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Knotweed Identification

Knotweed Seasonal Identification

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.


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?

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 of 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).

Knotweed Growing Through Tarmac
Japanese knotweed growing through hard standing
Japanese knotweed growing through concrete house foundations
Japanese knotweed growing inside house
Japanese knotweed growing through brick wall
Knotweed Identification Guide (5 Species)
Japanese knotweed quick Identification Card

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 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 Japanese Knotweed waste is transported by a licensed waste carrier. 


How can Japanese knotweed affect mortgages?

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.

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 knotweedFallopia 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. 

JKC - Japanese Knotweed Crown (Labelled) - Blk Txt v7.png
Giant Knotweed Leaf
Giant Knotweed Flower

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

Bohemian knotweed (Fallopia x bohemica)

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 veinsLeaves 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.

Dwarf Knotweed Leaf and Flower

Himalayan knotweed - Persicaria wallichii 

Himalayan knotweeds are less common in the Ireland. They have slender, elongated leaves and tapered to a point. 

Himalayan knotweed (Persicaria wallichii) - Leaves
Himalayan knotweed (Persicaria wallichii) - Leaf & Flower

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.

Himalayan knotweed (Persicaria wallichii) - Stem

Lesser knotweed - Persicaria campanulata

Lesser knotweed (Persicaria campanulata) Flower

Lesser knotweed has hairy stems and brown sheaths that persist at the basis of the leaf stalks. Lesser Knotweed is one of the least common knotweeds and differs from Japanese knotweed in its long thin ovate 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.

Lesser knotweed (Persicaria campanulata) Leaves

Identifying knotweed all year round

Knotweed will look different throughout the year. As the images below show, it important ant 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 in spring time

(March - May), the plant will start

to show new shoots emerging from

around the crown. 

Knotweed Seasons 5.png

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.

RICS Guidance

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. 

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.

  1. American skunk-cabbageLysichiton americanus

  2. Brazilian giant-rhubarbGunnera manicata

  3. Broad-leaved rushJuncus planifolius

  4. Cape pondweedAponogeton distachyos

  5. Cord-grassesSpartina (all species and hybrids)

  6. Curly waterweed - Lagarosiphon major

  7. Dwarf eel-grassZostera japonica

  8. FanwortCabomba caroliniana

  9. Floating pennywortHydrocotyle ranunculoides

  10. Fringed water-LilyNymphoides peltata

  11. Giant hogweedHeracleum mantegazzianum

  12. Giant knotweedFallopia sachalinensis

  13. Giant-rhubarbGunnera tinctoria

  14. Giant salviniaSalvinia molesta

  15. Himalayan balsamImpatiens glandulifera

  16. Himalayan knotweedPersicaria wallichii

  17. Hottentot-figCarpobrotus edulis

  18. Japanese knotweedFallopia japonica

  19. Large-flowered waterweedEgeria densa

  20. Mile-a-minute weedPersicaria perfoliata

  21. New Zealand pigmyweedCrassula helmsii

  22. Parrots featherMyriophyllum aquaticum

  23. Red algaGrateloupia doryphora

  24. RhododendronRhododendron ponticum

  25. SalmonberryRubus spectabilis

  26. Sea-buckthorn Hippophae rhamnoides

  27. Spanish bluebell Hyacinthoides hispanica

  28. Three-cornered leekAllium triquetrum

  29. WakameUndaria pinnatifida

  30. Water chestnutTrapa natans

  31. Water fernAzolla filiculoides

  32. Water lettucePistia stratiotes

  33. Water-primroseLudwigia (all species)

  34. WaterweedsElodea (all species)

  35. WireweedSargassum muticum

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