Bulletin #2511, Maine Invasive Plants: Japanese Knotweed/Mexican Bamboo, Fallopia japonica, Synonym: Polygonum cuspidatum (Smartweed Family)
Developed by the Maine Natural Areas Program and University of Maine Cooperative Extension
Threats to Native Habitats
Japanese knotweed is a robust perennial herb that emerges early in the spring and forms dense thickets up to nine feet in height. Thickets may be so dense that virtually all other plant species are shaded out. Large colonies frequently exist as monocultures, reducing the diversity of plant species and significantly altering natural habitat. Reproduction from rhizomes (horizontal underground stems), even small fragments, enables the plant to be easily transferred to new sites by flowing water and by soil used as fill. Unchecked, this plant can colonize extensively in riparian areas. Once established, it is difficult to remove.
Japanese knotweed is a fast growing, easily recognized herbaceous perennial that grows from three to nine feet in height and forms large thickets where it colonizes. The leaves are two to six inches long and broadly oval with somewhat squared bases and pointed tips. It is sometimes referred to as Mexican bamboo because of its large hollow stems. It produces tiny greenish-white flowers that grow in linear clusters along the stem. Flowers bloom from August to September and form shiny black-brown, three-sided seeds. In autumn, the leaves fall from the stout stems and the chestnut brown stems may remain standing for most of the winter, giving a false impression that the plant is woody. Reproduction is primarily vegetative with new shoots developing from extensive rhizomes. The plant most likely reaches new sites by transport of rhizome fragments. This plant may be confused with giant knotweed (Fallopia sachalinensis), which has a similar growth form, but is generally taller and has leaves with rounded leaf bases. Giant knotweed is also a non-native species with its origins in Asia. Consult a wildflower field guide or contact a natural resource professional for accurate identification.
This plant is most commonly found in moist, open habitats such as riverbanks, river islands, and disturbed wetlands; along road margins, and in areas with disturbed soils. Many colonies are escaped relicts of historical plantings and are located in or near towns or cities. Colonization of more natural habitats is facilitated by disturbance such as that caused by the scouring action of ice or high waters in rivers and streams.
Japanese knotweed is native to eastern Asia. It was brought to North America in the late nineteenth century, most likely for ornamental plantings. It has since spread into the wild over a large range that extends from Nova Scotia and Newfoundland south to North Carolina. In Maine, Japanese knotweed is documented in every county except Piscataquis and Hancock.
The best method of controlling this species is to prevent it from becoming established. It should be removed as soon as possible if it is found colonizing an area. Once well established, it can be eliminated by repeatedly cutting the stalks. Three or more cuttings in a single growing season can offset the growth of the rhizomes. An alternative is to cut it down repeatedly and apply glyphosate to the remaining part of the plant. Digging up the roots is not suggested because digging can lead to root fragments that can repopulate the area.
Josselyn Botanical Society of Maine. 1995. Checklist of the Vascular Plants of Maine, Third Revision. Orono, ME: Maine Agricultural and Forest Experiment Station.
Seiger, L. 1991. Element Stewardship Abstract for Polygonum cuspidatum. Arlington, VA: The Nature Conservancy in collaboration with the International Network of Natural Heritage Programs and Conservation Data Centers. Natural Heritage Databases.
The Nature Conservancy of Vermont. 1998. Invasive Exotic Fact Sheet: Japanese Knotweed. Montpelier, VT.
Gleason, H.A. and Cronquist, A. 1991. Manual of Vascular Plants of Northeastern United States and Adjacent Canada. New York: New York Botanical Garden.
For more information or for a more extensive list of references on invasive species contact:
Department of Conservation
#93 State House Station
Augusta, ME 04333-0093
University of Maine Cooperative Extension
495 College Avenue
Orono, ME 04469
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