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Japanese Knotweed Identification Guide (UK & Ireland)
To get a better understanding of Japanese knotweed, follow our guide to quickly identifying different knotweeds and deciphering the law surrounding this highly destructive plant.
In its native environment, Japanese knotweed is kept in check by native insects and fungus that feed on the plant.
Outside of its native environment, Japanese knotweed has become a menace on both residential and commercial properties. It has become prevalent throughout Ireland over the course of the last 100 years.
Knotweed may appear harmless, but they can break through asphalt, tarmac and poor quality concrete, cause damage to building foundations and retaining wall structures. This is why it is crucial to identify it early and remove it.
In addition to damaging residences or workplaces, it will also seriously depreciate the value of property, hinder mortgage loan applications and may affect insurance companies insuring a property infested with knotweed.
How to identify knotweed?
People often notice Japanese Knotweed but don’t understand what it is. At times Japanese Knotweed can easily be mistaken for similar fast growing garden plants. However once you know the main signs it becomes easier to recognise.
Japanese knotweed usually appears in clusters and grows very quickly during warmer months. It is mostly found along garden boundaries, near fences and around unused ground. If you are trying to identify Japanese knotweed there are a few key signs to look out for before calling a specialist.
There are several different types of knotweed and during each season their appearance can change, making identification problematic even for a qualified surveyor. It is important to be able to identify knotweed when purchasing a property and the extent of any infestation either on the property or on adjacent premises.
Japanese knotweed can be identified by its delicate creamy/white flowers and distinctive bamboo like stems which can reach up to 3 metres in height. When it first breaks through the ground it can be recognised by fleshy, red tinged shoots with large spade shaped leaves (resembling asparagus tips). When identifying Japanese Knotweed there are a few key signs to look for.
Key signs include:
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Bamboo-like stems – The stems grow upright and hollow, often with small purple speckles. From a distance they can look very similar to bamboo canes.
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Shield shaped leaves – The leaves are wide and flat with a pointed tip, often described as heart or shield shaped. They grow in an alternating zig-zag pattern along the stem.
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Creamy white flowers – Towards late summer the plant produces small clusters of creamy white flowers which hang slightly from the stems.
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Rapid summer growth – Japanese knotweed grows quickly once the weather warms up and can reach several metres in height during the growing season.
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Orange rhizome roots – Underground, the plant spreads through thick root systems known as rhizomes. When cut open these roots are usually orange inside.
Knowing these key signs will make identifying Japanese Knotweed much easier. Being able to identify Knotweed early makes a big difference when it comes to removing it. The plant spreads at a rapid rate and the longer it is left, the more harm it can cause due to the spreading of the plant through its underground rhizomes and becomes harder to control over time.
If you are unsure whether a plant is knotweed, it is always worth getting a professional opinion. Japanese Knotweed Killers offer fully qualified Inspection and removal services.
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Safe and legal Japanese knotweed removal
There are specific regulations surrounding the removal and disposal of the plant. The legal and safest way of removing Japanese knotweed is by enlisting a contractor to dispose of the plants, roots and soil to an approved landfill. Learn more about our services and costs here.
Unfortunately removing Japanese Knotweed isn't as easy as just digging it up and disposing of it like normal garden waste. Knotweed needs professional removal to ensure there is no chance of it regrowing. For this reason Knotweed can be a big risk for commercial properties. Commercial removals are usually a much bigger job as the plant will grow through an extensive underground rhizome system. Please get in touch with us if you need our commercial knotweed removal services, or need more information.
Did you know? Japanese Knotweed can grow to a height of 2 to 3 metres in just one season, and only 0.06g of root is needed for the plant to grow again, making it almost impossible to eradicate by digging up.
How can Japanese knotweed affect mortgages?
Mortgage lenders are legally able to refuse a mortgage on a property which has an infestation of Japanese knotweed. There are now detailed questions regarding invasive plant species on house sale documents. The seller must answer these truthfully and state whether there is a management plan in place to control the plant, if so a copy of this plan must be included with the property information form.
If Japanese knotweed is found on a property it can also affect the owner’s ability to obtain buildings insurance. Proprietors do not have to legally declare the presence of Japanese knotweed on a property to an insurer (unless asked), but they are obliged to control the plan and take the necessary preventative measures towards the weed damaging the building. (See RICS Guidance)
If the owner makes a claim and during inspection it is found that they have not done everything within their power to control the plant, an insurer has the right to refuse to pay out.
Different types of knotweed
Japanese knotweed - Fallopia japonica
The most common type of knotweed. They have spade shaped leaves and small creamy white flowers. They have one stem per node and have a zig-zag stem pattern.
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Giant knotweed is similar to Fallopia japonica but has larger leaves and is taller (up to 4.5 metres). Creamy white flowers appear in late summer/early autumn in dense panicles.
The leaves are pointed at the tip, heart shaped and grow to about 40 cm long and 27 cm wide.
Bohemian knotweed - Fallopia x bohemica
This is a hybrid between Japanese knotweed (Fallopia Japonica) and Giant knotweed (Fallopia sachalinensis). A tall herbaceous, rhizomatous perennial, with stems up to 3.5 m tall. Leaf blades of lower cauline leaves up to 25 x 17 cm.
Distinctive intermediate trichomes on the lower leaf epidermis are the best character for distinguishing it.
The leaves are larger than Japanese knotweed and heart shaped. Leaves are pointed with veins reddish purple when immature.
It has the same general growth form as Japanese knotweed, but the leaves are much larger and do not have the truncate bases so typical of Japanese knotweed.
Unlike Japanese knotweed, both sexes are found, in Ireland the hermaphrodites seem to outnumber the male-sterile plants. Clones extend by rhizome growth and may occupy considerable areas with some long established colonies in the West of Ireland and in Dublin
Florets are 1-2.5 mm long and functionally unisexual but with each male or female flower possessing the complementary but vestigial, organs of the other sex. Each floret has 5 petals and 8 stamens.
Dwarf Japanese knotweed - Fallopia japonica var. compacta
As the name suggests, Dwarf Japanese knotweed's are small and only reaches 1m – 1.2m in height. Dwarf knotweed is considered less invasive than Japanese knotweed.
Stems grow in nodes and grow in a zig-zag pattern
Their leaves have crinkled edges and a leathery texture with reddish veins. Leaves can vary in shape and are often in concave form.
White or pale pink flowers appear in late summer, which often mature to dark pink or red.
Himalayan knotweed - Persicaria wallichii
Himalayan knotweeds are less common in the Ireland. They have slender, elongated leaves and tapered to a point.
Can grow to a height of up to 1.8m and the stems are usually green with a zig zag shape from node to node.
Himalayan knotweeds have hairy stems and brown sheaths that persist at the basis of the leaf stalks.
Lesser knotweed - Persicaria campanulata
Lesser knotweed has hairy stems and brown sheaths that persist at the basis of the leaf stalks. The plant differs from Japanese knotweed with its long thin ovate shaped leaves and pink flowers.
The undersides of the leaves are much lighter in colour with small white hairs.
Lesser knotweed don’t have the distinctive zig zag shape between leaf stems that are found with Japanese knotweed and Giant knotweed variants.
Japanese knotweed is frequently misidentified because several common plants share similar leaves, growth patterns or climbing behaviour. Many property owners initially believe they have knotweed when the plant is actually a harmless species.
If you are unsure, a professional inspection is the safest way to confirm whether the plant is Japanese knotweed.
The following plants are commonly mistaken for knotweed.
Bindweed (Convolvulus arvensis)
Bindweed is a climbing vine often found growing through hedges, fences and gardens. Its leaves can resemble young knotweed leaves, but the plant behaves very differently. Unlike knotweed, bindweed climbs and twists around structures rather than growing upright bamboo-like stems. It also produces distinctive trumpet-shaped white or pink flowers.
Russian Vine (Fallopia baldschuanica)
Russian vine is a fast-growing climbing plant sometimes referred to as “mile-a-minute vine”. Because it grows aggressively and produces masses of small white flowers, it is sometimes confused with knotweed.
However, Russian vine grows as a climbing plant rather than forming thick upright canes like Japanese knotweed.
Himalayan Balsam (Impatiens glandulifera)
Himalayan balsam is another invasive species commonly found along riverbanks and damp ground.
It grows tall like knotweed, but the stems are hollow and smooth without the bamboo-like nodes. Himalayan balsam also produces distinctive pink or purple helmet-shaped flowers.
Dogwood (Cornus species)
Young dogwood plants can sometimes resemble knotweed shoots in early spring.
However, dogwood develops woody stems rather than hollow bamboo-like canes and produces clusters of berries later in the season.
Bamboo
Because Japanese knotweed stems resemble bamboo canes, bamboo plants are sometimes mistaken for knotweed infestations.
Bamboo grows in woody clusters with narrow leaves and persistent stems throughout winter, whereas knotweed dies back to brittle canes during the colder months.
When to Seek Professional Identification
Because several plants can resemble Japanese knotweed at certain stages of growth, misidentification is common.
If you suspect knotweed on your property or land, professional identification ensures the plant is correctly diagnosed and prevents unnecessary treatment or removal work.
Plants Often Mistaken for Japanese Knotweed
Identifying knotweed all year round
Knotweed will look different throughout the year. As the images below show, it is important to be able to identify what knotweed looks like as the seasons change.
The weed spreads faster
during the summer months
(June - August), where it can grow
up to 10cm a day.
In the autumn (September - November) as the weather turns colder, plants retreat leaving behind brown stems, although, new growth can continue as late as November.
Through the winter months (December - February) Japanese knotweed appears completely dead with brittle canes
littering the ground.
Starting in early spring time
(March - May), the plant will start
to show new shoots emerging from
around the crown.
Knotweed together with other alien invasive species, can be a major problem for property owners and developers, but is a treatable problem with the right specialist help. Japanese Knotweed Killlers will meet all legal obligations with specialist experience in dealing with invasive weed species.
Japanese knotweed Risk Categories (from RICS Guidance)
Category Descriptors
1. Japanese knotweed is within 7 metres of a habitable space, conservatory and/or garage, either within the boundaries of this property or in a neighbouring property or space;and/or Japanese knotweed is causing serious damage to outbuildings, associated structures, drains, paths, boundary walls and fences and so on. Further investigations by an appropriately qualified and/or experienced person are required.
2. Although Japanese knotweed is present within the boundaries of the property, it is more than 7 metres from a habitable space, conservatory, and/or garage. If there is damage to outbuildings, associated structures, paths and boundary walls and fences, it is minor. Further investigations by an appropriately qualified and/or experienced person are required.
3. Japanese knotweed was not seen within the boundaries of this property, but it was seen on a neighbouring property or land. Here, it was within 7 metres of the boundary, but more than 7 metres away from habitable spaces, conservatory and/or garage of the subject property.
4. Japanese knotweed was not seen on this property, but it can be seen on a neighbouring property or land where it was more than 7 metres away from the boundary.
Japanese knotweed can be a serious problem for home owners. There are no quick-fix herbicide treatments to ensure immediate Japanese knotweed eradication.
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Red Alga - Grateloupia doryphora
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Waterweeds - Elodea (all species)