Fallopia japonica

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R. japonica
Scientific Name
Reynoutria japonica
Sieb. & Zucc.
Scientific Name Synonyms
Fallopia japonica
(Houttuyn) Ronse-Decraene
Polygonum cuspidatum
Siebold & Zucc.
Fallopia japonica var. japonica
(Houtt.) Ronse Decr.
Common Names
Japanese knotweed


Fallopia japonica is an upright herbaceous perennial shrub reaching heights of 10 ft. (3 m). The semi-woody stem is hollow with enlarged nodes.
Leaves are alternate, 6 in. (15.2 cm) long, 3-4 in. (7.6-10 cm) wide and broadly-ovate. Leaf tips are abruptly acuminate and the bases are truncate.
Flowering occurs in late summer, when small, greenish-white flowers develop in long panicles in the axils of the leaves. Plants are dioecious (male and female flowers occur on separate plants).
The fruit are papery and winged, containing shiny, black, three angled achenes.
Ecological Threat
Fallopia japonica commonly invades disturbed areas with high light, such as roadsides and stream banks. Reproduction occurs both vegetatively (rhizomes) and seeds, making this plant extremely hard to eradicate. The dense patches shade and displace other plant life and reduce wildlife habitat. This plant is native to eastern Asia and was first introduced into North America in the late 1800s.

General Description

The following is based on descriptions by Fernald (1950)[1], Muenscher (1955)[2], Locandro (1973,1978)[3][4], and Ohwi (1965)[5].

Fallopia japonica is an herbaceous perennial which forms large clumps 1-3 meters high. It is fully dioecious and can reproduce by seed and by large rhizomes which may reach a length of 5-6 meters. The stout stems are hollow and bamboo-like, extend from an erect base and are simple or little branched and glabrous with thinly membranous sheaths. Leaves are broadly ovate, truncate to cuneate at base, abruptly cuspidate, 5-15 cm long, 5-12 cm broad, with petioles 1-3 cm long. Greenish white flowers 2.5-3 mm long, densely arranged in axillary panicles; 3 styles; 8-10 stamens with longitudinally dehiscing anthers. Fruiting calyx wing-angled, 6-10 mm long. Achenes shiny black-brown, 3-4 mm long, acutely trigonous. Male flowers have branched panicles on upright racemes with the distal end of the raceme in the highest position; individual panicles generally point up. Female flowers are drooping or decumbent with the distal end in the lowest position; individual panicles are not oriented in a particular direction. Both male and female flowers possess vestigial organs of the other sex. Fallopia japonica closely resembles Polygonum sachalinense (Reynoutria sachalinensis), an exotic species native to northern Japan and the Sakhalin Islands.[5] Polygonum sachalinense can be distinguished primarily by its larger size, greenish flowers and cordate leaves which gradually taper to the tip.[1] Fallopia japonica is known to hybridize with Polygonum sachalinense and with Fallopia baldschuanica.[6][7] Hybrids between Fallopia japonica and Polygonum sachalinense have been frequently mistaken for Fallopia japonica in the U.K..[8]

Stewardship summary

Fallopia japonica is widely distributed in much of the eastern U.S. In western Pennsylvania it already occupies hundreds of acres of wetlands, streambanks and hillsides and has spread along the banks of the Allegheny and Ohio Rivers and dominates the edges of many of the islands in these rivers.[9]It is present on at least two sites belonging to the Pennsylvania Chapter of The Nature Conservancy (Long Pond in the Poconos and Bristol Marsh, an urban preserve near Philadelphia)and has become a problem on creeks in suburban Philadelphia.[10] It is also a serious problem in Rock Creek Park, a national park in Washington D.C. In the U.K., it is considered a major weed and a threat to conservation, and it is legally prohibited to introduce Japanese knotweed into the wild.[11][12][13] Its early emergence and great height combine to shade out other vegetation and prohibit regeneration of other species.[14] Thus it reduces species diversity and damages wildlife habitat.[9][15][11] It does not appear to be a threat in undisturbed forest and other low light areas, but it is likely that, if unchecked, it will continue to expand its range in open habitats. Once Fallopia japonica has established it forms large, almost pure stands which are extremely persistent and difficult to eradicate.

Natural history

Fallopia japonica is native to eastern Asia. It was introduced from Japan to the United Kingdom as an ornamental in 1825, and from there to North America in the late nineteenth century.[16][17][18] In Japan, Fallopia japonica is widely distributed and is usually found in sunny places on hills and high mountains.[5][19] It is a dominant pioneeer in the primary succession of volcanic slopes and is frequently a colonizer in secondary succession.[20]

In the U.K., Fallopia japonica has spread extensively, occurring in virtually every 10 km square (Nature Conservancy Council, 1989). Stands range in size from individual plants to clumps more than 500 square meters.[11] Fallopia japonica has also become naturalized in much of central Europe.[14] In North America, it is widely found in the eastern U.S. and has been observed as far north as Nova Scotia and New Foundland, as far south as North Carolina, in much of the midwest and in the coastal areas of Washington and Oregon.[21][17][3] Large stands have been noted in western Pennsylvania, in particular along the banks of the Ohio and Allegheny Rivers and on the islands in these rivers.[9] Fallopia japonica spreads primarily along river banks, but also grows in wetlands, waste places, along roadways, and in other disturbed areas (Muenscher, 1950).[13][18]

Fallopia japonica can thrive in a wide variety of habitats. In Japan, it can grow on volcanic soils high in sulfur and having a pH less than 4.[18] In the U.S., it has been observed growing in a variety of soil types, including silt, loam, and sand, and in soils with pH ranging from 4.5 to 7.4.[4] Analyses of soils from 17 stands in Wales showed no correlation between stand size and vigor and soil chracteristics. The stands studied grew in soils with a broad range of pH, organic matter and nutrients.[11] In Japan, Fallopia japonica growth is slow, but steady in nutrient poor sites, and rapid in nutrient rich sites.[20] In areas where Fallopia japonica has been introduced, it is found primarily in moist, unshaded habitats. Distribution maps from the U.K. show that it is generally associated with regions of high precipitation.[18] Locandro (1973)[4] reports it growing on xeric as well as hydric sites in the U.S. Its distribution appears to be limited by light. It is found primarily in open sites, and its growth and abundance are depressed in shady sites (Seiger, unpublished data).[22]

Fallopia japonica flowers from July to October in Japan [5] and in August and September in the U.K. and North America.[2][1][18] It is pollinated by bees and other insects.[3][8] Seeds appear about two weeks after flowering (personal observation) and are wind dispersed.[23] In Japan, reproduction in Fallopia japonica is characterized by high seed production and low seedling survival, but plants have a very high probability of survival once established.[24] However, in the U.S., U.K, and Europe, seeds do not appear to be a significant mode of reproduction. In a study of the reproductive ecology of Fallopia japonica populations in New Jersey, Locandro (1973)[4] found viable pollen, but noted that fertile males were rare. Seedling germination was observed in the presence of males, but no seedling survival was recorded during five years of observation. In the absence of males, females produced empty achenes. In the U.K., fruit set is very rare. This was originally attributed to the rarity of male fertile flowers.[18] It has since been found that there are no male fertile forms of Fallopia japonica in England and that the pollen source is actually a hybrid between Fallopia japonica and Polygonum sachalinense.[8] Fallopia japonica also hybridizes with Fallopia baldschuanica.[8][6][7] In the U.S., hybrids morphologically similar to those between Fallopia japonica and Fallopia baldschuanica have been grown from seeds collected in the field, but seedling establishment has not been observed in the wild (Seiger, unpublished data).

The primary mode of reproduction in the U.S., U.K. and Europe is through extensive rhizomes which can reach 15-20 meters in length.[4][18] Dispersal can occur naturally when rhizome fragments are washed downstream by the current and deposited on banks or, as more commonly occurs, when soil is transported by humans as fill dirt.[3][18] Rhizomes can regenerate from small fragments, and have even been observed to regenerate from internode tissue.[4] Rhizomes can regenerate when buried up to 1 meter deep and have been observed growing through two inches of asphalt.[16][3] The ability of rhizomes to generate shoots was found to be affected by the source of rhizome fragments, fragment size and depth planted, the optimal depth being just below the surface.[4]

Fallopia japonica requires high light environments and effectively competes for light in such environments by emerging early in the spring and using its extensive rhizomatous reserves to quickly attain a height of 2-3 meters. Shoots generally begin to emerge in April and growth rates exceeding 8 cm per day have been recorded.[4] In addition, its deep root system gives it an advantage in foraging for nutrients and water, and contributes to soil stabilization on disturbed sites.[25][24] Hirose and Tateno (1984)[24] found that organic nitrogen levels on Mt. Fuji increased following colonization by Fallopia japonica on bare volcanic desert and concluded that Fallopia japonica contributes to the development of the ecosystem, in part, by acting as a nutrient reservoir.

Fallopia japonica is found on open sites and does not appear to be able to invade forest understory due to its high light requirements.[22] Studies of the very closely related Polygonum sachalinense indicated that Polygonum sachalinense plants grown in low light did not have higher photosynthetic rates than plants grown in high light, and thus would not be expected to adapt to sites with low light intensity.[26] Transplant studies of Fallopia japonica in closed understory sites showed poor survival and growth compared to open bank sites, confirming that it was environmental factors and not limitations on dispersal which exclude Fallopia japonica from understory sites.[27] Follow-up studies in the greenhouse showed that Fallopia japonica grown under light levels comparable to those found in the understory had significantly less rhizomatous reserves at the end of the season than did plants grown under full sunlight (Seiger, unpublished data).

Fallopia japonica occurs in much of the temperate U.S. Though not yet a major weed in the U.S., it is spreading, particularly in the eastern states. Dispersal is limited to areas where rhizome fragments from existing stands are washed downstream or soil containing rhizomes is transported by humans. Once established, it forms large, monospecific stands which displace virtually all other vegetation. Establishment can be prevented by monitoring for the introduction of Fallopia japonica and manually removing the entire plant. Small stands may be controlled by repeated cutting, which may need to be supplemented by revegetation once growth of Fallopia japonica has been reduced. At present, the only method to control large stands is with repeated application of herbicides. Complete eradication may not be possible.


Current control methods (mechanical, herbicidal) require continued treatment to prevent reestablishment of Fallopia japonica. It may be feasible to reintroduce competitors as an alternative to continued treatment. There is a need for more research on whether native species might serve effectively as competitors and methods of reintroduction. Only very preliminary work has been done towards developing a biological control for Fallopia japonica and much research remains to be done (see below).

The International Institute for Biological Control in conjunction with the National Agricultural Research center in Japan has produced a partial list of insect herbivores in Japan which are associated with Fallopia japonica. A number of pathogens have been collected from Japan by CAB International and are held at the International Mycological Institute.[28] Plans are under way to begin a study of Fallopia japonica in its native habitat.[29] Contact:

Dr. Simon V. Fowler
International Institute of Biological Control
Silwood Park
Buckhurst Road
Ascot, Berks, SL5 7TA, UK

Recent studies in Rock Creek park, Washington, D.C. indicate potential for control by mechanical means combined with revegetation (Seiger, unpubished data). Field tests will be conducted in 1992. Contact:

Leslie Seiger PhD
Biology Department
California State University, San Diego
9500 Gilman Dr.,
La Jolla, CA 92093-0346
Phone Number: 619-594-6328
E-mail: lseiger@sunstroke.sdsu.edu

In areas where Fallopia japonica has not yet become established, the focus of management should be to prevent establishment by monitoring areas for introductions of Fallopia japonica and eradicating newly established stands before they can become established.

Manual control consists of digging out the rhizomes or cutting the stalks. Digging is extremely labor intensive and tends to spread the rhizome fragments and promote disturbance and is not recommended.[11] Cutting, on the other hand, may be quite effective in eliminating Fallopia japonica. It has been observed that Fallopia japonica does not establish where grazing pressure is high.[11][13] In a review of control methods, Palmer (1990)[11] noted that eradication is not complete with cutting alone, but has been nearly achieved in some cases and should be feasible with persistence. A number of authors claim that cutting is ineffective.[21][16][30] These conclusions are based on the observation that shoots are regenerated following cutting. However, a greenhouse study found that cutting stems results in a significant reduction of rhizomatous reserves. The same study also found that cutting was equally effective at any time during the growing season prior to the beginng of senescence.[31] A study of the effects of repeated cutting showed that at least three cuts are needed in a growing season to offset rhizome production (Seiger, unpublished data). Manual control can be labour intensive, but where populations are small and isolated, may be the best option. No research has been done to test the effectiveness of burning. It may act similarly to cutting by removing above ground material.

Shading, particularly in conjunction with cutting, may be another useful means of control on smaller stands. Studies showing that F. japonica requires high light environments suggest that covering stands with black plastic or shade cloth may reduce growth. Pridham and Bing (1975)[16] state that applying several layers of black polyethylene film covered by asphalt, blocks or stones to a leveled soil surface may provide some control. However, they also note that F. japonica is able to emerge through asphalt. If shade cloth (or plastic) is to be applied without cutting, then, to prevent F. japonica from emerging through the covering, shade cloth should be placed over shoots after the plants have reached their full height or placed well above newly emerging shoots, or raised as plants grow.

A number of biocidal chemicals have been found to be effective against Fallopia japonica. Most of these are undesirable for use in conservation areas because they are nonselective, may be persistent in the soil and/or are not safe for use near water. One frequently used way to minimize the effects of non-selective herbicides on non-target species is to paint herbicides directly onto the target plants.[10] In the case of F. japonica, this would probably require prior cutting for easier access if herbicides are to be applied after the plants have reached their full height. Herbicides appear to be more effective when combined with cutting.[15][30]

Glyphosate [N-(phophonomethyl)glycine] has been found to be very effective against Fallopia japonica.[21][13][32] Glyphosate is a nonselective systemic herbicide with a short residual life.[33][32] Application is more effective in the fall when leaves are translocating to rhizomes.[33] The British Nature Conservancy Council (1989)[12] recommends applying 2.0 kg/ha in August with a prior cut in late spring or early summer. Glyphosate is available from Monsanto under the trade names Roundup® and Rodeo®. Only Rodeo has been approved for use near water.[34] Glyphosate has been used with limited success on some nature reserves in the U.K..[11] Repeated applications over several years may be necessary.[21][11][13]

The Nature Conservancy Council (1989)[12] also recommends picloram to be applied at a rate of 2.6 kg/ha in the spring. Picloram is a selective herbicide which is persistent in the soil. It must not be used near water, thus excluding its use in many of the areas where Fallopia japonica is a problem.[15][35] Dicamba (3,6-dichloro-o-anisic acid) has also been found to be effective against Fallopia japonica, but is persistent in the soil and nonselective.[16] A number of other herbicides have been tested against Fallopia japonica, both alone and in combination with other herbicides.[15][30] Herbicide may have to be used on stands that have been allowed to attain a large size. However, their use is not recommended in nature reserves because of their undesirable effects on other biota and the need for repeated applications to maintain control of Fallopia japonica.

Regardless of whether control is manual or chemical, as long as some rhizomes remain in the soil Fallopia japonica returns once management is relaxed.[11][12][13] It has been suggested that the reintroduction of effective competition might be possible.[36] Research has only recently begun on biological control. The herbivores and pathogens of Fallopia japonica in Wales have been examined for their potential as control agents.[28] A program is underway at the International Institute of Biological Control to identify biological control agents.[29] The genetic uniformity of Fallopia japonica makes it a good candidate for biological control.[8] Biological control may be necessary where Fallopia japonica has taken over vast areas as it has done in the U.K., but it may be years before a successful control agent can be found. The following individuals are familiar with Fallopia japonica and its control in natural areas:

Leslie Seiger PhD
Biology Department
California State University, San Diego
9500 Gilman Dr.,
La Jolla, CA 92093-0346
Phone Number: 619-594-6328
E-mail: lseiger@sunstroke.sdsu.edu

John Palmer
Richards Moorehead & Laing Ltd
3 Clwyd Street
Clwyd LL15 1HF, Wales

Jonathan Soll
The Nature Conservancy of Oregon
821 SE 14th Ave
Portland OR 97214
503-230-1221 x329 or jsoll@tnc.org


It is extremely difficult, if not impossible to eradicate large established stands of Fallopia japonica. However, establishment can be prevented fairly easily by removing Fallopia japonica before it becomes firmly entrenched. Areas known to be near established stands of Fallopia japonica, particularly those downstream from such stands, should be monitored for the introduction of Fallopia japonica. The following individuals have direct experience monitoriing Fallopia japonica:

Leslie Seiger PhD
Biology Department
California State University, San Diego
9500 Gilman Dr.,
La Jolla, CA 92093-0346
Phone Number: 619-594-6328
E-mail: lseiger@sunstroke.sdsu.edu

Robert Orchowski, Director, Air Quality, Environmental Affairs Unit
Duquesne Light
One Oxford Center
301 Grant Street
Pittsburgh, PA 15279

John Palmer
Richards Moorehead & Laing Ltd
3 Clwyd Street
Clwyd LL15 1HF, Wales
David Beerling
School of Pure & Applied Biology
University of Wales
PO Box 915
Cardiff CF1 3TL, Wales

Static files


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  8. Bailey, J.P. (1990). Breeding behaviour and seed production in Alien Giant Knotweed in the British Isles. In:The biology and control of invasive plants. Conference organized by the Industrial Ecology Group of the British Ecological Society at the University of Wales College of Cardiff, September 20-21, 1990. 8.0 8.1 8.2 8.3 8.4
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  10. Broaddus,L. (1992). The Nature Conservancy, Pennsylvania Chapter. Memorandum to Leslie Seiger, March 6,1992. 10.0 10.1
  11. Palmer, J.P. (1990). Japanese knotweed (Reynoutria japonica) in Wales. In: The biology and control of invasive plants. Conference organized by the Industrial Ecology Group of the British Ecological Society at the University of Wales College of Cardiff, September 20-21, 1990. 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 11.8 11.9
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  14. Sukopp, H. and U. Sukopp (1988). Reynoutria japonica Houtt. in Japan und in Europa. Veroff. Geobot. Inst.ETH, Stiftung Rubel, Zurich 98:354-372. 14.0 14.1
  15. Scott, R., and R.H. Mars (1984). Impact of Japanese knotweed and methods of control. Aspects of applied biology 5, Weed control and vegetation managemnet in forestry and amenity areas pp. 291-296. 15.0 15.1 15.2 15.3
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  17. Patterson, D.T. (1976). The history and distribution of five exotic weeds in North Carolina. Castanea 41:177-180. 17.0 17.1
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  19. Kanai, H. (1983). Study on the distribution patterns of Japanese plants (5): Accumulation of phytogeographical data of popular plants, points and measures. Journ. Jap. Bot., 59: 257-269.
  20. Hirose, T. (1984). Nitrogen use efficiency in growth of Fallopia japonica Sieb. et Zucc. Annals of Botany,54:695-704. 20.0 20.1
  21. Pauly,W.R. (1986). Summary of Mexican bamboo control methods (Wisconsin). Restoration and Management Notes, 4(1):37-38. 21.0 21.1 21.2 21.3
  22. Beerling, D.J. (1991). The effect of riparian land use on occurrence and abundance of Japanese knotweed Reynoutria japonica on selected rivers in South Wales. Biol. Cons. 55:329-337. 22.0 22.1
  23. Maruta, E. (1976). Seedling establishment of Fallopia japonica on Mt. Fuji. Jap. J. Ecol. 26:101-105.
  24. Hirose, T., and M. Tateno (1984). Soil nitrogen patterns induced by colonization of Fallopia japonica on Mt. Fuji. Oecologia 61:218-223. 24.0 24.1 24.2
  25. Nakamura, T. (1984). Vegetational recovery of landslide scars in the upper reaches of the Oi River, Central Japan. J, Jap. For. Soc. 66(8):328-332.
  26. Patterson, D.T., D.J. Longstreth, and M.M. Peet (1977). Photosynthetic adaptation to light intensity in Sakhalin knotweed (Polygonum sachalinense). Weed Science 25:319-323.
  27. Seiger, L.A. and H.C. Merchant (1991). Effects of site on survivorship and size of Fallopia japonica. Bull.Ecol. Soc. Am. 72(2 suppl.):247.
  28. Fowler, S.V. and D. Schroeder (1990). Biological control of invasive plants in the UK - prospects and possibilities. In: The biology and control of invasive plants. Conference organized by the Industrial Ecology Group of the British Ecological Society at the University of Wales College of Cardiff, September 20-21, 1990. 28.0 28.1
  29. Fowler, S.V. (1991). International Institute of Biological Control. Silwood Park, Buckhurst Road, Ascot, Berks,SL5 7TA, UK. Letter to Leslie Seiger, January 8, 1991. 29.0 29.1
  30. Orchowski, R.W. (1991). Knotweed control pilot program Brunot Island Wildlife Habitat Enhancement Project.Presented at the 3rd Annial Wildlife Enhancement Council Symposium, Washington, DC, November 19, 1991. 30.0 30.1 30.2
  31. Seiger, L.A. and H.C. Merchant (1990). The ecology and control of Fallopia japonica. Bull. Ecol. Soc.Am. 71(2 suppl.):322.
  32. Ahrens, J.F. (1975). Preliminary results with glyphosate for control of Fallopia japonica. Proc. Northeast.Weed Science Soc. 29:326. 32.0 32.1
  33. Lynn, L.B., R.A. Rogers, and J.C. Graham (1979). Response of woody species to glyphosate in northeastern United States. Proc. Northeast Weed Sci. Soc. 33:336-342. 33.0 33.1
  34. Bender, J. (1988). Element Stewardship Abstract for Lythrum salicaria. The Nature Conservancy.
  35. Gritten, R. (1990). The control of invasive plants in the Snowdonia National Park. In: The biology and control of invasive plants. Conference organized by the Industrial Ecology Group of the British Ecological Society at the University of Wales College of Cardiff, September 20-21, 1990.
  36. Eaton, J.W. (1986). Waterplant ecology in landscape design. In Ecology and design in landscape. The 24th symposium of the British Ecological Society, Manchester 1983. A.D. Bradshaw, D.A. Goode, E.H.P. Thorp (Eds.). Blackwell Scientific Publications. Oxford.

Additional References

  • Maruta, E. (1983). Growth and survival of current-year seedlings of Fallopia japonica at the upper distributional limit on Mt. Fuji. Oecologia 60:316-320.

Original Document

Element Stewardship Abstract; L. Seiger, 1991

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