Description ¹³

More info for the terms: adventitious, geophyte

Botanical description: This description covers characteristics that may be relevant to fire ecology and is not meant for identification. Keys for identification of giant and Japanese knotweed are available (e.g., [49,58,78,85,98,142]). For information on differentiating between giant, Japanese, and Bohemian knotweed, see the following: [7,43,153]. These 3 knotweeds are commonly confused [7,16,43,153].Giant knotweed is a stout, rhizomatous, perennial geophyte (review by [7]). Observations from the Czech Republic suggest that giant knotweed roots grow "deep" into the soil. Rhizomes have a diameter of 3 inches (8 cm) and may spread 50 to 65 feet (15-20 m) laterally [79]. Giant knotweed stems are generally clustered, erect [42], hollow, and up to 13 feet (4 m) tall. Leaves are alternate, oval to oblong, and large, frequently 12 inches (30 cm) long, with a cordate base [121]. Flowers are white [94] or greenish and borne on axial or terminal racemes or panicles. Seeds are triangular achenes [121] with 3-mm long nutlets [94].

Giant knotweed flowers.

Japanese knotweed is a stout, rhizomatous, perennial geophyte. Rhizomes are dark brown, knotty, up to 3 inches (8 cm) in diameter (review by [7]), and may spread laterally 23 to 65 feet (7-20 m) [115]. Japanese knotweed has a deep central taproot (reviews by [7,11]). Adventitious roots are white, fine, and thread-like. Stems are 3 to 13 feet (1-4 m) tall, glaucous, erect, and hollow (review by [7]). Japanese knotweed plants are often shrubby or woody at the base [98]. Leaves are alternate and broadly ovate (3 to 6 inches (8-15 cm) by 2 to 5 inches (5-12 cm)) with a truncate base [94]. Flowers are borne on axial or terminal racemes or panicles 3 to 6 inches (8-15 cm) long. Flowers are creamy-white. Seeds are tri-winged achenes (review by [7]).

Bohemian knotweed exhibits botanical characteristics similar to those of giant and Japanese knotweed, though it is intermediate in leaf size ([153], review by [7]).

Impacts and control ¹⁴

More info for the terms: allelopathy, cover, density, fire management, fitness, fresh, invasive species, litter, natural, prescribed fire, rhizome, shrub, tree

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 [53]), nutrient (review by [52]) and light ([107], reviews by [52,53]) limitation, and allelopathy [107,143].

Stand of Bohemian knotweed in Washington.
Photo courtesy of 10,000 Years Institute, www.10000yearsinstitute.org

Establishment of giant [136] and Japanese [72] 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 [77]. Changes in fungal assemblages were reported in areas with Japanese knotweed [72]. 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 [14].

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 [136].

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 [77].

Other ecological impacts of the 3 knotweeds include reduced recruitment of in-stream woody debris (review by [111]) and reduced habitat quality for wildlife (reviews by [52,87,111]). Establishment of these knotweeds may also increase the risk of streambank erosion [12] 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 [7]) or limit recreational access to riparian areas ([39], review by [52]). 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" [101].

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 [18]. 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 [75]. 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 [115]. For a review of control methods for Japanese knotweed in the United Kingdom plus information on preventing establishment, see [24].

Several sources suggest that the 3 knotweeds are difficult to eradicate [31,59,98,109,142] due to their extensive root and rhizome systems [115], the ability of multiple plant parts to regenerate vegetatively [31,115], sprouting immediately [115,116] or 1 to 3 years after treatment [116], and the large scale of stand establishment [115]. 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 [24]) to be effective. Careful disposal of removed plant parts is important to prevent downstream transport [31] 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 [109].

Fire: For information on the use of prescribed fire to control these species, see Fire Management Considerations.

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) [123]. 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) [122].

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 and by monitoring several times each year [64]. 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 [59].

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 [134]. See the Guide to noxious weed prevention practices [134] 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 [115]. 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 [115]. 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 [101]. Also in the United Kingdom, cutting Japanese knotweed stands increased both the lateral spread of clumps and stem density (review by [11]).

Hand-pulling was considered an ineffective means of Japanese knotweed control in the United Kingdom (review by [11]). 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 [5]. 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 [103]. 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) [15].

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 [103].

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 [74]. 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 [92]. One source from the Pacific Northwest reports that covering patches of the 3 knotweeds is not an effective control method [115].

Industrial composting methods kill all plant parts of Japanese knotweed, but home compost piles are not likely to reach lethal temperatures [32].

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 [132] 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 [105]. Insects from its native range may be useful in controlling Japanese knotweed ([145], review by [24]). In North America, signs of insect herbivory are low for Japanese [23] and Bohemian [107] 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 [52].

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 [21]. See the Weed control methods handbook [132] 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 [31].

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 [112]. 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 [109]. 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 [111].

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 [73]. 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 [116]. 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 [25].

National distribution ¹⁵

Canada

Origin: Exotic

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

United States

Origin: Exotic

Regularity: Regularly occurring

Currently: Unknown/Undetermined

Confidence: Confident

Sources and Credits

1. (c) own work, some rights reserved (CC BY-SA), http://commons.wikimedia.org/wiki/File:Fallopia-sachalinensis-00707.JPG
2. (c) Biopix, some rights reserved (CC BY-NC), http://www.biopix.com/photos/JCS-Fallopia-sachalinensis-60477.JPG
3. (c) Biopix, some rights reserved (CC BY-NC), http://www.biopix.com/photos/JCS-Fallopia-sachalinensis-60476.JPG
4. (c) Ori Fragman-Sapir, some rights reserved (CC BY-NC-SA), https://i1.treknature.com/photos/1990/polygonum_sachalinense1_236.jpg
5. (c) catherine_hosted, some rights reserved (CC BY-NC), uploaded by catherine_hosted
6. (c) Marion Zöller, some rights reserved (CC BY-NC), uploaded by Marion Zöller
7. (c) Kyle Jones, some rights reserved (CC BY-NC), uploaded by Kyle Jones
8. (c) David McCorquodale, some rights reserved (CC BY), uploaded by David McCorquodale
9. no rights reserved, uploaded by Daniel Atha
10. (c) Martin, some rights reserved (CC BY-NC), uploaded by Martin
11. (c) anonymous, some rights reserved (CC BY-NC), http://www.biopix.com/photos/JCS-Fallopia-sachalinensis-60475.JPG
12. (c) anonymous, some rights reserved (CC BY-NC), http://www.biopix.com/PhotosMedium/JCS%20Fallopia%20sachalinensis%2045164.jpg
13. Public Domain, http://eol.org/data_objects/24629923
14. Public Domain, http://eol.org/data_objects/24629940
15. (c) NatureServe, some rights reserved (CC BY-NC), http://eol.org/data_objects/29045450

More Info

• iNat taxon page
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