Japanese knotweed is an invasive non-native plant that can spread rapidly, crowding out other plants and causing serious damage to infrastructure. Its eradication requires knowledge and determination, as it can be very difficult to remove.
This guide is designed to help you identify this invasive species and plan how to remove it safely and permanently.
There are now legal requirements for the removal or disposal of Japanese knotweed, including the disposal of pieces of the plant. It’s now an offence to cause Japanese knotweed to grow in the wild, either by fly-tipping or allowing the plant to escape the confines of your garden.
Homeowners should be particularly vigilant, as Japanese knotweed in a garden must now be reported in a TA6 property information form. It can deter potential buyers and lead to banks refusing mortgage or remortgage applications.
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Identifying Japanese Knotweed
Japanese knotweed leaves are shovel-shaped (also described as a shield or heart-shaped) with a point at the tip. They are staggered on alternate sides of the stem (one stem per node), creating a zig-zag stem growth pattern. They’re a luscious green colour and grow up to 200mm long. See the images below for easy identification of the Japanese knotweed leaf.
Japanese knotweed stems can grow to 2-3 metres tall. They look similar to bamboo with nodes and a hollow stem that has very noticeable purple speckles. The leaves grow out from alternate nodes along the stem in a zig-zag pattern.
Bunches of stems emerge from ‘crowns’ where the roots (rhizomes) protrude above the ground. The stems’ bases are bright green with purple speckles and can be fairly thick, measuring around 5cm in diameter.
Knotweed stems are relatively easy to snap, revealing a complicated hollow interior. The leafless stems die back in the winter, leaving brittle red-brown or straw-coloured canes, as can be seen below.
Elongated clusters of creamy white flowers may appear towards the end of August and early September. The leaves will still be apparent and along with the flowers, it will create dense foliage. Flowers emerge on panicles, which are composed of dense clusters of miniature flowers on slender, 10cm-long spikes. Each spike’s individual flowers are around 0.5cm wide.
Almost all Japanese knotweed in the UK is a clone of the female plants introduced to the country in the mid-nineteenth century. Russian vine pollen can fertilise female knotweed plants, resulting in hybrid seeds. These are black, have a diameter of around 2mm, and resemble apple pips when split in half. Fortunately, most are sterile and rarely germinate into new plants.
Japanese knotweed has a permanent network of underground shoots called rhizomes, which are considered underground stems. Knotweed rhizomes snap easily and are dark brown outside and orange/yellow inside. Thin roots emerge from them, delivering water and nutrients to the rhizomes, which store starchy energy in the fleshy orange tissue.
The crowns can be quite large, measuring up to 40cm in diameter, with thick (typically 3cm in diameter) rhizomes extending outwards.
It is the knotweed rhizome that spreads the plant. Rhizomes can penetrate up to 3 metres into the ground, and the rhizome system can grow up to 7 metres outwards from the crown. As little as 0.7g of viable rhizome can give rise to a new plant. See the images below.
Early growth of Japanese knotweed
New shoots begin to emerge from crowns in late winter from thumb-sized buds that are dark red to pink in colour. The quickly spreading shoots resemble thick asparagus spears until the leaves unfurl.
However, Japanese knotweed shoots can also sprout from the ground a few metres from the nearest crowns, seemingly unnoticed. This growth from rhizome buds is significantly smaller and thinner than crown buds but similar in shape and dark red colour. Peony shoots are one example of a plant that can be mistaken for knotweed in early spring, however, peony leaves are very different to knotweed leaves.
New shoots from Japanese knotweed that have been treated with herbicides in the past may seem malformed. When glyphosate weedkiller has been applied, micro-leaves and micro-stems commonly form, resulting in what is known as ‘bonsai’ knotweed.
Other herbicides can cause stems and foliage to twist and curl. When crowns are removed by digging, stems that are smaller than typical develop. These are indications that the weedkiller is working and should be reapplied to the new season’s growth.
Japanese Knotweed through the Seasons
Identifying the way this weed changes through the seasons and knowing how to deal with it requires both knowledge and visual clues to help you identify it and tackle it for removal.
Key features of Japanese knotweed identification in Spring
- When new shoots emerge from crowns, they are fleshy and asparagus-like.
- Rhizome buds produce thinner and smaller dark red/purple shoots with new leaves that are frequently coiled up.
Key features of Japanese knotweed identification in Summer
- Knotweed stems are green with red/purple speckles and reach a maximum height of 2-3 metres.
- They resemble bamboo with nodes between stem parts plainly visible.
- Along the stems, the leaves grow on alternative sides, making a zig-zag pattern.
- The stems emerge from crowns that form dense bunches.
- Flowers bloom in the late summer (August/September).
Key features of Japanese knotweed identification in Autumn
- Stem bracts drop flowers and little (5mm) seed casings.
- Leaves become yellow and fall off — yellowing can start at the leaf margins and extend outward.
- Stems turn red and drop their leaves.
Key features of Japanese knotweed identification in Winter
- The leaves fall off and the shoots die back, leaving behind dead, straw-coloured, hollow stalks that resemble bamboo stems.
- Some knotweed branches might persist, or new shoots can sprout, during the winter months after moderate autumn with no frosts.
Removing Japanese Knotweed
To successfully remove Japanese Knotweed, it is crucial to assess the size of the infestation and make a long-term plan. It can take a few years of consistent effort to eradicate the plant.
Your plan can include a number of the following approaches. For example, spraying the plants with herbicides, then cutting down the dead plants and burning that debris, before disposing of the residue at a licenced disposal site.
The most successful methods of removing Japanese knotweed:
- Use of herbicides
- Cutting it down
- Digging it out
- Burning it
- Burying it
- Barrier protection
- Disposing of it safely
- Engaging a professional contractor
Which of these methods you employ will depend on the size of the infestation, timescale and budget. A small amount of Japanese knotweed can be tackled by the home gardener, but large-scale problems may need professional treatment.
1. Using Herbicides (weedkillers)
The most effective chemicals to combat Japanese knotweed are Glyphosate-based herbicides, such as Roundup Tree Stump. Glyphosate weedkillers for Japanese knotweed have been tested, tried, and confirmed to function with effective results. Always read the instructions for each chemical before using it.
Spray in the summer, while the knotweed is flowering, and before the plant dies back in the autumn. Give the herbicide at least a week to work, so it reaches the roots and rhizomes. Repeat the treatment until the weed is entirely dead, then cut out the dead plant material.
Using herbicide regularly, eradicating knotweed will take at least two years and the affected area will need to be watched carefully afterwards.
2. Cutting it Down
Japanese knotweed stems can be cut down close to the soil surface and then removed. This is a particularly good idea after The waste material can either be completely burned or buried in an impermeable container in a licenced disposal site.
Ensure that the item to be burned is exposed to extremely high temperatures, preferably in an incinerator, to ensure that no fragments survive the process.
Japanese knotweed debris should never be left unattended and must be disposed of safely, because the weed may easily re-grow even from cut rhizomes.
3. Digging it Out
After cutting down the knotweed stems, the next step is to dig out the weed’s root system. If the infestation isn’t too large, you can dig out the roots. Dig up as much as possible to prevent regrowth. The plant’s large root structure makes it difficult to eradicate, and regeneration is always a possibility.
The same restrictions and legislation apply to
This is normally only appropriate for large-scale Japanese knotweed infestations. Professionals use excavators to dig out the plants and their root system, to a depth of a metre. They then transport the plant material and affected soil to a landfill site that specialises in its disposal.
5. Burning knotweed debris
Controlled burning is the most effective and safest method of eliminating Japanese Knotweed material that has been cut down or dug up. You can burn Japanese knotweed debris on your property under controlled conditions.
It is easiest to burn the plant once it has dried out, although the root material and rhizomes may survive the procedure. It is important to burn it at high temperatures, to kill as much of the plant as possible.
You cannot, however, transport the rubbish to another location to be burned or take the burned rubbish to your council’s conventional landfill or recycling centre since cross-contamination will occur. You will need to use a licenced professional with permission to transport it to a specialised landfill site for disposal.
6. Burying knotweed debris
A professional company may pack the chopped Japanese knotweed waste into impermeable containers and bury it in a licensed landfill site.
No other waste material, especially organic material, should be buried with it, as this could lead to a new infestation in a different location.
7. Barrier Protection
Weed-suppressing textiles spread over the soil where Japanese knotweed has been removed effectively stop the renewal of old weeds and prevent new growth.
Denying the weed sunshine and moisture stunts its growth and aids in its eradication, however, this is a long process that requires patience. The key is to also cover the perimeter of the area, to prevent rhizomes from escaping sideways and causing new growth.
8. Disposing of knotweed safely
Avoid spreading contaminated soil. Tiny fragments of Japanese knotweed can create new growth in contaminated soil. You should dispose of the material at licenced disposal sites to avoid pollution. You can find locally licenced sites by calling your local council. Do not dispose of any living or dead Japanese knotweed with your green waste.
A registered professional will have suitable training to remove Japanese knotweed waste safely and securely.
9. Engaging Professional Help
If the knotweed is too stubborn or too widespread, it is best to seek help from an expert. Specialists have access to better weed killers and have the skills and knowledge to speed up the eradication process.
Japanese knotweed is a persistent perennial weed that takes time and effort to remove. After investing your time and money in removing this weed it’s essential to keep monitoring your garden for regrowth.
As the plant can regrow from even tiny pieces left in the soil it’s important to remain vigilant and watch out for any shoots. These may appear away from the original plants, because of the sizeable underground system of rhizomes and roots.
Any new shoots can be treated with herbicide to kill them off early. You can even paint herbicide onto individual shoots to get the chemicals into the root system.
Always store herbicides and weedkilling equipment safely.
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