Where Knotweed Grows
very well in moist, shaded areas such as stream banks, but will also
readily take over sunnier, drier sites, including abandoned gardens,
roadsides and other disturbed areas. The large store of carbohydrate
reserves in those rhizomes allows them to recover from pulling,
mowing, and even herbicide applications. For example, when you try
to pull out the bamboo-like stalks, you inevitably leave a piece of
rhizome behind. Any little piece of rhizome will sprout a new plant.
Don't Till It!
Whatever you do, resist the temptation to till an area where
Japanese knotweed is growing. Tilling
does more to spread Japanese knotweed than control it. I have seen
Japanese knotweed tilled under during construction projects, only to
have its stalks push right up through the blacktop! Although repeated
mowing should theoretically exhaust the carbohydrate reserves in the
rhizomes, the truth is you may be exhausted long before the Japanese
The 1 - 2 Punch
A combination of herbicide applications, cutting, and establishing
another crop in the area to compete with the Japanese knotweed works
best for long-term control. Turfgrass is a good option, because the
herbicides used to control broadleaf weeds in turf can also keep
Japanese knotweed in check once you get the worst of it under
Timing of Herbicide Apps
Since you did not identify which weed killer you used, it is hard to
say if it was one that effectively controls Japanese knotweed.
Timing may be another reason that your herbicide application was not
effective. Most herbicides work best on actively growing weeds that
are not under drought stress. Late summer and early fall herbicide
applications provide better control of perennial weeds such as
Japanese knotweed. This is also true for tough lawn weeds such as
ground ivy and violets, as well as woody weeds such as
tree-of-heaven (Ailanthus altissima) and poison ivy (Rhus toxicarium).
This is because in spring, the movement of those stored
carbohydrates is up and out to push new growth. Even systemic
herbicides will go with the flow rather than being absorbed and
translocated down to the roots and storage organs such as rhizomes.
Spring applications often burn the leaves and new growth back, but
Japanese knotweed just shrugs it off and starts growing again. In fall,
carbohydrates are moving back to the rhizomes for storage through
the winter. At that time of year, weeds readily absorb systemic
herbicides and take them down to the rhizomes very effectively.
Penn State's Roadside Vegetation Management Specialist Art Gover has
worked with PennDOT over the years to establish best management
practices for problem weeds such as Japanese knotweed. For Japanese
knotweed, control starts by cutting the growing plants halfway down
to the ground in mid-June.
Allow them to regrow until late August or early September, and then
make your herbicide application. By cutting the weeds back, you
force them to expend more of their carbohydrate reserves to regrow,
resulting in weaker weeds. Allowing them to regrow provides
sufficient leaf surface to absorb the herbicide when you spray.
Roundup and other brand names of glyphosate herbicides are labeled
control Japanese knotweed, but cannot be used near water because the
surfactant(s) they contain can be toxic to aquatic life. If you have
Japanese knotweed near water, use a product like Rodeo. It uses
glyphosate as the active ingredient, but lacks the problematic
surfactant. Rodeo will provide more effective control if you can add
a non-ionic surfactant that is labeled for use near water.
Surfactants help spread the herbicide over leaf surface evenly to
insure good coverage.
Types of Sprayers
It is more cost effective to purchase a pump-up sprayer and mix your
spray from concentrate than to purchase ready-to-use herbicide
formulations because you will need to make repeated applications. Be
sure to use a plastic pump-up sprayer; glyphosate reacts with
aluminum and creates an explosion hazard. Never use that spray tank
to apply insecticides or fungicides to plants you value because it
may contain enough herbicide residue to harm those plants. Reserve
it for herbicide applications only.
indelible marker, write "Weeds Only" on your pump-up
sprayer to avoid its accidental use for other purposes.
Mix the glyphosate formulation at the strongest rate allowed by the
label, but no stronger. You can encourage the knotweed to absorb the
herbicide even more effectively by adding a water-soluble fertilizer
such as Peter's 20-20-20 at one tablespoon per gallon when you are
mixing the herbicide solution. Spray the Japanese knotweed
thoroughly, but not to runoff. Do not worry about spraying the
undersides of the leaves. Glyphosate does not kill instantly. It
will take ten to fourteen days to see the effects of the spray.
Some practitioners prefer to cut Japanese knotweed back, and then
immediately treat the cut stump with glyphosate. Some even inject
the herbicide into the cut stem with a syringe. By directing the
glyphosate precisely at the target weed, you avoid broadcasting
glyphosate over a larger area, and perhaps damaging desirable
plants. Glyphosate is a non-selective herbicide that will damage any
green plant material it contacts. If you accidentally spray some on
a branch or small area of a desirable plant, pruning it off as soon
will keep the herbicide from damaging it.
The following spring, check and see how effective the control has
You will see some sprouts of Japanese knotweed coming up. You can
spot treat the individual sprouts with the glyphosate-fertilizer
combination, or cut them back by hand. By keeping after it and spot
treating or cutting the sprouts whenever you see them, you will
begin to exhaust the carbohydrate reserves in the rhizomes. In late
summer or early fall, lightly rake up the area to create a seedbed
to plant turfgrass, but do not till the area up deeply.
Rake in some
compost, mushroom manure or other aged organic matter to create good
growing conditions for the grass seed. If the area is sunny, a
combination of perennial ryegrass and Kentucky bluegrass will do
well. If it is shadier and soil drains well, use a shade mix with a
higher percentage of fine and/or creeping fescues. Remove any
knotweed shoots that appear by hand while the grass is growing in,
because herbicides can damage newly-planted grass.
The following spring, you can use an ordinary broadleaf weed
herbicide that is labeled for use on lawns to spot treat any
Japanese knotweed sprouts that come up. You can no longer use
glyphosate because it will kill the turfgrass along with the
knotweed. Repeat whenever you see Japanese knotweed shoots coming up
so that it does not gain the upper hand again. Mowing the area will
also help keep it in check.
Identifying garlic and wild onions
Learning about weeds
Biofungicide for tomato late blight