Japanese Knotweed Treatment Options

Overview

Knotweed can be controlled by both chemical and mechanical methods. In both cases, any effort on a particular plot needs to be continued for a minimum of five years because shoots can appear up to 60 feet away from the dormant root systems. 


The root systems often extend ten feet deep underground. Knotweed spreads mainly from small and large fragments of stems and roots (rhizomes); the seeds are usually sterile. This means that all cuttings of knotweed must be incinerated no matter which general method is undertaken. 


Knotweed must be discarded with general incinerated trash, never added to the compost pile. Large canes do not need to be bagged, but it is a good idea to bag small pieces to keep them from spreading. 

How Can I Get Rid of It?

Wetlands

If you are within 100 feet of a wetland (pond, ocean, stream, etc.), we can usually add you to an existing permit or help you to apply for the required permit. We then help you with every phase of the work. Please call Laura or Nan if you are near a wetland. Using herbicide in a wetland buffer zone without an approved plan is harmful to the area and is against the law. The Rockport Conservation Commission is very supportive of our efforts. 

Herbicides

Research shows that judicious application of glyphosate herbicide according to the protocols below can eradicate large fractions of knotweed infestations each year. As knotweed weakens, it may change form dramatically into a twisted, gnarly version of its former self! As the knotweed changes form, your chosen approach will need to evolve.

 

The herbicide glyphosate is applied in late August or early September, as near to flowering as possible when knotweed is most vulnerable and the herbicide is pulled down into the roots. It is much less effective if applied earlier in the year.  

Herbicide application

Mechanical Methods Alone

Mechanical methods without herbicide are most effective with small infestations of 50 stems or less where the infestation can be followed carefully. Mechanical control consists of cutting knotweed to approximately six inches high at least four times per growing season from June to October. All cuttings and fragments must be removed from the site and destroyed. 


However, the literature and our own research have shown that repeatedly cut knotweed responds by producing more small stems over a much wider area, so cutting alone should not be undertaken unless the area can be followed for many years. Cutting without removing all fragments will spread the infestation since each piece can start a new plant. 


The root clusters of knotweed at the surface may be dug up, bagged and destroyed to further discourage the plant. This is combined with the planting of native grasses and other species approved to prevent erosion and inhibit the growth of knotweed.