Britt Slattery, USFWS
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Japanese Knotweed
Polygonum cuspidatum
Origin: Eastern Asia
Background
Japanese knotweed was probably introduced into the United
States in the late 1800's. It was first planted as an
ornamental and has also been used for erosion control and
landscape screening. Japanese knotweed is a noxious weed
in the state of Washington.
Distribution and Ecological Threat
Japanese knotweed occurs across the continent from Maine
to Wisconsin, south to Louisiana and in scattered
locations in the Midwest and Western states. It can
tolerate a variety of adverse conditions, including deep
shade, high temperatures, high salinity and drought.
Knotweed is commonly found near water sources, such as
along streams and rivers, in low-lying areas, waste places
and utility rights-of-way and around old home sites. It
spreads quickly to form dense thickets that exclude native
vegetation and greatly alter natural ecosystems. Japanese
knotweed poses a significant threat to riparian areas,
where it can survive severe floods and rapidly colonize
scoured shores and islands. Once established, populations
are extremely persistent.
Description and Biology
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Plant: a member of the buckwheat family (Polygonaceae),
knotweed is an upright, shrubby, herbaceous perennial
that can grow to over 10 feet in height. Stems of
Japanese knotweed are smooth, stout and swollen where
the leaf meets the stem. Like all members of the
family, it has a membranous sheath surrounding the
joints of the stem.
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Leaves: although variable, leaves are normally about 6
inches long by 3 to 4 inches wide, broadly oval to
somewhat triangular and pointed at the tip.
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Flowers, fruits and seeds: minute greenish-white
flowers occur in attractive, branched sprays in summer
and are followed soon after by small winged fruits.
Seeds are triangular, shiny and very small, about 1/10
inch long.
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Spreads: primarily by seed and by vegetative means with
the help of long, stout rhizomes. It can be transported
to new sites by water, wind, as a contaminant in
fill-dirt, or on the soles of shoes. It often escapes
from neglected gardens and discarded cuttings.
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Look-alikes: Virginia knotweed (Tovara
virginica), prince's feather (Polygonum
orientale), and giant knotweed (Polygonum
sachalinense, non-native).
Prevention and Control
Japanese knotweed is an extremely difficult plant to
control due to its ability to re-grow from vegetative
pieces and from seeds. Mechanical and chemical methods are
most commonly used to eliminate it. Single young plants
can be pulled by hand depending on soil conditions and
root development. All roots and runners must be removed to
prevent re-sprouting. Glyphosate and triclopyr herbicides
have been used, applied either to freshly cut stems or to
foliage.
Native Alternatives
sweet pepperbush (Clethra
alnifolia)
USFWS
|
Virginia sweetspire (Itea
virginica)
Britt Slattery, USFWS
|
maleberry (Lyonia ligustrina)
R. Harrison Wiegand
|
silky dogwood (Cornus amomum)
Chris Miller, NRCS
|
fragrant or shining sumac
(Rhus aromatica or copallina)
R. Harrison Wiegand
|
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