How to Kill Japanese Knotweed

Japanese knotweed is the most feared plant in British gardens, and for good reason. This aggressive invasive weed can devalue your property, stop mortgage applications in their tracks, and land you in legal trouble if it spreads to neighbouring land. An estimated 1.4 million UK homes are affected by knotweed, making it a problem that many gardeners will face at some point.

The good news is that Japanese knotweed can be controlled and eventually eradicated. But it requires the right approach, the right timing, and a realistic understanding of what you’re dealing with. DIY attempts with household products almost always fail and can make the problem significantly worse.

How to Identify Japanese Knotweed

Correct identification is essential before you begin any treatment. Japanese knotweed has several distinctive features that set it apart from similar plants.

In spring, reddish-purple shoots emerge from the ground, looking somewhat like asparagus spears. These grow rapidly, sometimes up to 2cm per day, developing into tall bamboo-like stems that reach 2-3 metres by late summer. The stems are hollow with distinctive purple speckles and prominent nodes at regular intervals.

The leaves are the most recognisable feature. They’re heart-shaped or shield-shaped, bright green, and arranged in a zigzag pattern along the stems. Each leaf typically measures around 14cm in length with a flat base. In late summer, creamy-white flowers appear in hanging clusters, similar to astilbe.

Japanese knotweed is sometimes confused with Russian vine, Himalayan honeysuckle, or certain persicaria species. The key differences are the bamboo-like spotted stems and the alternate leaf arrangement. If you’re unsure, many specialist companies offer free identification services from photographs.

Why Japanese Knotweed Is So Difficult to Kill

Understanding why this plant is so problematic helps explain why proper treatment is essential. Japanese knotweed survives through an extensive underground rhizome network that can spread 3 metres deep and 7 metres wide from the visible plant. These rhizomes are the plant’s energy store and survival mechanism.

The rhizomes can remain dormant in soil for many years, ready to regenerate when conditions are right. Even a fragment as small as 0.5 grams, roughly the size of a fingernail, can grow into a new plant. This is why disturbance and poor disposal methods often spread knotweed rather than eliminate it.

The plant’s ability to exploit weaknesses in hard surfaces adds to its fearsome reputation. While recent research suggests structural damage is less common than previously thought, knotweed shoots can push through cracks in concrete, tarmac, and brickwork. This potential for property damage is what concerns mortgage lenders.

The Legal Position on Japanese Knotweed

Japanese knotweed is listed under Schedule 9 of the Wildlife and Countryside Act 1981. This makes it illegal to plant Japanese knotweed or cause it to grow in the wild. Importantly, allowing knotweed to spread from your property onto neighbouring land or into wild areas could constitute an offence.

It is not illegal simply to have knotweed growing in your garden, and you don’t have to report its presence. However, you do have a responsibility to prevent its spread. Local councils and police can issue Community Protection Notices under the Anti-social Behaviour Crime and Policing Act 2014, compelling you to control knotweed that is affecting neighbours’ quality of life.

The same legal framework applies to Himalayan balsam, another Schedule 9 invasive species that spreads rapidly along waterways and gardens.

The disposal of Japanese knotweed is also legally regulated. It’s classified as controlled waste under the Environmental Protection Act 1990 and must be disposed of at a licensed landfill site, buried to a depth of at least 5 metres. You cannot simply put it in your garden waste bin or take it to a standard recycling centre.

Japanese Knotweed and Property Sales

If you’re selling a property with Japanese knotweed, you must declare it on the TA6 property information form. This standard conveyancing document asks specifically about knotweed, and deliberately providing false information could result in the buyer suing you after completion.

The presence of knotweed doesn’t automatically prevent a sale, but it does complicate matters. Mortgage lenders use a risk assessment system based on RICS (Royal Institution of Chartered Surveyors) categories ranging from A to D. Category A indicates knotweed is already causing damage, while Category D means it’s visible on adjoining land but poses limited risk.

Many lenders will decline applications outright if untreated knotweed is present. However, most will consider lending if you can provide a professional treatment plan with an insurance-backed guarantee (IBG) from a PCA (Property Care Association) or INNSA (Invasive Non-Native Specialists Association) registered company. These guarantees typically last 5-10 years and transfer to new owners.

Why DIY Methods Fail

Before discussing what actually works, it’s worth understanding why so many DIY approaches fail. Desperation leads many gardeners to try household remedies, but these almost always make the situation worse.

Diesel, bleach, and vinegar do not kill Japanese knotweed. They may damage the visible growth, but they don’t penetrate the deep rhizome system where the plant’s energy is stored. The knotweed simply regrows, often more vigorously. Using diesel also contaminates your soil and breaks environmental regulations.

Burning knotweed removes the top growth but leaves the rhizomes completely unaffected. Cutting it back has the same problem and risks spreading fragments that can root elsewhere. Even covering with plastic sheeting just encourages the rhizomes to spread laterally, seeking gaps.

Standard weedkillers from garden centres rarely contain sufficient concentration of active ingredients to affect the rhizome system. They often just cause dormancy, where the plant appears dead above ground but remains very much alive below, ready to regrow.

What Actually Kills Japanese Knotweed

Effective treatment requires a systemic herbicide that travels from the leaves down into the rhizomes. Glyphosate-based weedkillers are the standard treatment, but timing and application method are critical to success. For the most stubborn infestations, the strongest weed killer formulations give the best results.

The best time to treat Japanese knotweed is late summer to early autumn, after the plant has flowered but while the leaves are still green and actively transporting nutrients down to the rhizomes for winter storage. Treating earlier in the year may stunt growth but won’t kill the root system.

For established infestations, expect treatment to take at least 3-5 years of annual applications. Each year, the treatment weakens the rhizome system further until it’s exhausted. You’ll notice progressively weaker regrowth each spring until eventually nothing emerges.

Professional treatment typically involves either foliar spraying or stem injection. Stem injection delivers herbicide directly into the hollow stems, which is useful near water or where spray drift could damage other plants. For most garden situations, careful spraying of the foliage is effective.

If you need the knotweed gone immediately, excavation is the only option. This involves removing all contaminated soil to a depth of at least 3 metres and disposing of it at a licensed facility. It’s expensive, typically several thousand pounds, but provides immediate results.

A Realistic Treatment Plan

For most homeowners, a herbicide treatment programme offers the best balance of effectiveness and cost. Treat annually in late summer or early autumn when the plant is actively growing. Use a professional-strength glyphosate weedkiller at the recommended concentration.

Allow the herbicide to work for at least 3 weeks before removing any dead growth. Cut stems a minimum of 10cm above ground level and leave the crown intact. Dead brown canes can be composted on site, but never compost any plant material that might contain live rhizome.

Continue treating any regrowth annually until no new shoots appear for at least two consecutive growing seasons. Even then, monitor the area carefully, as dormant rhizomes can regenerate years later if disturbed.

If you’re planning to sell your property, consider engaging a professional company to provide a treatment plan with an insurance-backed guarantee. This documentation is often essential for mortgage approval and gives buyers confidence that the problem is being managed properly.

Similar persistent weeds like bamboo, bindweed, and ground elder require the same patient, systemic approach. Quick fixes don’t exist for plants with extensive root systems.

Frequently Asked Questions

Is it illegal to have Japanese knotweed in my garden?

No, it’s not illegal to have Japanese knotweed growing on your property. However, it is illegal to plant it or cause it to spread into the wild or onto neighbouring land. You have a responsibility to control it and prevent spread.

Will Japanese knotweed stop me getting a mortgage?

Not necessarily. While some lenders refuse all applications where knotweed is present, most will approve mortgages if you provide a professional treatment plan with an insurance-backed guarantee from a PCA or INNSA registered specialist.

Can I remove Japanese knotweed myself?

You can treat it yourself with professional-strength glyphosate, but DIY methods using household products like bleach, vinegar, or diesel don’t work. Improper treatment can make the problem worse and leaves you without the documentation needed for property sales.

How long does it take to kill Japanese knotweed?

Professional herbicide treatment typically takes 3-5 years of annual applications to exhaust the rhizome system completely. Excavation provides immediate removal but is significantly more expensive.

Do I have to declare Japanese knotweed when selling my house?

Yes. The TA6 property information form requires disclosure of Japanese knotweed. Providing false information can result in the buyer suing you after completion. Being honest and having a treatment plan in place is the best approach.

If Japanese knotweed is threatening your property value or blocking a sale, don’t waste time on ineffective DIY remedies. A professional-strength systemic weedkiller applied at the right time of year is your best route to getting this invasive plant under control for good. For permanent weed control strategies across your whole garden, see our complete guide.