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What does it look like?
Images A-H below show what Japanese knotweed looks like below and above ground throughout the year.
Where: A = dense rhizome (root) growth at 1-2m depth; B = detail of rhizome growth; C = cross section through living
rhizome (note orange colour); D = dormant stem buds in early spring; E = developing stem buds in spring; F = late
spring stem growth; G = full growth in mid summer; H = dead stems during winter dormancy.
Scientic name: Reynoutria japonica var. japonica (previously Fallopia japonica)
Other Schedule 9 Species: Dwarf knotweed, Giant knotweed, Bohemian (hybrid) knotweed
What is Japanese knotweed?
Japanese knotweed is the most common of four invasive
knotweed taxa present in the UK (Japanese and Dwarf
knotweeds being varieties of the same species):
1a Japanese knotweed (Reynoutria japonica var. japonica)
1b Dwarf knotweed (R. japonica var. compacta)
2 Giant knotweed (R. sachalinensis)
3 Bohemian (hybrid) knotweed (R. × bohemica)
Where do invasive knotweeds grow?
Japanese knotweed is able to grow in most (if not all) soil
conditions found in the UK, though these species show
a strong preference for man-made habitats. Common
habitats where invasive knotweeds are found include
roadsides, waste ground, railway embankments and
cuttings, mine spoil-tips and along rivers and streams.
Public information on invasive species in Wales
Japanese knotweed
How do invasive knotweeds spread?
Invasive knotweeds are long-lived (rhizome-forming
perennial), competitive dominant plants that spread by
direct rhizome (root) growth and from leaf, stem and
rhizome fragments of the parent plant. Pieces of rhizome
smaller than 1cm in length can grow into a new plant.
Legislation
Because of negative impacts on the UK environment
and economy, all invasive knotweeds are listed under
Schedule 9 of The Wildlife and Countryside Act
1981 which means it is an offence to plant or cause
these plants to grow in the wild. Also, knotweed plant
material is a ‘controlled waste’ under the Environmental
Protection Act 1990 which means it can only be
transported by a registered waste carrier to suitably
permitted or exempt sites. Transfer notes shall be kept
for each load for a minimum of 2 years.
Identication - Japanese knotweed
Identication - distinguishing between invasive knotweeds
Key identication features
Invasive knotweeds look quite similar, though there are clear differences in the leaves and stems in particular.
All invasive knotweed stems have distinctive nodes (stem segments); one leaf is held at each node, attached
alternatively upwards along the stem (i.e., the leaves are not paired opposite one another at each node).
Common
name Japanese knotweed Dwarf knotweed Giant knotweed Bohemian (hybrid)
knotweed
Height 2-3m in height Up to 1m in height Up to 4m in height 2.5-4 m in height
Lower leaf
shape & size
10-15cm long Up to 10cm long Up to 40cm long Up to 25 cm long
Lower leaf
description
Broadly oval, attened
at the base – feels
quite tough to the
touch with a smooth
lower side
Broadly oval, attened
at the base – feels
quite tough to the
touch with a smooth
lower side
Broadly oblong, heart-
shaped at the base –
feels quite delicate with
a furry lower side
Intermediate leaf
features – lower
surface of the leaf
often feels quite coarse
Stems
Red ecking Red ecking No red ecking Some red ecking
Flower colour
& growth
Creamy white,
branched
Creamy white,
unbranched
Greenish white,
branched
Creamy white,
branched
Flowering
period August-October August-October August-September August-September
Invasive knotweed lifecycle - control and management
Research
In 2018 Jones et al. published the rst set of results
from the world’s largest ongoing eld-trial on Japanese
knotweed control.
The research tested a number of methods of chemical
and physical (mechanical) control. Results were unable
to show that physical methods were effective and that,
of the plant protection products currently available, use
of the herbicide glyphosate at the time when the plant
is most vulnerable to its effects, is the most effective
means of controlling invasive knotweeds. Note that the
growth stages in the diagram below are approximate and
dependent upon invasive knotweed species, weather
conditions and altitude.
Importance of the knotweed lifecycle
Timings of invasive knotweed growth stages are shown
below along with recommended timings for treatment.
Results of the Jones et al. (2018) research found that
glyphosate-based herbicides are highly effective only
if they are applied from mid-summer into late autumn
(depending on the weather). This is when resources start
to be drawn down from above ground growth into the
rhizome; if these herbicides are applied sooner in the
year, herbicide is not transported down into the rhizome
and there is little to no impact on above ground growth in
subsequent years.
Timing of invasive knotweed growth stages and treatment application
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Growth stages: approximate - species, weather and altitude dependent
Bud development
Stem growth
Green leaves
Flowering
Die back
Dormancy
Treatment application - glyphosate-based herbicide
Biannual foliar spray First spray Second spray
Stem injection
Annual foliar spray
Don’ts and Do’s of knotweed treatment
Note
This list of Don’ts and Do’s for knotweed treatment is
based on research by Jones et al. (2018).
Don’ts
• Don’t consider undertaking knotweed treatment yourself
unless you have the appropriate skills and experience.
There are anecdotal reports of mortgages being declined
as a result of inadequate/poorly undertaken treatment.
• Don’t think there is a quick x – invasive knotweeds
are resilient weeds that cannot be controlled by
herbicide in a single year – any treatment strategy
should be long-term and target both the above ground
and extensive below ground parts of the plant.
• Don’t undertake treatment methods which are ineffective
and risk spreading invasive knotweeds further. Physical
control methods were shown by Jones et al. (2018) to be
the least effective of the treatments tested and were also
the most likely to result in further spread. Physical control
methods include: cutting using hand tools, strimmer,
mower, ail, thrasher; pulling; surface covering with
geomembrane/geotextile; composting and burning.
• Don’t try digging out the knotweed yourself, it is easy to
miss parts of the rhizome and spread it and you cannot
dispose of this plant material along with your garden
waste – in fact, it is illegal to do this.
• Don’t use unsuitable herbicides – results of the Jones
et al. (2018) research showed that of the herbicide
products currently available, glyphosate applied at the
time when invasive knotweeds are most vulnerable to its
effects is the most effective control treatment. Anyone
using professional herbicides must hold a recognised
professional training certicate.
• Don’t use animals to control knotweed by grazing –
previous research has shown that this does not control
invasive knotweeds.
• Don’t try untested methods of control – these do not work,
but are likely to make the problem worse.
Contractor accreditations
When looking for a contractor, the following accreditations
and registrations are recommended:
• Amenity Forum Membership
• BASIS Professional Register
• BASIS Amenity Training Register
• BASIS Nominated Storekeeper (NSK) Professional
Register
Membership of one of the following trade associations is
recommended:
• Property Care Association (PCA)
• Invasive Non-Native Specialists Association (INNSA)
Do’s
• Ignore the hysteria – Jones et al. (2018) showed that
knotweed can be controlled using glyphosate at the right
time of year, though this can usually take three to ve
years.
• For control and management of invasive knotweeds,
based on the research by Jones et al (2018), it is
recommended to use one of the following control methods:
1. Biannual foliar spray – half of maximum permitted
application rate (see product label) applied twice per year.
2. Stem injection – maximum permitted stump treatment
application rate (see product label) applied once in the rst
year of treatment; however, it used signicantly (15 times)
more herbicide than either foliar spray treatment.
3. Annual foliar spray – maximum permitted application
rate (see product label) applied once per year; performed
almost as well as biannual foliar spray and stem
injection, but signicantly less labour intensive and
more sustainable in the long term.
Please note that this leaet is for information only. If you have any specic legal concerns
regarding Japanese knotweed, we recommend that you take legal advice.
Copyright statement
Images contained in this document are copyright of Advanced Invasives Ltd -
© Advanced Invasives Ltd | 2020. All rights reserved.
Any redistribution or reproduction of images contained in this document in any form is
prohibited other than the following:
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use only
• you may copy the content to individual third parties for their personal use, but only if you
acknowledge this document as the source of the material
You may not, except with our express written permission, distribute or commercially exploit
the images. Nor may you transmit or store the images in any other website or other form of
electronic retrieval system.
Don’ts and Do’s of knotweed treatment
• Monitoring - check for signs of regrowth and if this
appears, seek expert advice. The images below show
deformed Japanese knotweed regrowth following
treatment with glyphosate-based herbicides.
• Once knotweed has been effectively controlled using
herbicide don’t disturb this land by digging, for
example, as it is likely to come back; even if the above
ground parts of the plant are dead, the below ground
rhizome system may not be.
• If you need to remove knotweed from your land quickly or
need to stop it spreading below ground on to neighbouring
land, contact a suitably qualied contractor to undertake
physical remediation (e.g. installation of barrier
membranes).
• Use the right control methods – it is an offence to cause
invasive knotweeds to grow in the wild and they can cause
a private nuisance. This means that allowing actions
which cause the spread of knotweed from your land may
constitute a criminal offence and/or may give rise to liability
under a private nuisance claim.
What does knotweed treated with herbicide look like?
Images I-K below show what deformed Japanese knotweed regrowth looks like following treatment with glyphosate-
based herbicides.
Sources cited
1 Advanced Invasives Caerphilly CBC (2019) 2 Akeroyd BSBI (2014) 3 Bailey et al. Biol. Invasions (2009) 4 Bailey & Conolly Watsonia (2000) 5 Beerling et al. J. Ecol (1994) 6
Brabec & Pyšek Folia Geobot. (2000) 7 Environment Agency (UK) (2013) 8 Gerber et al. Biol. Conserv (2008) 9 Grime John Wiley & Sons Ltd (2001) 10 HM Government (2015)
11 Hocking et al. BSBI (2019) 12 Jones PhD Thesis (2015) 13 Jones et al. Biol. Invasions (2018) 14 Jones & Eastwood Pest Manag. Sci. (2019) 15 Stace C&M Floristics (2019)
This information leaet has been produced
for the Welsh Government by
Advanced Invasives Ltd
Cite as:
Jones D (2020) Public information on invasive species in Wales:
Japanese knotweed. Welsh Government.