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Pest Management -- Invasive Plant Control

Japanese Knotweed -- VT Class B Noxious Weed

***Conservation Practice Job Sheet VT-595***

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**Japanese Knotweed (*Polygonum cuspidatum)* Japanese Knotweed, leaves**

**Japanese Knotweed**

Japanese knotweed, also known commonly as 'bamboo', is a native of Japan
that was brought to the US from Britain in the late 1800's as an
ornamental. It quickly naturalized and spread throughout the Northeast.
It is found throughout the US and in all of New England.

Japanese knotweed is a shrub-like, herbaceous perennial (but dies back
to ground each fall) that can grow to ten feet in height and form dense
thickets that exclude native vegetation and greatly alter natural
ecosystems.  It poses a significant threat to riparian areas, where it
can survive severe floods and is able to rapidly colonize scoured shores
and islands.  Once established, populations are extremely persistent and
difficult to control.

Japanese knotweed is most commonly found in areas with full sunlight and
where the soil has been disturbed. It is often seen along stream banks
(erosion and deposition areas), roadways and waste places. Knotweed
reproduces sexually as well as vegetatively through an extensive network
of rhizomes (roots that can sprout new stems) that may spread up to 65
feet from the parent plant.

Knotweed may colonize new areas through wind dispersed seed as well as
through transported root and stem fragments as small as ½ inch. Knotweed
is often transported to new sites in floodwaters and as a contaminant in
fill (along roads).

Japanese knotweed's stout, hollow, bamboo-like stems and the large (3 to
6 inches long), broadly ovate, alternate leaves are distinctive. Tiny
white or greenish-white flowers develop in August and September and grow
in numerous linear clusters that form a mass of white over the plant
when in full flower (see picture above). The plant is insect pollinated.
Frost-killed stems turn bronze colored and may remain upright through
winter.

**Similar Natives**

There are no similar natives in Vermont.

**Control**

Japanese knotweed control should take a watershed perspective, from the
headwaters downstream, as the primary dispersal mechanism is by water
(both seeds and plant fragments). Control of knotweed must be well
thought out due to the extensive root system and sprouting ability as
well as the site limitations. Knotweed frequently infests riparian areas
near streams which add complexity to any control plan.

Manual, mechanical and chemical methods are all useful to varying
degrees in controlling knotweed. Removing or killing plants will provide
increased light at the site which may lead to a surge of sprouts in the
following year. Prepare to monitor and control these outbreaks for
multiple years.

As with all invasive species, Japanese knotweed is most effectively
controlled by recognizing their appearance early and removing isolated
plants before they begin spread.

**Biological Control**

Biological controls of knotweed are being studied.

**Mechanical or Manual Control**

Mechanical controls include grubbing or pulling seedlings, rhizomes,
mature plants, and repeated clipping. Manual control takes special
precautions because any live plant part (1/2 inch or larger) may sprout.
Plant parts must be disposed of properly and must not be allowed to
enter waterways. Stems and roots must be contained or dried with little
or no soil contact or they may sprout. Do not compost plant parts.

Digging or pulling (uprooting) will eliminate a portion of the root
system but not all. Pull the root crown and as much root as possible.
Each time new sprouts are seen (look after a week and at least 20 feet
from the plant) uproot them and get as much root as possible. This
method is only feasible on very small patches.

Because disturbed, open soil can support rapid invasion by other
invasives, managers must monitor their efforts at least once per year
and repeat control measures as needed. Limit soil disturbance whenever
possible.

Hand cutting, mowing or other methods must be applied every 2-3 weeks
from April through August. Cutting should continue after August but less
frequently. Prevent plants from exceeding 6 inches in height. Again,
dispose of plant parts properly or they will sprout.

Covering with heavy plastics and geotextile fabrics has had some success
but is costly and requires pulling, cutting or herbicide treatment near
the edges.

Cut stems to the ground surface and then cover the stand being sure to
extend coverage at least 10 feet beyond the farthest stems. Weigh down
the edges and monitor for sprouts. Leave cover in place for at least two
growing seasons.

Any type of manual control will require a strong commitment from the
manager because of the aggressive growth characteristics of knotweed.
The treatments will need to continue, as described above, for at least
three years but probably much longer depending on the size of the patch.
Manual control of knotweed is best suited to ecologically sensitive
areas and isolated small patches where there is a commitment to avoid
herbicide use.

**Prescribed Burning**

There is little information about the efficacy of burns.

**Chemical Control**

Herbicide applications to knotweed must be carefully planned and
implemented as the stands are typically near surface waters. Methods
include spraying, wicking, injecting, and pouring. Integrating control
techniques, such as cutting in the spring and applying herbicide in late
summer, may be a good alternative for the site.

Glyphosate (brand names Roundup, and for use near waterbodies,
Rodeo**^1^**) is a nonselective herbicide which kills both grasses and
broad-leaved plants while triclopyr**^3^** (brand names Garlon**^1^**,
Pathfinder**^1^**, and others) is a selective herbicide that kills
broad-leaved plants but does little or no harm to grasses.

**Foliar Treatment:** It may be necessary to precede foliar applications
with stem treatments to reduce the risk of damaging non-target species. 
Although it is generally best to apply foliar spray in late summer when
the plant is translocating nutrients, this is not usually practical for
knotweed which may be 10 feet tall late in the season. From a practical
standpoint, the best time to foliar spray is when the plants are 3-6
feet tall**^4^**. These stands will require follow up treatments later
in the growing season. Apply a 2% solution of glyphosate or
triclopyr**^3^** and water to thoroughly wet all foliage**^2^**.  Do not
apply so heavily that herbicide will drip off leaves**^2^**. A 0.5%
non-ionic surfactant is recommended in order to penetrate the leaf
cuticle, and ambient air temperature should be above 65 ºF.**^2^**
Foliar applications appear to be a reasonably efficient approach (1 to 4
treatments over two seasons) to obtain control over small and medium
size knotweed patches. Larger patches will often require treatment over
several years and combinations of manual and chemical control
methods**^4^**. Do not cut down treated plants for at least a full
growing season.

**Cut stem treatment: ** Use this method in areas where plants are
established within or around non-target plants or where vines have grown
into the canopy.   Cut the live stem about 2 inches above ground level
(between the lowest nodes).  Immediately apply a 25% solution of
glyphosate (e.g., Roundup, or use Rodeo**^1^** if applying in or near
wetland areas) or triclopyr**^3^** (e.g., Garlon) and water to the
cross-section of the stem.**^2^**  A subsequent foliar application of
glyphosate may be require to control new seedlings and re-sprouts. 

**See Best Management Practices from Soll (2004) for additional
information and specifics on injection and cut and pour techniques.**

**^1^** -- Class A Restricted Use Herbicides

**^2^** -- From Alien Plant Invaders of Natural Areas Fact Sheets (NPS)

**^3^** -- Greater than 2% Triclopyr herbicide is Class A - restricted
use in Vermont.

^4^ -- From Controlling knotweed in the PNW, J. Soll 2004.

**Important Note**

Mention of specific pesticide products in this document does not
constitute an endorsement. These products are mentioned specifically in
control literature used to create this document.

**Disposal**

Stem and root fragments as small as ½ inch can sprout so special care
must be taken to contain the plant parts when using manual control. Do
not allow plant parts to enter waterways during control. Limit soil
contact when drying the plant parts. Small plants may be hung in trees
to prevent re-rooting. Cut stems may be piled on a raised platform,
brush pile or tarp for drying. Do not compost plant materials as they
may sprout and then spread. Piles may be burned. Do not remove soil or
plant material from the site unless being disposed of in a landfill.

**Information and Recommendations compiled from:**

-   The Nature Conservancy - Fact Sheets (and references therein)

-   Invasive Plant Atlas of New England (IPANE)

-   CT NRCS Invasive Species ID Sheets

-   Literature Review for USFWS (Draft) -- SLCVP TNC 2006

-   Vermont Invasive Exotic Plant Fact Sheets

-   Alien Plant Invaders of Natural Areas (NPS)

-   Controlling Knotweed in the Pacific Northwest. TNC of Oregon. J.
    Soll, 2004.

> <http://tncweeds.ucdavis.edu/moredocs/polspp01.pdf>