TOP
TIPS FOR CONTROLLING JAPANESE
AND GIANT KNOTWEED IN
YOUR YARD
KNOW HOW IT SPREADS Japanese and giant knotweed both spread primarily vegetatively by the growth and fragmentation of rhizomes. Once established, knotweed spreads aggressively through an extension of the rhizomes. Lateral expansion rates of 6 to 8 feet per year are not uncommon. Very small fragments of rhizome (as small as 1-2 inches) and fresh stem material are able to produce viable shoots and roots. Stems in water or on damp soil may produce viable plants within 6 days. Knotweed also grows from seeds, which are produced in large numbers (when male and female plants are present) and are dispersed by wind and water.
DON'T DIG! Do not try to dig up the rhizomes. It is nearly impossible to be sure you've gotten all fragments (see # 1 above). Rhizomes can also be very large ~ as much as 60 feet in length! The digging required to completely remove all fragments of rhizome would cause such disturbance and potential erosion that it would only encourage sprouting of remaining fragments or growth of new invaders.
REPLANT WITH NATIVE OR NON-INVASIVE PLANTS It is important to rapidly establish native or non-invasive plants to control knotweed and stabilize the soil. Knotweed seedlings do not compete well with other vegetation. Consult your neighbors, local nurseries, garden clubs, or the internet for suggested plantings. Always check first for the potential invasiveness of a plant to be sure you don't give yourself more problems!
CUT
REPEATEDLY Repeated
cutting of the stems reduces
vigor and can eventually reduce
the root reserves in some
cases, particularly with small,
isolated populations. Cutting
is effective at any time during
the growing season, but only
when done repeatedly as it
will result in new shoot emergence.
Cutting greatly reduces the
reserves in below-ground rhizomes.
At least three cuts are needed
in one growing season to offset
rhizome production. Repeated
cuttings must be continued
until knotweed stops resprouting
and must be checked regularly
for any regrowth. Cutting
is most effective when used
in conjunction with either
shading or herbicide (see
below).
It
is very important to bag
and properly dispose of all
cut stem fragments (see
# 1 above)!! If cut stems
cannot be burnt, recommendations
include carefully sealing
them in plastic bags or spreading
and drying them in the sun,
away from water or exposed
soil. (Remember even moist
stalks in the center of a
pile can resprout.)
COVER & SHADE
IT Some
success has been noted with
cutting combined with shading.
After cutting near the soil
surface, stands can be covered
with sheets of plastic.
There is some early evidence
that clear plastic has greater
results due to the sun baking
the rhizomes. Be very careful
to secure plastic, particularly
near streams and rivers,
so that it cannot be washed
or blown away. Please consult
handout from Delaware River
Foundation for more information
on their use of plastic
sheeting.
DECIDE
ON HERBICIDE? Controversial?
Yes! Effective? Yes! We do
not currently recommend the
use of herbicide along the
banks of the Upper Delaware
River due to potential risks
to native fish or rare and
endangered freshwater mussel
populations. If you decide
to use herbicide you must
read the labels carefully
and take great caution when
working near water (for example,
painting or injecting cut
stalks instead of spraying
would be more accurate). Currently
the most effective means for
controlling large stands of
knotweed is a combination
of cutting and spraying with
an herbicide containing glyphosate.
There are several commercially
available products including
Roundup TM (for dry, upland
sites), Rodeo TM (for in or
near wet areas), and other
competing products. Glyphosate
is a non-selective, soil binding
herbicide that will kill all
vegetation, so it is important
not to let spray contact any
native or desirable vegetation.
Follow all instructions on
product label or consult a
licensed herbicide applicator.
Your local Cooperative Extension
Office can provide more information
on the use of herbicides and
we strongly suggest you learn
as much as possible. Cut stalks
in early June to draw reserves
out of rhizome. In August
or when stalks reach approximately
4 ft. tall, spray to coat
leaves but not so heavily
that herbicide drips off the
leaves. The timing of
cutting and herbicide application
is very important. You will
need to regularly check for
any regrowth or reinfestation
(from new fragments or seeds)
and spot treat.
AVOID
EXPERIMENTING WITH HOUSEHOLD
SOLUTIONS Always
take caution when using any
harmful substance -- not only
herbicides can damage soil
and kill native vegetation
and wildlife. Seemingly harmless
applications of vinegar, bleach,
or other household products
can be even more damaging
to the environment than herbicide.
Please research and learn
as much as possible before
trying any new treatments.
SUPPORT
RESEARCH FOR NATURAL CONTROLS There
are currently studies being
conducted on biological controls
(fungal agents or herbivorous
insects that control the plant
in its native habitat). Biological
controls are generally studied
for several years before release
to predict what, if any, other
plant species might be at
risk from the control agent.
Even after approval and release,
biocontrols generally take
5 or more years to show a
noticeable effect and will
not eradicate the weed, simply
reduce it to more manageable
levels. Although biocontrol
has been practiced throughout
the world for many years,
it is a relatively new concept
to many people. Read more
about this fascinating topic
at http://www.cabi.org/
BIOSCIENCE/japanese_knotweed_alliance.htm ,
and follow their links.
KEEP
LEARNING & TEACHING It's
important to continue to search
for and share information
regarding the control of knotweed.
New studies are being published
all the time, and much like
human health, the more we
learn, the more treatment
options we have. It is often
combinations of recommended
control methods that work
best. Your property has its
own, unique qualities that
either enhance or detract
from the growth of knotweed.
Experiment and learn what
works best for you, then share
with your neighbors and educate
the uninformed about this
troublesome plant and how
it is harming the environment.
There are many fact sheets
and websites where individuals
can learn more.
AVOID
PLANTING IT Knotweed
is still sold and exchanged
for ornamental use. If you
live in an area where knotweed
is a problem, avoid introducing
any varieties that may cross
with the invasive variety
to create new troublesome
hybrids. Know knotweed's many
names: Japanese Knotweed ( Polygonum
cuspidatum, Fallopia japonica
or Reynoutria japonica ),
Giant knotweed ( Polygonum
sachalinensis ),
Japanese or Mexican bamboo,
fleece flower, fallopia, lace
plant, and Sally/donkey/gypsy/wild
rhubarb.
This
tip sheet was prepared by
DRIPP (the Delaware River
Invasive Plant Partnership)
for a workshop in the Upper
Delaware River region of NY
and PA. It is intended to
be accompanied by other detailed
handouts. Please contact Joanne
Steinhart at 570-643-7922,
ext 12 or jsteinhart@tnc.org
for more info.
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