Control Methods

Effective Methods
Controlling knotweed takes persistence and diligence. Since knotweed plants are spread easily downstream, begin control at the furthest upstream infestation when treating along waterways. Digging, cutting, or covering plants can actually make knotweed grow stronger, so manual or mechanical control methods are not advised. Current research indicates that plants that have been disturbed by activities such as repeated mowing, bull dozing, or other forms of aggravation tend to be harder to control. It is believed that this could be a possible defensive strategy caused by the plant’s root system.

A man spraying along the beach for weeds
Our field technicians use an adaptive approach that utilizes a combination of foliar spray of imazapyr and/or glyphosate, stem injection of glyphosate, and stem bending. The exact method used is dependent upon the site, characteristics of the infestation, and according to Integrated Pest Management (IPM) principles. Treatment occurs annually from July to September/early October. Foliar application of 1% imazapyr or 4% glyphosate is applied using backpack sprayers, while 3 ml of glyphosate is used for stem injection.


“An important step in the control process is to 1st inventory knotweed plants that are presently growing on your property. Knowing the extent of infestation enables you to develop and implement an appropriate control strategy suitable for your property.”
Dead weeds in a field after being treated with chemicals
Recommended Methods
Chemical Control
Be advised that if knotweed is near water, a permit and special pesticide applicators license may be required. Native plants, fish, and other aquatic life may be harmed if herbicides are used improperly. Please check with your local noxious weed program or the Washington State Department of Agriculture about the proper use of herbicide.
Apply a systemic herbicide to actively growing plants July - September when plants have reached their full height. Due to the extensive root system knotweed plants can develop over time, it is important that only systemic herbicides be used to control knotweed. Systemic herbicides are herbicides that are able to translocate throughout the entire plant, delivering chemical from the leaves down to the root system. Contact herbicides are not recommended since they only kill the above ground part of the plant (leaves and stems.
2 chemicals that are effective if applied properly are imazapyr (e.g. Polaris, Habitat) and glyphosate (e.g. Roundup, Aquamaster). Rate, timing, and careful application are keys to effective and safe control, so please read and follow all product label directions carefully. The best time to spray is late August when plants are in flower, or before leaf drop occurs (September - October). It may take several weeks for chemical applications to show results but this is okay since systemic chemicals are meant to work their way slowly throughout the plant to ensure an effective kill of the entire plant. Being said, it is important to avoid high percent mixes as applying too much chemical often results in only the “top kill” of the plant. A 1% mix of imazapyr has proven to be very effective, with estimated results of 90% control of plants when properly applied.

Note: Regardless of the size of infestation, expect new shoots for up to several years. Please contact our office at 253-845-9770 if you have any questions regarding spray applications.
Stem Injection
Another effective chemical control strategy is injecting a measured amount of herbicide directly into the hollow stem of a knotweed cane. This method can be very effective and reduces damage to possible off-target vegetation that can occur when spraying chemicals. This method is however time consuming, uses more chemical, and can only be done to plants with a stem diameter of at least a 1/2 inch. Due to the higher amount of chemical used, there is also a limit on how many canes can be treated per acre so be sure to read and follow directions listed on the chemical lab.
Individual injection of weed stems
Currently, glyphosate is the only chemical that may be used for this treatment method. Labeled products include Aquaneat, Aquamaster and Roundup Pro Concentrate. Although you can also expect about 90% control with this method, some re-growth will more than likely occur the following year that will be too small to inject. Such re-growth will need to be controlled by spraying.

Please contact our office at 253-845-9770 if you have any further questions regarding stem injection.