How to Effectively Treat Japanese Knotweed
Digging is not effective: Small fragments of knotweed roots can start new plants. Knotweed can spread roots several feet deep under ground and over 10 feet across - so digging up a plant is not effective - and can spread knotweed from root fragments. It is also possible for discarded canes to start new plants if the canes have not been thoroughly dried out.
Herbicides: Herbicides can be very effective with eradicating knotweed if performed optimally. We have had great success using a combined technique of injecting canes that are large enough to inject (about 3/4" diameter) with 2 ml of concentrated glyphosate (e.g. Accord or Rodeo, RoundUp Pro, and Aquamaster are approved for injection) using a JK Injection Tool with a short needle with marking system and pen, and spraying the leaves of plants too small to inject with 5% glyphosate (with surfactant).
Identify Knotweed: Make sure you are able to identify knotweed. Knotweed that has not been treated in previous years will have a bamboo like stem for medium to large plants, and typical broad leaves. Knotweed that has been treated in previous years often comes up very deformed, with numerous small leaves and without a bamboo like stem. Check out the slideshow which includes pictures of both untreated and deformed/previously treated knotweed.
Watch APIPPs video Keep Knotweed Out and also video Keep Phragmites Out .
Injection Method: Advantages of injection include being the most effective, not exposing surrounding plants to herbicide, and being able to be used even during windy or rainy conditions (when foliar spray should not be used). We have found a 2 ml injection very effective (some labels say use up to 5 ml, others say 5 ml) - though for giant canes use two squirts of 2 ml. Make sure the injection gun is calibrated to 2 ml. Apply a little silicone from the tube to the moving parts if needed for them to move smoothly and so that no air leaks in when the trigger is released. Fill the cannister with the concentrated pesticide. It works best if you leave the cannister attached to the gun when it needs refilling, and refill it prior to when it is totally empty to reduce air in the system. At the end of the day, the cannister can be emptied either back into the pesticide container or into the backpack sprayer, then add water to the cannister and spray into the backpack sprayer until empty. The needle does not have to pierce through both sides (the liquid can leak out if so), so a short needle is preferable. For Rodeo/Accord, not more than 3550 canes per acre should be injected with 2 ml each. The marking pen should be positioned above the needle (it will get clogged with herbicide if below). If the marking pen system is not working, manually mark each stem after it is injected. The injection can be done between any internodes, so select a site that is convenient to reach with a large enough cane (usually between 2nd and 4th internode). Inject at least a few inches above an internode so the herbicide won't leak out the injection hole. Each stem or cane of the knotweed needs to be treated. Stems too small to be injected need to have their leaves sprayed. Do not spray leaves of plants that have been injected.
When to treat: Knotweed should only be treated after the plants start to flower (about early August). Treating before then will likely not kill all the root system. After treatment the plants will start to look sick after several days. There is no need to cut the plants after treatment, but if desired for cosmetic reasons, the canes can be cut a few weeks after treatment (to allow enough time for herbicide transfer to the root system).
Ideal treatment includes injection of large canes and spraying leaves of plants too small to inject after they start to flower, coming back about 4 weeks later to treat any plants that were missed, then coming back the next year, and again the year after to treat any remaining plants. In our experience the first treatment should kill over 90% of the plants, the second year nearly all the rest, with a third, fourth and sometimes fifth year needed for large stands.
Spraying large plants leads to deformed plants coming up the next year, and normal plants returning over a few years. Spraying tall Giant Knotweed proved nearly totally ineffective, while injecting Giant Knotweed canes eradicated the plants.
Another possible technique is "cut-stem" in which plants are cut a few feet above ground and herbicide placed in the open stalk. Our experience is that this technique is not effective, with deformed plants coming up the next year. It also requires disposal of the cut canes.
Another possible technique is cutting plants in May, then spraying a few months later. They will not grow as tall and have more leaves, so spraying in August or September will be easier. However, this requires disposing of the cut canes, and we do not have enough experience with this to comment on its effectiveness. While not ideal, this is a reasonable method if an injection gun is not available.
New York regulations: In New York State, for non-farm property herbicides can only be applied on one's own property by the owner, or by a certified category 3A commercial pesticide applicator, technician, or apprentice. To become certified in New York, one must have training (which can be on-line) or education prior to being able to sit for a two part test - on general pesticide (closed book) and on category 3A (open book), which are given about monthly in various locations across the state. You need obtain the Core Manual and category 3A (ornamental and turf) manuals from Cornell University Cooperative Extension, as well as copies of state pesticide regulations. You need a NYS drivers license, or a NYS photo ID prior to taking the test. Once you've passed the test and paid a fee, you will become a technician getting a photo ID, and after a year's experience and taking a 6 hour on-line training be able to apply to become a certified category 3A applicator ($200 yearly fee). Technicians are not allowed to use restricted pesticide (e.g. Accord or Aquamaster), which is needed for applications within 100 feet of wetlands (e.g. marsh/bog). Pesticide application records must be submitted to the NYS DEC by Feb 1 the following year. Applicators must have a written copy of label information in their possession when they are making applications.
Unrestricted herbicides: Accord XRT II, RoundupPro (2.5 gallon containers) and Roundup Super Concentrate (1 gallon containers) are not restricted and are not approved for use within 100 feet of wetlands (e.g. marsh/bog), and can be purchased through ACE hardware stores or Arborchem. These have surfactant, so do not need added surfactant to use for foliar spray.
Restricted herbicides: Accord, Aquamaster, and Rodeo are restricted and approved for use within 100 feet of wetlands (e.g. marsh/bog), and can be purchased by certified applicators through Arborchem in 2.5 gallon containers. These do not have surfactant, so surfactant (e.g. Chemsurf 90 which can be purchased through Arborchem) needs to be added when they are diluted down to 5% glyphosate for foliar spray. The dilution is 1 to 4 pints of Chemsurf 90 per 100 gallons. Adding 1 tablespoon (15 ml) of Chemsurf 90 per gallon is equivalent to about 3 pints per 100 gallons.
For foliar spray of 5% glyphosate, a good back-pack sprayer is the SP2 SP Systems Sealed Pison Backpack Sprayer - 4 Gallon which can be obtained through Northern Tool or Arborchem. Since RoundupPro is 41% glyphosate, Accord XRT II and Roundup Super Concentrate 50.2% glyphosate, and Accord 53.8% glyphosate, different dilutions are needed to get to 5% glyphosate. Adding 16 ounces of RoundupPro, 13 ounces of Accord XRT II or Roundup Super Concentrate, or 12 ounces of Accord to one gallon of water yields 5% glyphosate. We have not found that blue dye is helpful to add to the spray.
Signs: Special New York pesticide application signs need be placed around treated areas and left in place for 24 hours.
Applicators: Some certified category 3A applicators the RIIPP has used include Dr. Douglas Johnson (volunteer), Ryan Burkum, Lenny Croote, Eric and D. Avery Menz, Kathy Vanselow, Zack Simek, George Spak, Jerry Charbonneau. The Adirondack Park Invasive Plant Program (APIPP) has done much eradication efforts, originally by Steve Flint, then Brendan Quirion, and now by Zack Simek as applicators.
Please make donations payable to: Hamilton County SWCD - RIIPP, and mail to: HCSWCD PO Box 166, 103 Country View Drive Lake Pleasant, NY 12108 with your name, phone, address, and email.
Contact 518-548-3991, firstname.lastname@example.org with questions.