WVU Update
Published as a Special Insert for Farm Bureau News
Spring 2015
Identification and control of invasive knotweeds
I
ntroduced from Asia in the late 1800s, invasive knotweeds are
prevalent throughout West Virginia, especially along stream and
river banks.
Based on their ability to spread
and regenerate through extensive
underground rhizomes and to displace
native plants, they are considered to
be highly invasive and obnoxious.
Identifying knotweed
Invasive knotweeds are recognized
by their hollow stems, tall-growing
habit, prominent
leaves, and
vibrant autumn
displays of white
flower clusters.
There are three
knotweeds:
1) giant knotweed Japanese
with large, heart- knotweed can
shaped leaves
be identified by
the size of printer the shape of its
paper; 2) Japanese leaves.
knotweed with
spade-shaped leaves half the size
of printer paper; and 3) Bohemian
knotweed with characteristics that
fall in between.
Similar approaches may be followed
to manage all species of invasive
knotweeds. Age and extent of the
infestation, sensitivity of the area to
a control method, and availability of
equipment and professional services
should be taken into consideration
prior to managing knotweeds.
Mechanical control
Mechanical knotweed control
methods are time-consuming and
labor-intensive. Frequent removal of
top-growth by repeated cutting may
eventually kill a knotweed colony.
Digging, grubbing, hoeing, and
cultivating are not recommended
unless all the underground parts are
completely removed and burned.
Chemical control
Chemicals provide a cost-effective
method of knotweed control if applied
carefully and consistently. Established
stands may require multiple herbicide
applications over several years. For
best results, cut back knotweeds
after complete emergence in spring,
allow them to regrow, then apply
an herbicide in late summer or early
fall. Do not apply the herbicide
under drought-like soil conditions.
Effective herbicides for knotweeds
are 1) glyphosate (Roundup® with
various formulations for terrestrial
use or Rodeo®/Shore-Klear ® when
spraying near water) and 2) imazapyr
(Arsenal for terrestrial use or Habitat
for aquatic use).
If using 4 lb/gal (41%) glyphosate
formulation, a 4% spray solution
(5.1 oz product/gal water) may be
used. If using a 5.5 lb/gal (49%)
formulation of glyphosate, a 3% spray
solution (3.8 oz product/gal water)
may be used. Add a surfactant
(sticker) such as methylated seed
oil at the rate of 1.25 oz/gal water,
especially if a generic formulation
is used. Spray to wet most (>80%)
of the foliage without creating
dripping droplets.
Due to its persistent nature and
potential to cause injury to nearby
vegetation, imazapyr is best applied
by a professional. Foliar applications
require a 2% solution (2.6 oz product/
gal. water) of Arsenal or Habitat, with
surfactant at 1.5 oz/gal of water for
Arsenal, and 2.5 oz/gal of water for
Habitat.
– continued on page iii –
Insert Provided by WVU Extension Service and Davis College of Ag., Natural Resources, and Design
Spring 2015
West Virginia Farm Bureau News 13