Covering or smothering the 3 knotweeds has had variable results. Field observations in New Jersey documented one Japanese knotweed plant emerging through 3 inches (8 cm) of asphalt . A horticultural journal reports that giant and Japanese knotweed may push through 2 inches (5 cm) of asphalt but suggests that smothering giant and Japanese knotweed plants with several layers of black plastic topped with asphalt, gravel, or patio stones may be an effective control method . One source from the Pacific Northwest reports that covering patches of the 3 knotweeds is not an effective control method .
Industrial composting methods kill all plant parts of Japanese knotweed, but home compost piles are not likely to reach lethal temperatures .
Biological control: Biological control of invasive species has a long history that indicates many factors must be considered before using biological controls. Refer to these sources: [137,149] and the Weed control methods handbook  for background information and important considerations for developing and implementing biological control programs.
Managers in the United Kingdom are actively pursuing biocontrol programs for Japanese knotweed . Insects from its native range may be useful in controlling Japanese knotweed (, review by ). In North America, signs of insect herbivory are low for Japanese  and Bohemian  knotweed. One source identified a number of potential insect, slug, and snail biocontrol agents for "knotweeds" in North America, though field observations in New York, Oregon, and Washington suggested that herbivore damage was low .
Chemical control: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but they are rarely a complete or long-term solution . See the Weed control methods handbook  for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.
Several sources suggest that herbicide application is the most effective means of controlling the 3 knotweeds in North America [31,73,109,111,112], though their establishment in riparian areas presents challenges to chemical control programs [73,111] and multiple years of treatment are needed [31,109,110]. For reviews of Japanese knotweed response to herbicides and other chemicals see the following sources: [7,11,101]. For information on intensive programs to chemically control the 3 knotweeds in North America, see the following sources: [31,40,73,109,110,111,112].
The 3 knotweeds may sprout in the growing season following some herbicide applications [31,110,115]. In Washington, numerous Bohemian knotweed root crowns sprouted the growing season following foliar herbicide application. Follow-up cut-stem treatments and stem injection treatments in the 2nd and 3rd season after initial treatment successfully killed plants, though some stems still survived into the 4th season .
Along the Hoh River in northwestern Washington, herbicide injection treatments reduced the number of Bohemian knotweed plants, but the resulting shift from large, multi-stemmed clumps of Bohemian knotweed to small, single-stem clumps made location of plants and subsequent control efforts difficult . Effective management required large crews, intensive surveys and mapping, and multiyear efforts, all with the possibility that floods or high water might transport plants from upstream to control sites or expose buried rhizomes . Managers also reported some concern that the injected herbicide could spread into adjacent soils and harm nearby native vegetation. Managers concluded that Bohemian knotweed eradication would likely take ≥10 years in this area .
Integrated management: Several sources report the use of integrated management techniques in the control of the 3 knotweeds. Along roadsides in New York, one author recommended management strategies that included both direct control of Japanese knotweed and the promotion of native vegetation . In riparian areas in northwestern Oregon, managers used patch size, patch location, time of year, and landowner preference to determine control strategies for Japanese and giant knotweed. Integrated methods included foliar and stem herbicide application, spring stem cutting, and manual removal and digging of rhizomes. Adaptive management led to a 89.6% reduction in number of stems over 8 years of treatments. However, control efforts were not successful in eradicating any patch with >573 stems through the use of any treatment regime, even after up to 9 treatments . Reports from Europe suggest that using a combination of mechanical digging and herbicide application led to a greater reduction in Japanese knotweed plant density, plant height, stem diameter, and number of leaves than either treatment alone .
Plant growth: Japanese knotweed exhibits rapid growth . Over the New Jersey growing season of approximately 80 days, the average rate of Japanese knotweed growth was 3.28 inches (8.35 cm)/day . In the Pacific Northwest, Japanese knotweed stems reached 10 feet (3 m) by June . Giant knotweed also exhibits fast growth . In the Czech Republic, early spring giant knotweed shoots grew an average of 1 inch (3 cm)/day for 20 days, then increased growth to approximately 2 inches (5 cm)/day . In the Pacific Northwest, giant knotweed stems reached 15 feet (4.5 m) by June . Several sources report that Japanese knotweed grows better in full sun than partial shade ([19,81,103], reviews by [11,38,102]). See Successional status for more information on this topic.
Populations of one or more of these 3 knotweeds may cover extensive areas. In Washington, DC, 2 streamside stands of Japanese knotweed extended 282 feet (86 m) and 190 feet (58 m), respectively . Approximately 25% of individual Japanese and Bohemian knotweed plants surveyed in Belgium covered >0.02 acres (100 m²) .
In the United Kingdom, one Japanese knotweed plant occupied approximately 245 acres (100 ha) (S. Hatherway personal communication cited in ). This extensive growth plus the identification of all Japanese knotweed plants in the United Kingdom as genetically identical suggests that it might be one of the largest vascular plants on earth , prompting the newspaper headline: "Largest female on earth could strangle Britain" (review by ).
These 3 knotweeds grow in dense stands and often become the dominant vegetation ([73,87], review by ). Two- to 3-year-old giant knotweed plantings produced enough "root runners" that no vegetation of any kind could grow under their mat (review by ). Japanese knotweed stands in Colorado were characterized as "dense thickets of rank growth" . In New York, vegetative cover of roadside Japanese knotweed was 81.3% in study plots with a total plant cover of 84.6% . In Michigan, Japanese knotweed established in dense thickets >7 feet (2 m) tall . In southwestern Washington, managers treated over 300,000 Japanese knotweed stems in 500 acres (200 ha) of treatments . In northwestern Washington, giant knotweed stem density in low terrace and floodplain forests ranged from 0.0 to 8.8 stems/m² .
The young shoots of Japanese knotweed are edible to humans and can be steamed and eaten like asparagus [36,56,69,78]. Tender parts of Japanese knotweed rhizomes may be peeled, boiled, and buttered like potatoes . The leaves of Japanese knotweed were used as a substitute for tobacco by troops in World War II (review by ).
The knotweed genus (Polygonum) produces a wide variety of secondary compounds that have been evaluated for medicinal properties including cancer prevention and antifungal and antimicrobial activity (reviewed by ). The rhizomes of both Japanese and giant knotweed have been used to induce menstruation, as a cathartic, and as a laxative in China, and for painkilling and to reduce bleeding in Japan . Japanese knotweed is made into "Itadori", or "well-being", tea in Asia because it contains high levels of the compound resveratrol, reported to reduce the risk of cancer, heart ailments, and health issues related to estrogen imbalance .
In the Pacific Northwest, Japanese and giant knotweed are valued for honey production. They provide a late-season flower source and the resulting honey is higher in antioxidants than honey made from other plants . Laboratory experiments determined that extracts from giant knotweed may have the potential to be useful for restricting the fish pathogen pasteurellosis, preventing large economic losses in the aquaculture industry . Giant and Japanese knotweed are also used for firewood and match sticks (review by ). In rehabilitation efforts, Japanese knotweed has been used as a sand dune stabilizer (review by ) and as a heavy metal accumulator on contaminated soil (review by ).