At a glance
Japanese knotweed (Reynoutria japonica) is unlike any other garden weed in the UK. It has legal implications, can affect your ability to sell your home, and has been known to cause structural damage to buildings, walls and hard surfaces. Getting rid of it is not a weekend job – it takes years of consistent treatment – but it is achievable, and the sooner you start the sooner it’s gone.
How to identify Japanese knotweed
- Stems – hollow, bamboo-like with purple speckles. Can reach 2-3m in a single season.
- Leaves – large, heart-shaped, arranged alternately up the stem. Flat base to the leaf rather than pointed.
- Flowers – clusters of small creamy-white flowers in late summer (August-September).
- Rhizomes – underground roots are orange-brown inside when cut. Can extend 7 metres horizontally and 3 metres deep.
Never treat without correct identification. Several plants look similar including Russian vine and bindweed. If in doubt, use the UK government’s free ID service or consult a professional before treating.
Your legal obligations in the UK
- Not illegal to have it on your property – contrary to popular belief, simply having it in your garden is not a criminal offence
- Illegal to allow it to spread to neighbouring land – under the Wildlife and Countryside Act 1981, causing it to grow in the wild is an offence
- It is controlled waste – under the Environmental Protection Act 1990, knotweed and contaminated soil must be disposed of at a licensed landfill site – not composted, burned or put in a skip
Treatment options
| Method | Effectiveness | Cost | Time to eradicate | Verdict |
|---|---|---|---|---|
| Glyphosate stem injection | Very high | £20-50/season DIY | 3-5 years | Most effective DIY |
| Glyphosate foliar spray | High | £15-30/season | 4-5 years | Easier than injection |
| Professional treatment | Very high | £1,500-5,000+ | 3-5 years | Comes with insurance certificate |
| Physical cutting alone | Low | Free | Never – makes it worse | Never use alone |
Stem injection in late summer (August-September) is the most effective DIY method. Glyphosate solution injected directly into hollow stem nodes travels into the rhizome system with better kill rates and less environmental impact than foliar spraying. The plant must not be cut before treatment – wait at least 2 weeks after any cutting.
Eradication timeline
- Year 1 – treat in August/September. Rhizome survives, new growth returns next spring
- Year 2 – regrowth typically 50-70% reduced. Treat again in summer
- Year 3 – regrowth significantly reduced. Continue monitoring and treating any new shoots
- Year 4-5 – most infestations effectively controlled. Continue monitoring for occasional regrowth
Impact on property sales and mortgages
Japanese knotweed must be declared in property sales in the UK – it is included in Law Society TA6 forms that sellers must complete honestly. Many mortgage lenders refuse to lend on properties with knotweed, or require a professional management plan with insurance-backed guarantee. A professional treatment certificate makes a significant difference to mortgageability.
When to call a professional
- Infestation over 10 square metres or multiple areas
- Planning to sell within 5 years – lenders often require professional management plans
- Knotweed within 7 metres of foundations or services
- It has spread to a neighbour’s property
- DIY treatment hasn’t produced results after 2 seasons
Check credentials before hiring a professional. Look for members of the Property Care Association (PCA) or Invasive Non-Native Specialists Association (INNSA). A good company provides a written management plan with a guarantee period of at least 5 years.
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Japanese knotweed is beatable with the right approach and patience. Start treatment this summer, document everything, and be consistent. For more on persistent UK garden weeds read our guide on how to get rid of bindweed permanently.
