Japanese Knotweed Control Homeowner How To Guide 

Identification

The first step is understanding you have a Knotweed problem. Thankfully Knotweed is pretty easy to identify. One can cross reference the plethora of images available online with the vegetation in their landscape, or use a plant ID app like Picture This to confirm they have Japanese Knotweed.

Method of Control

Understand that the best method for controlling knotweed will involve the use of Glyphosate (Round Up). In our research and practical experience managing knotweed, Glyphosate is the least impactful, least persistent, and most effective option we have at successfully managing its growth. Our recommendation is a foliar (aerosol application to plant leaves) of Glyphosate to as much of the exposed leaf material as possible. Stem injections are an option but require more specialized tools and use much higher concentrations of herbicide per square foot than foliar applications.

Planning

Upon confirming ID you will want to plan for its management. Knotweed has a very specific control method that is season and growth dependent. It is imperative that one plan for the proper timing of each step ahead of time to allow for maximum control.

Phase 1

June Cutback. During the month of June, knotweed can be cut back to the ground in order to reset its growth. This is to be only done if you plan on treating it that same season. If you are not going to treat it the same season, it is best to leave it alone as cutting it down and not treating will only encourage its spread outwards.

The material that is cut down should be kept in the same place where it was cut. The reason for this is knotweed and knotweed stem fragments have the ability to reroot after cutting. Moving them away from the area of treatment could further spread the problem. We recommend keeping them in the same area incase anything does reroot it will easily be treatable during herbicide applications. Only after the stalks have turned brown and completely dry may they be removed offsite.

Phase 2

August-September herbicide treatment. The only time Knotweed will actively take herbicide down to its roots is during ‘The Window’ which falls during its Pre Flower, Flower, and Post flower growth cycle. Commonly in Massachusetts this is during the August to mid/late September time frame. We recommend performing the application early August if you want to avoid pollinators on the flower with the option of a mid to late September follow up treatment to control any lingering growth. Waiting too late in the season will also be ill advised as the plant needs at least 2-3 weeks before loosing its green color and before first frost in order to completely take up the herbicide application.

Glyphosate Application

Always read the label on whatever herbicide you are using. On the label it will outline the recommended PPE for applying the herbicide. We always recommend long sleeve shirt, pants, closed shoes, nitrile/rubber gloves, safety glasses, and N95 mask suitable for liquid aerosol as a minimum guideline. Always, always, always READ THE LABEL, the label is the LAW for the herbicide’s use.

In order to apply an effective foliar Glyphosate application, you will need to make your own tank mix as all the Glyphosate formulations available ready to use are not strong enough. You will want to purchase a formulation of Glyphosate that is 41%-54% which then can be diluted in water to the proper strength. Here is a common off brand Glyphosate formulation of 41% available through Amazon and is appropriate for NON wetland/aquatic application. These come in sizes ranging from 16oz to 1 gallon. The benefit of this Glyphosate formulation is that is does not need the addition of a surfactant for the application. We recommend not getting too much and purchasing what you need for this each season’s work. Working around wetlands will require an aquatic formulation of glyphosate with also an aquatic surfactant to improve the herbicide’s stickiness to the plant. All invasive work in Massachusetts, especially using herbicide that occurs in or near wetlands in falls under your city/town’s Conservation department and will likely require permitting.

You will want a glyphosate formulation of 2% Active ingredient applied to the leaves of Knotweed. In the case of 41% glyphosate, you would add 7oz of Glyphosate concentrate to 1 gallon of water to reduce the Active ingredient % down from 41% to 2%. The addition of blue marking dye is a great tool to better get a sense of your coverage especially when you are returning to a site that you have just sprayed but the herbicide application has dried enough to not be noticeable. For larger areas this is very helpful to prevent over application or missing untreated areas.

Expect approximately 1 gallon of formulation to cover 300-600 sq/ft. Measuring out the size of the patch will allow you to plan for the size of sprayer needed as well as amount of Glyphosate to purchase. We recommend making one gallon of formulation and applying it to the patch and take a rough measurement after that tank is finished to get a sense of the coverage area 1 gallon provides for you and make the remaining needed tank mixes accordingly.

A good sprayer will be needed to apply the Glyphosate to the leaves of the Knotweed. Ideally all visible leaf material will be covered in your herbicide formulation. Backpack sprayers have the benefit of having one hand be able to pump the sprayer while the other hand is using the application wand to apply the chemical. For small patches where hand pumping will not be as much of a concern, a 1-2 gallon carry sprayer should be appropriate.

Additional Resources

One of the best resources we have found online is the Facebook Group World Wide Japanese Knotweed Support Group. Here there are plenty of discussions and information on controlling knotweed at your home.