Japanese Knotweed: The Definitive GuideJune 21, 2017
Last modified: May 26, 2020
The Definitive Guide to Japanese Knotweed (2017 Edition)
This guide has everything you need to know about Japanese Knotweed.
So, If you want to learn about how to identify and remove knotweed, you’re in the right place.
“This content is endorsed by Paul Beckett, Director and co-founder of Phlorum, a multi-disciplinary environmental consultancy, specialising in Japanese knotweed removal and offering free identification of Japanese knotweed for both domestic and commercial premises.”
-Paul Beckett, co-founder of Phlorum
“PBA Solutions have been pleased to contribute information for the Garden Buildings Direct – Definitive Guide which provides a good basis for understanding the rights and wrongs of Japanese knotweed treatment. Members of the Property Care Association, PBA Solutions are dedicated to the professional treatment and management of non-native invasive weeds in the UK.”
–PBA Solutions, Japanese knotweed specialists
Everything you need to know about Japanese Knotweed
This aggressive knotweed has plagued British countryside, striking fear into homeowners and causing devastating impacts on a property.
But what makes Japanese Knotweed so unusual from other garden weeds?
This knotweed takes first place as the UK’s most invasive and destructive plant.
Known to have ravaged through tarmacked drives and trespass homes, it can creep underneath houses and reach up through your floorboards.
In an attempt to control its growth, it’s costing the government thousands of pounds every year.
It spreads up to 1.5m each month!
Although it produces seeds, it’s extremely rare for the seeds to germinate.
The most common method of dispersal is the spreading of rhizome fragments (shoot-like-roots).
The crown can survive drying or composting and will produce new canes when it comes into contact with soil or water.
To dispose of knotweed canes it is important the stems are cut above the crown, rather than pulling as this will dislodge the crown.
Living crowns usually have growth buds which are obvious in late spring and which have a characteristic pink colour.
Did you know, you could be breaking the law?
In the UK, there are two main pieces of legislation surrounding Japanese Knotweed.
The Wildlife and Countryside Act 1981 states that,
It is an offence to plant or let Japanese Knotweed grow in the wild!
Section 14(2) of the Wildlife and Countryside Act 1981 also states,
“If any person plants or otherwise causes to grow in the wild any plant which is included in Part 2 of Schedule 9, he shall be guilty of an offence”. (Japanese knotweed is a Schedule 9 listed plant).
An offence under the Wildlife and Countryside Act can result in a criminal prosecution.
Environmental Protection Act 1990 classifies the weed as ‘controlled waste.’
This means by law you must dispose of it safely at a licensed landfill site.
Soil containing rhizome, the root-like stems of the plant, is regarded as contaminated.
It must be disposed of an official landfill site and buried to a depth of at least 5 metres.
According to the Environmental Protection Act (EPA) 1990, any controlled waste, must be disposed of at appropriately licensed landfills.
If you deposit, treat or keep controlled waste without a licence it is an offence under Section 33 of the EPA.
A violation of the Environmental Protection Act can result in an unlimited fine.
You can also be held liable for costs incurred from the spread of Knotweed into adjacent properties.
The Control of Pesticides Regulations 1986 requires any person who uses pesticides,
“…to take all reasonable precautions to protect the health of human beings, creatures and plants to safeguard the environment and to avoid contaminating water.”
If you plan on using pesticides near water, seek approval from the Environment Agency first.
Allowing it to spread from your land can make you liable to third-party litigation and/or civil prosecution.
Not disposing of Japanese knotweed in accordance with the Environmental Agency Regulatory Position Statement ‘Treatment and disposal of invasive non-native plants: RPS 178’ could easily result in spreading the knotweed and therefore breaking the law.
Identify Japanese knotweed in seconds if you know how!
Without guidance, this type of knotweed isn’t always easy to identify.
If you have suspicions of raging knotweed in your garden, it’s important to find out before it becomes a serious problem for your garden and your wallet.
Luckily, our identification guide will help you to recognise the different features of the plant.
So, you can easily distinguish it from other knotweeds.
We’ve even included images showing how it changes each season.
Take a look at our detailed Japanese knotweed identification guide below!
Here are four ways to identify the dreaded knotweed:
#1 It’s got distinctive leaves
The first way to recognise Japanese Knotweed is by its shovel-shaped leaves.
With a point at the tip and staggered on the stem, you can see a zigzag growth pattern.
Bright green in colour, the leaves can grow up to 200mm long!
Take a look at Japanese knotweed pictures below for easy identification of the leaves.
#2 Stems that grow up to 3m tall!
It’s no wonder Japanese knotweed is so swamped in parts of the country.
The stems alone can grow up to 2-3 metres tall.
Not dissimilar to bamboo, with nodes and purple speckles, the leaves shoot out from the nodes in a zigzag pattern.
At the mature stage, the stems are hollow and not woody.
However, in winter they become brittle and will easily snap.
#3 Look for creamy-white flowers
An abundance of creamy-white flowers appears towards the end of August and early September.
The clusters grow to approximately 0.5cm wide and up to 10cm long.
The leaves will still be apparent, creating dense foliage.
Have a look at what Japanese Knotweed flowers look like below.
#4 Its roots can grow under your house
Rhizomes are the underground part of the weed.
These root-like stems are dark brown on the outside and orange/ yellow inside.
They can grow to a depth of 2 metres and extend up to 7 metres horizontally!
This can be a real problem if situated near your home, as the roots will grow underneath.
As little as 0.7g of viable rhizome can give rise to a new plant.
See the images below to assist in identifying knotweed rhizomes.
Now learn how knotweed changes appearance all year round
It grows fastest in the Spring
The fastest Japanese knotweed growth is during the months of Spring.
new shoots burst from crown buds and appear somewhat like asparagus. New shoots emerging from rhizome buds some distance from the crowns appears with a red/ purple colour.
In late spring, canes can reach up to 3 metres (10 feet) high.
Watch out for the shovel-shaped green leaves in Summer
In Summer, the leaves are green and shovel shaped.
In late summer and early autumn, small clusters of white flowers will appear.
The stems are mostly hollow and bamboo-like with leaves having a distinctive zigzag appearance.
Stems start turning brown towards Autumn
In Autumn, a dense covering of leaves will remain.
However, by this point, they are starting to turn yellow.
The plants stay at 2-3 metres tall, with the hollow stems starting to turn brown.
In Winter the yellow leaves fall off and stems become brown
At the beginning of winter the knotweed canes die off and the weed becomes dormant.
The leaves then turn brown and fall off.
If the area hasn’t been treated, often previous year’s decomposition can be seen underneath.
The plant can cause serious Property Damage
One couple lost £250,000 after Japanese Knotweed invaded their home!
Matt and Sue fell in love with a brand new four bedroom house, in their dream area of town.
They purchased the house without hesitation and soon started a family.
After a short time, they began noticing a plant which was growing rapidly in their garden.
The growth didn’t slow down and they started to wonder if this wasn’t just any regular weed.
It began ravaging through their drive and soon invaded their much-loved home, growing through skirting boards and reaching up through the floorboards.
They called the council, who sent an environmental specialist to evaluate the damages.
It was on such a large scale that Matt and Sue were advised to get a solicitor and take legal action.
Devastatingly, they had lost more than £250,000 on their property!
Watch the video below to hear how Matt and Sue became victims to Japanese Knotweed.
Japanese Knotweed Removal Requires Informed Knowledge
Japanese knotweed removal can be tricky if you don’t know what you’re dealing with.
As it only takes a fragment to start a new plant, removal isn’t as simple as digging it up.
If Japanese Knotweed is in your garden, you’ll want to eradicate the knotweed entirely.
Some methods of removal are more effective than others, although costs can vary!
DIY: Getting rid of Knotweed yourself
It is possible to dig out the plant yourself, but it does pose some risks – notably, you’ll not be able to get a guarantee, which most mortgage providers demand when lending knotweed-affected-property.
Due to the depth that the rhizomes can penetrate, regrowth usually occurs.
You should consider some DO’s and DON’TS before taking on knotweed yourself.
#1: Don’t spread contaminated soil
Japanese knotweed is classed as ‘controlled waste.’
As it is so easily spread, this means it requires disposal at licensed landfill sites.
You should never include the knotweed with normal household waste or put out in green waste collection schemes.
#2: Do get to the root of the problem
Remove as much root as possible, then repeatedly destroy regrowth.
This way the energy reserves in the remaining underground parts will be gradually exhausted.
The process can take up to several years to be effective.
#3: Don’t allow Japanese knotweed to spread to neighbourhood properties
Talk to your neighbours about the knotweed problem and any treatment you have planned.
A barrier may be needed as you can be held responsible if it encroaches onto their property.
#4: Don’t do any DIY removal if you plan on selling your home
If you plan to buy or sell a property invaded with a Japanese knotweed, be careful.
Do not undertake a treatment plan you cannot guarantee.
Mortgage lenders usually demand assurances from professional companies that the problem is under control.
Non-Chemical Removal Includes Plant Sucking Bugs!
A plant sucker (psyllid) is being released in the UK as a means of biological control.
It is currently only being released at a handful of trial sites and is not available to gardeners.
If successful, the insects will be released nationally and become widespread in Britain.
Over the next five to ten years, if effective, Japanese Knotweed could be under control!
You can slow the growth of knotweeds!
Some gardeners use non-chemical methods such as weed suppressant fabrics.
Groundcover or landscaping fabrics can be laid over the recently cleared soil.
This will suppress regrowth of old weeds and prevent new weeds from establishing.
However, each weed suppressant fabric has its own advantages and disadvantages.
Spun materials are made from plastic fibres bonded together
The sheet can be used in most cases, both short and long term.
But are best covered with a protective mulch of bark or gravel.
It’s lightweight and easy to cut but cheaper versions don’t last long.
They can ruck into folds where soil accumulates and weeds grow.
Tougher versions, such as Plantex, are expensive.
Woven materials are sheets of woven plastic strands
They can also be used for the long-term on beds, borders and paths.
They are available in different grades, varying in toughness, weight and durability.
Do not need covering with mulch, although mulch may improve their appearance.
Plastic sheeting can be used in areas to suppress weeds for short periods.
They are cheap and easy to cut with scissors.
Picking holes in the surface will allow water to penetrate, although the downsides are that weeds can grow through.
When using weedkillers, always follow the instructions to make effective use of the product and minimise any
risks to people, pets and the environment.
Glyphosate is the simplest method for gardeners
Glyphosate-based weedkiller is the most effective method for home gardeners.
Cut away any old stems during the previous winter to allow good access.
As with other weeds, the most effective time for spraying is at the flowering stage in late summer.
However, at 7ft high (2.1m) or more it can be tricky to spray at this stage!
To make eradication easier, allow the plant to grow about 3ft (90 cm) in May, and then spray it with the weedkiller.
There will be regrowth and therefore the second application in mid-summer is necessary.
Check during September and if it has grown again, spray again before growth begins to die down in the autumn.
Check again the following Spring and avoid spray coming into contact with other plants.
Glyphosate-treated knotweed will often produce small-leaved, bushy regrowth 50-90cm (20in-3ft) in height.
This treated regrowth is very different in appearance to the ordinary knotweed.
It usually takes at least three to four seasons to eradicate Japanese knotweed using glyphosate!
Professional contractors who have access to more powerful weedkillers can speed up the process.
Hiring a Removal Specialist Can Be Necessary for Extreme Cases
Specialist companies offer Japanese knotweed removal.
They usually report on risk for mortgages with suggested treatment plans and offer insurance-backed guarantees where required.
The British Association of Landscape Industries (BALI) also has a directory of members offering invasive weed control.
The Property Care Association has a code of practice for managing invasive weeds and
members are thoroughly vetted and able to provide management plans acceptable to Mortgage
If your knotweed is too extreme to tackle yourself, always seek help from specialists.
This can be a costly procedure, though, Japanese knotweed removal costs vary anywhere from £3,000-£5000 upwards.
More Invasive Plants you may have in your Garden
1. Giant hogweed can grow up to 10ft tall!
This type of weed can be harmful to the skin if its sap makes contact.
Whilst perhaps not as difficult to control as Japanese knotweed, it needs intervention to prevent spreading.
is often the ideal option, as is simply digging it out.
If dealing with Giant Hogweed, you must cover your arms and legs as the sap can cause a nasty reaction.
2. Rhododendron ponticum is destroying wildlife
Although attractive, it has an extremely negative impact on both wildlife and the ecology of the site which it inhabits.
It has been found that it can significantly reduce the numbers of earthworms, birds and plants in the area.
It is also considered to be toxic to herbivores.
Other habitats of small mammals, such as the dormouse, have also been badly affected due to the smothering effect of the rhododendron on native plants.
Currently, the seemingly most effective control method is by injecting the stems with a herbicide.
This supposedly kills the plant within six months and makes the process of control far more precise.
3. Himalayan balsam produces 800 seeds a year!
Despite a display of rather pretty pink flowers, can be extremely difficult to get rid of if it takes hold in your garden.
Producing 800 seeds every year, they can survive up to two years whilst floating in rivers.
It’s extremely tolerant of shady positions, Himalayan balsam is extremely invasive.
Similarly to Japanese knotweed, Glyphosate can be used for chemical control.
Cutting the flower heads or pulling up the stems prior to it setting seed is the most environmentally friendly way of control.
4. It’s an offence to buy or sell New Zealand pigmyweed
New Zealand pigmyweed grows primarily in water or soil with a heavy water content near to the sides of lakes/streams etc.
It can reduce the oxygen available to fish and frogs.
Like Japanese knotweed, it is an offence to plant it – as well as an offence to buy or sell it.
Non-chemical control using Glyphosate is the most effective.
Top 4 Frequently Asked Questions!
1. Will digging it up and burning it get rid of it?
No. This doesn’t work for Invasive species like Japanese Knotweed.
They are good at surviving and digging them up can cause them to spread to another area.
(Perhaps, your neighbours!)
This can cause all sorts of problems, including legal action against you!
Contact a suitably qualified contractor for advice to tackle the problem.
2. Will Japanese knotweed come back?
There is always a possibility of some regrowth, especially following herbicide treatment.
The effectiveness depends on the plants uptake of the chemical.
If regrowth occurs on a property, this should be treated again.
3. Can I get a mortgage if I have Japanese Knotweed on my property?
Yes, but there are risks.
You may require a treatment programme including an insurance-backed guarantee from a contractor.
Japanese Knotweed can decrease the value you of your home.
4. How does it spread?
The plant spreads by vegetative means, rather than by seed dispersal.
Rhizomes may grow from either an existing crown, where previous growth has taken place or from a cut stem.
New outbreaks of knotweed generally result from fragments of rhizomes within soils being moved from site to site.
This can be because of gardening and DIY projects, fly-tipping, or by natural processes.
It’s important to dispose of any fragments of the plant at a suitable landfill.
Cool Bonus: Download your PDF Growth Cycle so you can stay clued up how this invasive plant rapidly spreads.