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Japanese Knotweed Identification
To get a better understanding of knotweed, follow our guide to quickly identifying the different types of knotweed 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.
The weed may appear harmless, but it can break through poor quality concrete and tarmac causing damage to building foundations and retaining wall structures.
This will not only damage the stability of a residence or workplace, but it will also seriously depreciate the property’s value, hinder mortgage loan applications and may affect insurance companies insuring a property infested with Japanese knotweed.
How to identify Japanese Knotweed?
There are different types of Japanese knotweed and during each season their appearance can change, which makes identifying them problematic even for a qualified surveyor. It is important to be able to identify Japanese knotweed when purchasing a property and the extent of the infestation.
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.
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The law and Japanese Knotweed
Generally having any invasive species listed under annex 2 of the S.I. No. 477/2011 - European Communities (Birds and Natural Habitats) Regulations 2011, on your property does not break the law.
If the plant spreads to a neighbouring property or public land, it becomes a criminal offence and could result in a fine and/or inprisonment, depending on the severity of the offence.
Did you know? Japanese knotweed can lie dormant underground for up to 20 years, and it can take up to five years for poison to kill it completely.
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.
As a new plant can grow from a root the size of a finger nail; it is vital that all stages of Japanese knotweed removal are completed properly, and any knotweed waste is transported by a licensed waste carrier.
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.
As a new plant can grow from a root the size of a finger nail; it is vital that all stages of Japanese knotweed removal are completed properly, and any knotweed waste is transported by a licensed waste carrier.
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 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 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.
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
One of the most common types 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.
Japanese knotweed is a highly invasive species and listed under the European Communities (Birds and Natural Habitats) Regulations 2011, which makes it illegal to spread this species.
It is frequently found on roadsides, riverbanks, brownfield sites, and urban areas in Ireland. Prefers moist, well-drained soils, often thriving in disturbed areas.
Japanese knotweed can cause significant ecological and structural damage. It is a highly resilient plant with its extensive rhizome system making it very difficult to eradicate, and enabling it to spread rapidly in disturbed areas from very small fragments.
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'sare small and only reaches 1m – 1.8m in height.
Their leaves have crinkled edges and a leathery texture.
The leaves 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.
It 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 knotweed have hairy stems and brown sheaths that persist at the basis of the leaf stalks.
Lesser knotweed - Persicaria campanulata
Lesser knotweed are less invasive but still pose a problem. They can grow to a height of 60-90cm.
The flowers are tiny, pale pink bell shaped and produced in clusters on short spikes.
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.
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.
Starting in early spring time
(March - May), the plant will start
to show new shoots emerging from
around the dead 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 Killers will meet all legal obligations with specialist experience in dealing with invasive weed species.
European Communities (Birds and Natural Habitats) Regulations 2011 non-native invasive plant species A-Z (Updated 2017)
There are currently 35 invasive plant species listed in the European Communities (Birds and Natural Habitats) Regulations (annex 2, Part 1)...
Click on a species from the following list to find out more regarding non-native species subject to restrictions under Regulations 49 and 50.
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American Skunk-Cabbage - Lysichiton americanus
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Red Alga - Grateloupia doryphora
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Waterweeds - Elodea (all species)
Non-Native Plant Species identified as High Risk on Ireland's Biodiversity List...
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Risk score
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