Glyphosate is by far the most commonly used herbicide to treat Japanese Knotweed in the United Kingdom; however, there are still numerous homeowners who continue to ask the very same question:
Can you kill Japanese Knotweed with glyphosate permanently?
Yes, glyphosate can kill Japanese Knotweed. However, this can only happen if it is properly applied at the right time (i.e., during optimal growing conditions), and as part of a multi-season treatment program.
Japanese Knotweed is a lot more than just another garden weed. This aggressive invasive perennial plant has several characteristics that make it difficult to remove, including its extremely fast rate of growth, deep taproots and an extensive underground network of rhizomes.
If you attempt to use chemical treatments on Japanese Knotweed without knowing about the growth habits of this plant, it can lead to regrowth, spread to adjacent properties, and an increased cost for controlling it.
This article explains how glyphosate works, when it should be applied, what the law requires, and why professional Japanese knotweed control often produces more reliable outcomes. If you are looking for a broader overview, see our full guide on how to kill Japanese knotweed.
Japanese Knotweed: Quick Facts
Introduced to the UK in the 19th Century as a decorative plant, Japanese Knotweed has been identified as one of the leading invasive plant species that affects property.
Identification features include:
- Shoots emerge from the underground rhizome in early spring (red fleshy)
- Reach up to 1.5m by May and 3m by June
- Hollow stem with purple-red spots and visible nodes
- Leaves are lance-shaped and alternate on the stem in a zigzag manner
- White clusters of flowers appear in late summer/early Autumn
To find our more check out our Japanese Knotweed Identification Guide.
Japanese Knotweed can grow in:
- Gardens
- Roadsides
- Abandoned land
- River banks and stream sides
When the above-ground parts of the plant die, it still has the ability for the roots below the surface to continue to function. If you are unsure, you can use our free Japanese knotweed identification service to upload photographs for review.
Table of Contents
Understanding Japanese Knotweed
The first introduction of Japanese Knotweed into the UK occurred during the 19th Century when it was used as an ornamental plant, and was on sale throughout the UK. Today, Japanese Knotweed is identified as one of the most problematic invasive plant species impacting both residential and commercial property.
How Japanese Knotweed Grows
The growth of Japanese Knotweed above ground is merely half the story. A dense underground system of Rhizomes and Roots extends below the surface of the ground for over 1 Metre in depth and over several metres in width.
Early spring brings the emergence of fleshy red stems from the soil surface. Stems on this new plant will be able to grow up to 1.5 metres tall by May and up to 3 metres tall by June. The characteristic speckled red/purple colouration and node markings are easily identified on this hollow stem; alternatively, spade-shaped leaves that grow along the length of each stem in an alternating (zigzag) fashion can also provide identification.
The flowering takes place at the end of the growing season in late summer through to early Autumn, with white/creamy flower heads forming at the top of the stems in dense clusters.
However, the visible stem and leaves are not the primary concern. The plant’s root system is what enables rapid growth and long-term survival.
Japanese Knotweed can produce new plants from a rhizome of just 0.2g in weight; even the smallest of fragments carried on soil or with waste can cause the infestation to expand.
- It is possible for sections of Japanese Knotweed as small as 1cm to develop into entirely new plants.
- Japanese Knotweed spreads very quickly through the transport of Rhizome/ Stem fragments to other areas.
- Japanese Knotweed will lie dormant in the soil for long periods of time prior to growing again.
- The root structure of Japanese Knotweed extends well below the ground, typically 1m or more beneath the surface of the soil.
Is glyphosate right for your situation?
Answer a few quick questions to see which approach is usually most suitable.
Why Is Japanese Knotweed Difficult to Kill?
Japanese Knotweed is resilient because:
- It spreads through rhizomes beneath the ground.
- Roots store energy reserves.
- Digging can fragment the root system.
- Disturbed soil can stimulate regrowth.
- Dead stems do not mean eradication.
Cutting, mowing, or digging out Japanese Knotweed is rarely effective and can stimulate its root system to grow more vigorously. Digging is labour intensive and can worsen the issue by distributing fragments of root material through surrounding vegetation.
This is why systemic herbicide treatment is usually required to rid the land of established infestations.
Japanese Knotweed poses a serious threat to ecosystems by outcompeting native vegetation.
In some circumstances, Japanese Knotweed can cause structural damage to buildings and infrastructure if left untreated.
How Glyphosate Works on Japanese Knotweed
Glyphosate is a systemic herbicide. Once applied to the leaves or injected into the stem, it is absorbed and transported throughout the plant, including into the rhizomes and deeper roots.
Glyphosate works by systemic action, targeting the underground rhizome system and exhausting the plant’s stored energy.
It works by disrupting growth processes within plant tissue, gradually weakening the plant’s root system over successive growing seasons.
Unlike contact-only weedkillers, glyphosate based products move internally through the plant’s vascular system. Many DIY products fail because they only damage visible growth rather than exhausting underground rhizomes.
When applied correctly, glyphosate can kill Japanese Knotweed by:
- Penetrating leaves or stem tissue
- Moving into the plant’s root system
- Depleting stored energy in rhizomes
- Preventing sustained regrowth
However, complete eradication typically requires at least three growing seasons.
It typically takes at least three growing seasons to kill Japanese Knotweed using glyphosate-based herbicides fully.
When Is the Best Time to Apply Glyphosate?
Timing is critical.
The optimum time to treat Japanese Knotweed is late summer or early Autumn after the plant has flowered. During this period, nutrients naturally move downwards from leaves into rhizomes in preparation for winter.
Glyphosate applied in late Summer to early Autumn after flowering will be able to enter the root system with greater ease than if it was applied earlier in the growing season.
Early application can lead to lower levels of success. Applying control measures after the first frost will typically result in minimal control as the plants’ physiological process slow dramatically.
Best time to apply glyphosate
Select a season to see why timing matters.
Methods of Applying Glyphosate
There are two primary methods used for Japanese knotweed control.
1. Foliar Spray
Spraying involves applying herbicide directly onto the leaves.
This method is suitable for:
- Dense stands
- Large infestations
- Open ground with minimal surrounding vegetation
Spray applications must be carefully controlled to avoid affecting native plants and other plants in the garden.
Overspray can create a detrimental effect on surrounding vegetation. Where Knotweed is close to neighbouring land or sensitive wildlife habitats, spraying may not be appropriate.
2. Stem Injection
Stem injection is a targeted method that involves injecting herbicide directly into each hollow stem.
Stem injection:
- Reduces risk to other plants
- Minimises spray drift
- Is suitable for residential garden settings
- Is often preferred near boundaries
Because it targets the individual stem, it offers greater control where Knotweed is close to neighbouring properties.
How Long Does Glyphosate Take to Work?
Glyphosate does not kill Japanese Knotweed instantly.
Typical timeline:
Year 1:
Visible dieback. Reduction in stem height and vigour.
Year 2:
Further weakening. Fewer new stems emerge.
Year 3+:
Spot treatment and monitoring to prevent regrowth.
Some infestations may require follow up treatment beyond three growing seasons depending on soil conditions, root mass, and historical disturbance.
Legal Responsibilities in the UK
It is not illegal to have Japanese Knotweed on your own land.
However, under the Wildlife and Countryside Act 1981, it is illegal to plant or cause Japanese Knotweed to grow in the wild.
Allowing Knotweed to spread onto neighbouring land can lead to civil claims. Under the Crime and Policing Act 2014, local authorities may issue Community Protection Notices where invasive species create ongoing problems or anti social behaviour concerns.
Japanese Knotweed is also classed as controlled waste under the Environmental Protection Act 1990. Any removal of waste material must be taken to a licensed landfill site or permitted facility.
Failure to control Japanese Knotweed can lead to reduced property values and difficulties in selling a home or obtaining a mortgage.
It is illegal to allow Japanese Knotweed to spread into the wild or onto neighbouring properties. Read our Japanese Knotweed and the Law in the UK page for more information.
UK legal and compliance checklist
Tick what applies to your situation.
Can You Still Buy Glyphosate in the UK?
Glyphosate is still legally available to buy in the UK.
Although:
- glyphosate is banned in some areas by local councils as a weedkiller on public land
- there is a requirement for permission (environmental consent) before using it near water.
- the person applying professional grade glyphosate has to have appropriate qualifications.
In addition to the above, when treating weeds that are growing in proximity to water courses or designated protected areas, you will need to obtain written permission from the relevant environmental authority before applying any treatment.
All glyphosate products to be used by professionals must be applied by personnel who have successfully completed the necessary qualifications.
A professional grade Glyphosate product will be generally needed to effectively treat an area of land which has already been overrun with weeds.
Prior to using glyphosate on or very close to any body of water, you will need to identify a product formulation which is classified as being low toxicity. In many cases, additional approvals will also be required from the environmental agency responsible for ensuring protection of the watercourse itself.
Glyphosate vs Excavation
Glyphosate treatment is:
- Cost effective
- Causes less disruption to the garden than excavation
- Applicable in most gardens
Excavation involves:
- Digging out contaminated soil
- Removing large quantities of controlled waste
- Transporting to a licensed landfill site
- Substantial ground disruption
Excavation may be an option for development sites with the need for immediate removal. In addition, excavation is very labor-intensive and produces a large quantity of waste materials.
A well-planned and structured herbicide program in many residential situations will provide a proportional form of control.
Root Barriers and Containment
Root barriers can help contain the spread where Knotweed is close to structures. However, root barriers do not kill the plant.
Containment without treatment does not achieve eradication and may allow the root system to remain viable for many years.
Is Glyphosate Safe?
Glyphosate use can raise some reasonable concerns about wildlife and the environment.
However, when used in line with the manufacturers’ recommendations and those of the UK regulatory authorities, glyphosate has been approved for use.
Longer term studies also appear to indicate that glyphosate-based treatments will likely have a lower overall environmental impact than traditional (physical) management methods.
There are however some potential risks associated with glyphosate usage:
- Spray drift must be managed at all times
- Approved formulations are recommended for use close to water
- Wildlife exposure to glyphosate must be kept to an absolute minimum
A competent operator will reduce or eliminate many of the potential risks associated with using glyphosate.
What Is the 7 Metre Rule?
Mortgage lenders have for many years included reference to a ‘7m rule’ in their risk assessment documentation. The 7 m rule is actually a guideline used by professionals when undertaking property valuations as opposed to being a legal requirement.
Management plans that show that treatment is being carried out professionally are normally acceptable if the presence of the species within 7 meters of a habitable building would otherwise be considered an issue.
In these cases, a Japanese knotweed survey and a management plan is often required by lenders.
Can Roundup Kill Japanese Knotweed?
Some over-the-counter products contain glyphosate. However, lower concentration formulations may require repeated applications and careful timing.
Many contact-only weedkillers will not kill Japanese Knotweed because they do not reach the plant’s root system.
Professional treatment typically uses stronger, regulated herbicide formulations applied as part of a controlled programme.
For a comparison of products, see our guide to the best weed killer for Japanese knotweed.
Why Early Intervention Matters
As time goes by, and the Knotweed continues to be uncontrolled, the size and spread of its root system will increase.
The delay of treatment of Knotweed will lead to:
- Increase costs
- Increased potential of Knotweed spreading
- Increased difficulty to remove
- Impact on value of your property
In certain circumstances Knotweed could potentially cause structural damage to a property and also impact the ability of you as a homeowner to secure a mortgage.
Early intervention will help minimize the long term disruption caused by Knotweed.
When Should You Seek Professional Treatment?
It would be recommended that you use professional services for removal in the following situations:
- Where Knotweed is located close to the boundary
- When you are purchasing/selling a property
- Where a lender is requiring documentation
- Where DIY methods have failed
- Where Knotweed has existed at your property for an extended period of time.
Our professional Japanese Knotweed removal programs are structured, documentated and comply with current UK legislation.
Final Thoughts
Glyphosate kills Japanese Knotweed when applied regularly and with precision. Glyphosate targets the root system of the plant and reduces the underground rhizome structure through repeated applications during successive growing seasons.
Japanese Knotweed is still considered a highly resistant invasive plant species in the UK. Therefore, it is essential for effective control of the plant that treatments are implemented using both patience, appropriate timing, and knowledge about proper use.
While there may be some homeowners who are looking for long-term assurance regarding their Japanese Knotweed infestation, this will likely require them to seek professional advice in order to ensure their treatment plan is both compliant and proportional and provides adequate documentation.
If you would like additional information or schedule a consultation, hiring a professional allows you to create a treatment plan that complies, is proportionate to the severity of the infestation, and adequately documents the treatments performed.
