Japanese & Himalayan Knotweed Case Study

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This picture shows before and after pictures of the infestation of a mix of invasive plant species.

There is a mix of both Himalayan Knotweed and Japanese Knotweed that runs along the edge of a tree line. The bulk of the infestation is of Himalayan that has longer leaves. The Japanese Knotweed can be seen on the right hand side of the infestation and has a lighter heart or shield-shaped leaves.

This picture shows before and after pictures of the infestation of the invasive plant species known as Himalayan Knotweed. 

This picture shows before and after pictures of the infestation of the invasive plant species known as Himalayan Knotweed. 

These pictures are from a site in Rhayader that had multiple sites of infestations of a mix of both Himalayan Knotweed and Japanese Knotweed. The bulk of the infestation is of the Himalayan Knotweed variety and there were small patches Japanese Knotweed.

I was originally contacted to supply general weed control services around the building and kerb edges. I became quickly that there was evident a much bigger problem. The site owners were completely unaware that there was a knotweed problem.

It transpired that there were two very large infestations and four smaller infestation sites. It was then  agreed that a treatment plan would be put into place to prevent a further spread of the problem and to eradicate the infestation.

From the first treatment that was applied in the in late July there was a marked affect in the plants. However the infestation was that dense it became apparent that there were some patches that needed a second treatment. A second application of herbicide was applied in August to all areas of the infestation. 

Within a short period time the whole site was looking very dead. It was therefore left until the winter time when the plant would naturally die off before cutting down all the dead stems. This was so that the infested area would be completely bare and the following years spring regeneration would be very evident when the fresh shoots start to appear.

Year 2 of Treatment Plan and Inspections:

The following Spring a number of site inspections were carried out. From the thousands of stems of the original infestation, only a handful of fresh regeneration stems had appeared. I would estimate that this was approximately a 98% success rate. This was much better than expected, as I would have expected only an estimated success rate of about 70% on an infestation of that size and density.

Of the regeneration in the second year of the treatment plan. There were only three Japanese Knotweed stems and less than ten Himalayan stems scattered about the various original sites. From the three Japanese Knotweed Stems, one of them was in a flesh location.

A milder does of herbicide was sprayed on the fresh regrowth and the areas where the infestation were to ensure that all vegetation was killed off. This was to keep the areas completely clear. The fresh stems died within a couple of months. The site continued to be monitored incase any further stems appeared during the second year. Luckily no further stems have appeared. 

I have had an early inspection of the site an at this stage there is no sight of any fresh stems. I will in the next month carry out the first actual monitoring inspection of a series through the third years treatment plan.

The site will continue to be monitored for the next few years incase any of the roots have laid dormant and suddenly start to reappear. Knotweed does have a habit of surprising you. But I am hopeful that the site will soon be deemed clear. 

I will keep this page update with the site progress. 

Year 3 of Treatment Plan and Inspections:

There have been 3x site inspections during year three (Mid Spring, Summer, and Autumn) and there have not been a single Himalayan or Japanese Knotweed Stem seen on site. So there has been no requirement to apply any treatments.

Year 4 of Treatment Plan and Inspections:

There will be 3x site inspections planed.