The short answer
Yes — glyphosate is the standard herbicide for Japanese knotweed, but only as a repeated, professionally applied programme over several seasons. It is a systemic weedkiller: applied to the leaves or injected into the stem, it is drawn down into the rhizome. It works best applied in late summer or early autumn, when the plant is moving energy underground before dormancy. One application will not work, and near water it must be applied by a qualified operator under Environment Agency rules.
Glyphosate is effective against knotweed because it is translocated — the plant carries it from the point of contact down into the root system. The skill lies in the timing, the method and the patience: the rhizome is deep and resilient, so a single dose simply causes surface dieback while the plant regrows the following year. Here is how the two main application methods work and when each is used.
Glyphosate treatment at a glance
- Type Systemic, translocated herbicide
- Methods Foliar spray or stem injection
- Best timing Late summer / early autumn
- Duration Around 3 seasons + monitoring
- Near water Operator must hold EA permission
How glyphosate works on knotweed
Glyphosate is absorbed by green tissue or injected directly into the hollow stem, then moved with the plant’s own sugars down into the rhizome. The aim is to deliver enough active ingredient to the underground store to prevent regrowth. Because the plant naturally moves energy downward in late summer and autumn, this is the window when a translocated herbicide travels furthest into the rhizome — which is why timing matters so much.
Foliar spray vs stem injection
| Method | How it is applied | Best for |
|---|---|---|
| Foliar spray | Sprayed onto the leaves | Large open stands away from sensitive plants |
| Stem injection | Measured dose into each cut stem | Sites near desirable planting or watercourses |
- Foliar spray covers a lot of ground quickly but risks drift onto wanted plants.
- Stem injection is slower and more precise, targeting each cane — preferred where spray drift would be a problem.
Why it takes years
The rhizome can be metres across and store enough energy to push up new canes for several seasons even after the visible plant looks dead. A professional programme therefore repeats treatment over around three growing seasons, then monitors. ‘Dormant’ is not the same as ‘dead’ — see how long it takes. If you need it gone faster, the alternative is excavation.
Professional vs DIY
Domestic glyphosate products are weaker and rarely achieve a full kill on an established stand. Professional contractors use higher-concentration products under controls and keep records — which a mortgage lender will want to see. For the wider picture of options and cost, see how to kill knotweed and removal cost.
This page is general information, not application advice. Always follow the product label and use a qualified contractor, especially near water.
Treat it at the right time, the right way
Glyphosate only works as a timed, repeated programme. A PCA-accredited contractor will treat in the correct season and document it for your lender.
Frequently asked questions
How many times do I need to apply glyphosate?
An established stand needs repeated treatment over around three growing seasons, not a single application. The rhizome stores energy and pushes up new growth each year until enough herbicide has been translocated to kill it.
When is the best time to apply it?
Late summer to early autumn, when the plant is moving energy down into the rhizome before dormancy. This carries the herbicide deepest into the root system, which is where the kill must happen.
Is stem injection better than spraying?
Stem injection is more precise and is preferred near watercourses or desirable plants because it avoids spray drift. Foliar spray treats large open stands faster. The choice depends on the site.
Can I use shop-bought weedkiller?
Domestic glyphosate is usually too weak to fully kill an established stand and is hard to time and apply correctly. Professional products and methods give a far better result and a documented record for lenders.
Sources & further reading
- gov.uk — Prevent the spread of Japanese knotweed (treatment guidance)
- Environment Agency — Using herbicides in or near water: rules and permits
- Property Care Association (PCA) — Invasive Weed Control Group code of practice
- Environment Agency — Managing Japanese knotweed on development sites code of practice
This guide is general information, not a site-specific survey or legal advice. Japanese knotweed treatment and removal should be assessed by a PCA-accredited specialist before you act.