Japanese Knotweed
(Polygonum cuspidatum)
Julie Doll* and Dr.
Jerry Doll
"Japanese
knotweed {Japanese bamboo} is one of the most persistent,
durable, hardy plants known" (Locandro 1978). This quote
does not exactly give those with an infestation of this weed a
good feeling about their problem; unfortunately, it is only too
true. The spread of Japanese knotweed has been swift and
unstoppable. Not only an issue in the United States, the
eradication of this weed has stumped many a farmer, gardener,
and herbicide in many countries. The problem is such in the
United Kingdom that the Loughborough University is sponsoring a
workshop in late 1998 for the sole purpose of discussing
eradication and control of Japanese knotweed.
The
official name of Polygonum cuspidatum is Japanese
knotweed but it is also known as Mexican bamboo, crimson beauty,
Japanese fleece flower, Reynoutria fleece flower, Kontiki
bamboo, and Japanese polygonum. Some of these common names arose
because this species has been used as an ornamental plant in
many countries.
The
taxonomy and nomenclature of Japanese knotweed have changed
often over time, resulting in references to it in three genera: Polygonum,
Reynoutria, and Fallopia. These many names are the
result of a history of confusing taxonomy. In 1777, material
brought from Japan was named Reynoutria japonica and then
in 1846 additional plant material from Japan was named Polygonum
cuspidatum by Siebold and Zuccarini. Only in 1901 was it
discovered that the two plants were identical (Beerling et al.
1994). Taxonomic confusion continues today as in North America
the official Latin name is Polygonum cuspidatum while in
the British Isles it is Fallopia japonica.
History
Japanese
knotweed is native to Japan, China, and parts of Korea and
Taiwan. It was introduced to the United States prior to 1890. By
1900 it was established in the Eastern United States. It
appeared in Tennessee in the 1940’s and in 1966 was listed as
"one of the most persistent and aggressive of all perennial
weeds" (Remaly 1997). Unfortunately, the plant was still
used after this as an ornamental and for other uses. In this way
it easily spread across the United States. It occurs from the
Northeastern states to California (Uva et al. 1997). Nova
Scotia, Newfoundland, Canada, Europe, Russia, and most of Asia
also report having Japanese knotweed . Even today it is
suggested as a useful ornamental plant. However, some caution
that it should not be planted unless you can "surround
plantings (of Japanese knotweed) with concrete, blacktop or
brick" (MacKenzie and Smith 1990).
Description
Japanese
knotweed is a rhizomatous perennial that can reach 6 feet or
more in height. Many times it looks like a woody shrub and grows
in dense clumps where little or no other vegetation can survive.
The leaves are alternate on the stem, generally heart-shaped
with rounded lobes at the base, 2 to 3 inches wide and dark
green above while a paler green below. The leaf veins are often
reddish and the petioles are 1 inch long and ridged. The plant
has the characteristic ochrea of the buckwheat family at
the junction of the petiole and stem. Stems are reddish,
ridged, hollow and jointed and very bamboo-like and become woody
with age.
This
species is dioecious which means individual plants have only
male or female flowers. The flowers are small, white-cream,
bloom in late summer, and are produced in elongated erect
clusters that come from the leaf axils and may be 4 to 5 inches
long. The flowers on male plants are essentially erect while the
female flowers are more droopy or decumbent. While there have
been instances of male plants producing seeds, the vast majority
of the seed are produced by female plants. The fruit is a
three-winged calyx that contains one dark brown, triangular
seed, 2 to 4 mm long and 2 mm wide.
Japanese
knotweed is easily identified by the tall, reddish, hollow
stems, the ochrea on the stems at the base of the petioles, the
heart-shaped leaves and the aggressively spreading rhizomes that
result in large areas infested with only this species.
Habitat
Japanese
knotweed tolerates adverse conditions such as dense shade, high
temperatures, high salinity, drought, and floods. It grows in a
wide variety of soil types and pHs and can be found virtually
anywhere: near water sources, in low lying areas, waste plants,
utility and highway right-of-ways, abandoned home sites, and
near gardens. It is a special threat to riparian areas because
it can survive severe floods and can colonize the coastal shores
and islands (Remaly 1997).
The plant
continues to quickly move from fence rows, roadsides, and
railroads into crop fields. It is the type of plant that thrives
on moist, well-drained, nutrient rich soil but does not need
these conditions to survive (Fishel 1998). It is found mostly in
habitats influenced by man; it spreads much more in these areas
as opposed to undisturbed areas (Beerling et al. 1994.)
Established plants along roadsides can be broken by nature,
erosion, or road work and work themselves into drainage systems
and spread. It is easily spread by soil excavation sites and any
other soil disturbance that would spread the hearty rhizome
segments (Locandro 1978).
Growth and
Biology
Japanese
knotweed is a stout plant that grows quickly and aggressively.
Colonies can be as large as three acres (Locandro 1978). The
root system consists of thick rhizomes that send shoots far as
45 to 60 feet away from the parent plants. While Japanese
knotweed spreads rapidly from rhizomes, plants produce viable
seeds that can be spread by water, transported with fill dirt,
and be blown by the wind (Remaly 1997). Plants flower in late
August and into September and some colonies have a mixture of
male and female plants. However, as plants are dioecious, some
clumps of Japanese knotweed originated from a single rhizome and
have only one type of flower. In the United Kingdom, nearly all
Japanese knotweed infestations have only male flowers as this
was the type widely planted for ornamental purposes. On female
plants, the fruit remains on the flower stalk for only a short
while. While female clumps usually set seed, seedlings are
rarely, if ever, seen in existing infestations.
The plant
is very susceptible to spring and fall frosts (Uva et. al 1997).
Late spring frosts seriously damage emerged shoot growth but new
ones soon reappear. Plants can also be seriously damaged by high
winds when the leaves are newly formed. The rhizome mass sends
up numerous shoots from spring to late summer (April to August)
that easily emerge through heavy mulch and even on occasion
asphalt (Uva et al. 1997). The shoots may grow as much as two to
four inches a day in the spring (Jennings and Fawcett 1980).
Plants
quickly form dense colonies that few other species can survive
in. In the United Kingdom, plants produced 11 to 16 ton/acre of
above ground dry matter; rhizome weights ranged from 5 to 14
ton/acre. Thus this is one of the most productive terrestrial
plants in the country (Beerling et al. 1994). Plants respond
very favorably to fertilizer but growth is greatly reduced by
droughty conditions.
Rhizomes
are semi-woody and internodes are 3 to 4 inches apart. Even
rhizome segments with only 0.25 oz of fresh weight are capable
of forming new plants. Under very moist conditions even the stem
internode tissue can develop and initiate roots and shoots (Locandro
1978). While the aerial parts of plants die with the first fall
frost, rhizome buds over winter just below the soil surface and
form new shoots next season.
Uses
Japanese
knotweed is not seen as an undesirable weed by all. It is a
popular ornamental across the world. When used as an ornamental
it is often called fleece flower and is often given a glowing
recommendation by magazines and catalogs. People have no idea
that soon after it is planted in their flower gardens, the issue
becomes more of how to eradicate than propagate the plant.
In Japan
it is used extensively to hide garbage dumps, pigpens, waste
areas, etc. (Jennings and Fawcett 1980). In sandy seashore areas
Japanese knotweed is often used to stabilize soil as it can
easily withstand the spray and wind and low soil nutrients as
few plants can (Locandro 1978). The plant is edible; when the
newly emerged shoots are picked they can be prepared like
rhubarb. It is said to have a unique almond flavor. Bee keepers
have planted it for its abundant nectar secretion (Locandro
1978). The plant has some medicinal purposes and has been used
as a fodder crop, but rhizomes are reported to be toxic to some
species of livestock (Beerling et al. 1994). During World War
II, Japanese knotweed leaves were used as a substitute for
tobacco.
Management
Harvesting
or cutting the above ground growth of Japanese knotweed has
little effect on the rhizome biomass (Beerling et al. 1994).
Cutting may actually increase the lateral spread and stem
density of the weed. Mowing the entire infestation once a week
would be ideal. However, as this is often impractical, mowing
every two to three weeks helps reduce the biomass and approach
eradication. The entire area must be mowed in some cases for two
years for this approach to succeed (Uva et al. 1997). Burning
does not suppress Japanese knotweed infestations.
Mechanical
disturbance (hand hoeing, rototilling, disking) at 5- to 10-day
intervals for two seasons may achieve eradication but must be
done consistently. The technique of grubbing may be used when
the population of plants is small or in areas that herbicides
are prohibited. For this to be effective, a digging tool must be
used to remove the entire plant, including the rhizome and root
system. Careful monitoring for several months is essential be
sure that no root segment was missed that would allow the
infestation to regenerate. Based on these observations, it is
apparent that control is almost impossible without herbicide
use.
In areas
where herbicides can be used, a program with dicamba (Banvel or
Clarity) or glyphosate (Roundup or Touchdown) may be effective.
Triclopyr (Garlon) and clopyralid (Stinger and Transline) are
also reported to have activity on Japanese knotweed (Remaly
1997). For maximum effectiveness, add a surfactant to the spray
solution and treat plants when temperatures will be above 65 F.
One
application of an herbicide will not kill the entire infestation
if that is the only approach used. Repeated applications in a
growing season should be done. Even then weed regrowth in the
spring is likely (Beerling et al. 1994). The best approach is an
integrated program that uses more than one strategy. We
successfully eliminated an infestation in southern Wisconsin by
the following sequence of practices.
 Japanese
knotweed was allowed to grow unchecked until July.
It was then chopped
back to ground level. Plants regrew as expected.
When regrowth was 3 to
4 feet tall in early September, a 2% Roundup solution was
applied to the foliage to the point of runoff.
The site was hand
spaded 8 to 12 inches deep in early October.
Monitoring the site
the next year found no live Japanese knotweed plants.
This may
have been an unusually successful event but indicates the wisdom
of using a multiple strategy approach to suppressing Japanese
knotweed infestations when feasible. Planting a highly
competitive grass species and using dicamba as needed to control
escaping plants after following the above sequence of practices
is suggested.
In
summary, do not purchase nor plant Japanese knotweed as an
ornamental. If infestations are present adjacent to land you
own, pay particular attention to its rate of spread. If plants
begin encroaching on your land or into your fields, try and
eradicate the source of the infestation as described above.
References
Beerling, D.J., Bailey, J.P., Conolly, A.P. 1994. Biological Flora of the
British Isles. Fallopia japonica (Houtt.) Ronse Decraene. J.
Ecology 82:959-979.
Fishel, F.
1998. Japanese knotweed finding its way into Missouri crop
fields. Univ. Missouri Integrated Pest and Crop Managnt. Newsletter. Vol. 8, No. 10. June 12. p. 48.
Locandro,
R.R.1978. Weed Watch: Japanese Bamboo. Weeds Today. Fall. pp.
21-22.
Jennings, V.M. and R.S. Fawcett. 1980. Japanese
Polygonum. Iowa State
Univ. Coop. Exten. Ser. Pm. 762. 2 p.
MacKenzie,
D.S. and R. C. Smith. 1990. Polygonum cuspidatum var. compactum.
Amer. Nurseryman. July 15 issue.
Remaly, T.
1997. Tennessee Exotic Plant management Manual. Great Smoky
Mountains National Park. 1 p.
Uva, R.
H., J.C. Neal and J. M. DiTomaso. 1997. Weeds of the Northeast.
Cornell Univ. Press. Ithaca, NY.
* Student
Assistant, Dept of Agronomy, University of Wisconsin-Madison
October,
1998 |
