How to Get Rid of Japanese Knotweed

Japanese knotweed is an invasive perennial plant known for its rapid growth and extensive root system. This guide explains how to identify it and manage it safely and effectively.

What is Japanese Knotweed?

Japanese Knotweed was introduced to the UK in 1825 and widely planted as an exotic garden ornamental before the invasive nature of the plant became clear. Japanese Knotweed is probably the most invasive plant in Britain and is scheduled under the ‘1981 Wildlife and Countryside Act’ so that it is an offence to plant or cause it to grow in the wild. In addition under the Environment Protection Act (1990) Japanese Knotweed is classified as ‘controlled waste’ and must be disposed of at a licensed landfill site in accordance with the ‘Environment Protection Act (Duty of Care) Regulations 1991’.

How does Japanese Knotweed Spread?

Fleshy, red shoots appear in early spring from an extensive underground network of stems (rhizomes). If you’re learning how to spot Japanese knotweed, these early shoots are one of the first warning signs. The shoots can reach 1.5 m by May and up to 3 m by June, making it easier to understand how to spot knotweed as it matures.

Long racemes of cream flowers appear in late summer, before dieback occurs in autumn, leaving dry, leafless stems through winter. When learning how to identify Japanese Knotweed, these are also helpful signs! The rhizomes from a single plant can extend up to 2 m deep and 7 m across, which highlights how bad Japanese knotweed can be when left untreated.

In the UK, Japanese knotweed rarely produces viable seed, but spreads aggressively through its root system. This is a key reason why Japanese knotweed is bad, as even small fragments can regrow and spread rapidly.

Many people ask, is Japanese knotweed dangerous? While it is not harmful to touch, what is bad about Japanese knotweed is its ability to spread uncontrollably, damage structures, and create legal and financial issues. This is also why homeowners often worry about finding Japanese knotweed on a house, as it can affect property value and sales.

How Do You Treat Japanese Knotweed?

Treating Japanese Knotweed with Roundup ProActive & Roundup ProVantage 480

As with most broad-leaved perennials, optimum control is achieved after flowering in late summer. This is important when considering how to control Japanese knotweed effectively.
Treatment late in the season is most effective because glyphosate moves deep into the rhizome system. This is a key factor in how to kill knotweed plant properly, as targeting the roots is essential for long-term success.
Japanese knotweed is sensitive to frost, so treatments should be applied before the first frost. When planning how to control knotweed, timing is critical to ensure maximum effectiveness.
Chemical control near water requires Environment Agency approval. Always follow regulations when deciding how to kill knotweed safely and legally.
Clearing previous year’s growth in winter helps ensure even regrowth, making treatment easier. Treated plants may take 2–6 weeks to show symptoms, so patience is essential when learning how to kill knotweed.
For established infestations, a long-term plan is required. This answers the common question: can Japanese knotweed be controlled? Yes - but it often takes multiple years of consistent treatment and monitoring.
Japanese knotweed growing within 7 m of a residential boundary can significantly impact property value. This reinforces how bad Japanese knotweed is in property terms, and why early action is important.

Below are effective methods for those wondering how to control Japanese knotweed long-term.

Foliar Spray After Flowering
This is the optimum time for treatment, when flowers appear in late summer or autumn. Spraying both sides of the leaves ensures better absorption, improving results when considering how to kill knotweed effectively.

Two Foliar Sprays at 1 Metre Stem Height
Spray when plants reach 1–1.5 m in late May, then repeat later in the season. This method is useful for dense growth and supports a broader strategy for how to control knotweed over time.

Weedwiper
Using a handheld weed wiper is highly effective when exploring how to get rid of Japanese knotweed without affecting nearby plants. It is especially useful for targeted applications and small regrowth areas.

Stem Injection
Stem injection is one of the most precise answers to how to kill knotweed plant at the source. Herbicide is injected directly into the stem, making it ideal for small infestations or sensitive areas.

Method	Dose rate - Roundup ProVantage 480	Timing
Overall foliar spray	3.75 L/ha or 20 ml per litre of water in a standard knapsack sprayer fitted with 200 L output nozzles. Spot-treated leaves should be covered in spray droplets but not running off	1) Apply to flowering stems from late summer, using long lances 2) Apply late May with a follow-up treatment to re-growth at 1-1.5 m height before the first frost. If stems are taller consider cutting back and treating re-growth
Hand held weed-wiper	1 part Roundup ProVantage to 3 parts water	Stems can be wiped from 1-3 m height in late summer
Stem Injection Kit	1.5 ml Roundup ProVantage per stem	Apply to flowering stems from late summer through October for best results

PG Tip - Using a blue marker dye such as Amvista Blue when controlling Japanese Knotweed, can really help to see where you have been when spraying, weed wiping or stem injecting.

Please speak to a Progreen Technical Advisor for information on methods on how to get rid of Japanese Knotweed - contact us.

FAQs

How deep are Japanese knotweed roots?

The roots of Japanese Knotweed can grow up to 20cm a day and can reach depths of 3m in soil.

Is it illegal to have Japanese Knotweed in your garden?

No it is not illegal to have Japanese Knotweed to grow in your garden however, it is your duty to ensure that the plant does not spread into neighbouring properties as this could face prosecutions/fines. It is illegal to allow the Japanese Knotweed to spread in the wild.