| SOURCE: The following information on this
species is taken, in part, from the Delaware River Invasive Plant Partnership,
Invasive Species Fact Sheets, http://www.paflora.org/DRIPP.html.
Modifications include additional pictures from www.invasives.org
and http://invasives.eeb.uconn.edu/ipane/,
as well as additional resource information. Check the website links for future
updates. The last update for this fact sheet is April, 2002. |
NATIVE RANGE:
Japan
DESCRIPTION:
Japanese knotweed and giant knotweed are herbaceous perennials that form large
colonies of erect stems that can reach 9 feet in height. They spread by
vigorous rhizomes (horizontal stems that grow just below the soil surface).
Japanese knotweed and giant knotweed are very similar in appearance and are
known to hybridize. The best character for separating them is the shape of the
leaf base, those of Japanese knotweed are truncate (squared-off) at the bottom,
while those of giant knotweed are heart-shaped.
Height -
Individual stems are 3-9 feet tall depending on the vigor of the colony.
Stem
- The hollow, bamboo-like stems are erect and unbranched or with a few branches
toward the tip. Despite their size, knotweed stems are annual; they die back to
the rhizome at the end of the growing season. New shoots emerge in April and
grow rapidly; early in the season they can grow 3-4 inches per day.
Leaves
- Leaves are alternate on the stem, simple, 4-6 inches long and almost as wide,
and dark green. Japanese knotweed leaves are abruptly squared-off (truncate) at
the base; those of giant knotweed have a heart-shaped base. Both narrow to a
pointed tip.
Flowers
- Both Japanese knotweed and giant knotweed have numerous small, greenish-white
lowers that are produced in late summer. Japanese knotweed bears only male or
female flowers on a given plant. Giant knotweed blooms have both male and
female parts in the same flower. However, appearances can be difficult to
interpret as both the male and female flowers of Japanese knotweed have
vestigial organs of the other sex present.
Fruit and seed
- The seed (technically a fruit called an achene) of both knotweeds is shiny
black, 3-angled, and about 1/6 inch long. It is enclosed in a winged calyx that
contributes to its buoyancy. The seeds have no dormancy requirement and
germinate readily.
Roots
- Roots are present along the rhizome and can extend quite deeply into the soil
making knotweed effective in preventing erosion.
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Photos: Leslie J. Mehrhoff, University of Connecticut, IPANE,
http://invasives.eeb.uconn.edu/ipane/ and Jil W. Swearingen, USDI
National Park Service, www.forestryimages.org.
Counterclockwise: Fruits (Jil Swearingen), Flowering plants, Comparison of
leaves of P. sachalinense (right of each pair) and P. cuspidatum,
Stand of young plants, cross-section of hollow stem.
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DISTRIBUTION
AND HABITAT:
Japanese knotweed is native to Japan; giant knotweed comes from Sakhalin Island
in northern Japan. They were introduced into North America for ornamental use
in the late 1800s. Japanese knotweed is now widely naturalized in Europe and
North America. In the east it extends from Newfoundland to North Carolina. It
is also widespread in the Midwest and in coastal areas of the Pacific
Northwest. It is most commonly found lining the banks of creeks and rivers
where it often forms an impenetrable wall of stems; it also occurs in wetlands,
waste ground, and along roads and railroads. In Pennsylvania knotweed has also
been extensively planted at strip mine reclamation sites.
EFFECTS OF
INVASION:
Dense stands of knotweed exclude other plant species leading to very limited
biological diversity in infested sites.
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Photo: Leslie J. Mehrhoff, University of Connecticut, IPANE,
http://invasives.eeb.uconn.edu/ipane/
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REPRODUCTION
AND METHODS OF DISPERSAL: Japanese knotweed and giant knotweed both spread vegetatively by
the growth and fragmentation of rhizomes. Even a 1-2 inch-long piece of rhizome
dislodged by flooding can initiate a new colony when it is deposited
downstream. Knotweed also grows from seeds, which are produced in large numbers
and dispersed by wind and water. Seed viability is high, and seed bank
densities have been measured at 220-1758 seeds per square meter. Highest
germination rates occur on exposed mineral soil.
NATIVE ALTERNATIVES
FOR REVEGETATION OF STREAM BANKS:
The following species are suggested for establishing native plant cover after
knotweed has been removed: shrubs - winterberry holly (Ilex verticillata),
spicebush (Lindera benzoin), buttonbush (Cephalanthus occidentalis),
silky willow (Salix sericea), pussy willow (Salix discolor), American
elderberry (Sambucus canadensis), alder (Alnus serrulata and A. incana ssp.
rugosa); herbaceous species- riverbank rye (Elymus riparius),
wild-rye (Elymus villosus), big bluestem (Andropogon gerardii),
switch grass (Panicum virgatum), wingstem (Verbesina alternifolia),
joe-pye-weed (Eupatorium fistulosum and E. maculatum), boneset (Eupatorium
perfoliatum).
AUTHORS:
Ann F. Rhoads and Timothy A. Block, Morris Arboretum, University of
Pennsylvania.
For more information
on Japanese and Giant knotweed, please contact:
Cornell University, Biological Control of Weeds--Garlic Mustard,
http://www.invasiveplants.net/invasiveplants/biologicalcontrol/default.asp
Delaware River Invasive Plant Partnership,
http://www.paflora.org/DRIPP.html
Invasive Plant Atlas of New England,
http://invasives.eeb.uconn.edu/ipane/
Mid-Atlantic Exotic Pest Plant Council, Inc., http://www.ma-eppc.org
National Invasive Species Information Center,
http://www.invasivespeciesinfo.gov
Ohio Department of Natural Resources, Division of Natural Areas and Preserves,
http://www.dnr.state.oh.us/dnap/invasive/10japknotweed.htm
Plant Conservation Alliance, Alien Plant Working Group, Weeds Gone Wild Fact
Sheets, http://www.nps.gov/plants/alien/fact/pocu1.htm.
Plant Invaders of Mid-Atlantic Natural Areas,
http://www.nps.gov/plants/alien/pubs/midatlantic/
Southeast Exotic Pest Plant Council; Invasive Plant Manual,
http://www.se-eppc.org/manual/
The Bugwood Network, MA-EPPC Plant List,
http://www.invasive.org/maweeds.cfm
The Nature Conservancy Invasive Species Initiative,
http://tncweeds.ucdavis.edu/esadocs.html
USDA Forest Service, Northeastern Area, Invasive Plants: Weeds of the Week,
http://www.na.fs.fed.us/fhp/invasive_plants/weeds/
USDA - NRCS PLANTS Database, http://plants.usda.gov/
Virginia Natural Heritage Program Fact Sheet, Japanese knotweed,
http://www.dcr.virginia.gov/dnh/invlist.htm
Virginia Tech Weed Identification Guide,
http://www.ppws.vt.edu/weedindex.htm
REFERENCES:
McCormick, L. H. and T. W. Bowersox. 1998. Eradication and control of Japanese
knotweed at the Staple Bend Unit, Allegheny Portage Railroad National Historic
Site. Penn State School of Forest Resources, University Park, PA.
Niewinski, A. T., T. W. Bowersox, and L. H. McCormick. 1999. Reproductive
ecology of giant (Polygonum sachalinensis) and Japanese (Polygonum cuspidatum)
knotweed. National Park Service Technical Report NPS/PHSO/NRTR-00/079.
University Park, PA.
Reeder, Kathleen Kodish and Brian Eick. 2001. Northeast parks' regional
strategy to control knotweed. Park Science 21: 33-35.
Rhoads, Ann Fowler and Timothy A. Block. 2000. The Plants of Pennsylvania: An
Illustrated Manual. University ofPennsylvania Press, Philadelphia, PA.
Rhoads, Ann Fowler and William McKinley Klein. 1993. The Vascular Flora of
Pennsylvania: Annotated Checklistand Atlas. American Philosophical Society,
Philadelphia, PA.
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