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Controlling Japanese Knotweed

Persistence & correct timing are keys to control

By: Sandy Feather 2007
Penn State Extension


Q. I live near a utility right-of-way, and have weeds creeping in from the site. I have identified one of the worst weeds as Japanese knotweed, but have had no success getting rid of it. I've cut it down, pulled it out, and sprayed it with weed killer, but it keeps coming back. Do you have any suggestions?

A. Japanese knotweed (Polygonum cuspidatum) is also known as Mexican bamboo or Japanese bamboo ...and probably a few things unsuitable for publication here. Its jointed stems and aggressive spread certainly make it resemble a true spreading bamboo species. Japanese knotweed spreads primarily by fleshy underground stems known as rhizomes, although it also produces viable seeds.


Where Knotweed Grows

It does 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 knotweed.


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 control.


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.


Herbicides for Knotweed

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.
  

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Using an 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.


Trunk Treatments

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 as possible will keep the herbicide from damaging it.


Follow-Up

The following spring, check and see how effective the control has been. 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.

compost
Compost

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.

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