How Japanese knotweed spreads
It spreads through
its crown, rhizome (underground stem) and stem segments, rather than its seeds.
The weed can grow a metre in a month and can cause heave below concrete and
tarmac, coming up through the resulting cracks and damaging buildings and
roads. Studies have shown that a 1cm section of rhizome can produce a new plant
in 10 days. Rhizome segments can remain dormant in soil for twenty years before
producing new plants.
What to do if you see Japanese
If you have Japanese
knotweed on your land you may be causing a private nuisance to surrounding
properties. Using our guidance you should control the Japanese knotweed to
prevent further spreading.
If Japanese knotweed
on a neighbouring property is causing a nuisance to you, we would always recommend
that you co-operate with the landowner and seek to control the problem
amicably, rather than resort to legal action.
This is an issue
under Common Law and the Environment Agency has no powers in this situation.
The Wildlife and
Countryside Act 1981 states that it is an offence to "plant or otherwise
cause to grow in the wild" any plant listed in Schedule nine, Part II of
the Act. This lists over 30 plants including Japanese knotweed, giant hogweed
and parrot's feather.
It's down to
landowners to control these plants, but they don't have to remove them.
However, causing the plants to spread by removing or disposing of them
incorrectly would be illegal.
The police are
responsible for investigating any offences and each police force has a wildlife
liaison officer who can be contacted. If the police can't take action, a civil
action may have to be taken against a person to ensure that the invasive plants
on their land are controlled.
How to control Japanese knotweed
If you wish to remove
the Japanese knotweed before the area is used for development you should
closely follow the advice we provide in the knotweed code of practice, or
employ a contractor to do so for you.
If you are using a contractor to remove the waste for you,
they must be registered with the EA as a waste carrier.
Chemical control
Japanese knotweed is sensitive to a range of herbicides. The
most effective time to apply herbicides to Japanese knotweed is in late summer.
This is much more damaging to the underground rhizome system than applying
herbicides in spring. 
A qualified person
should carry out the treatment and contractors must have a National Proficiency
Tests Council (NPTC) certification. The EA permission is needed before
herbicides can be used in or near watercourses.
Herbicide treatment
may have to be used for at least three years before Japanese knotweed stops
growing back. Even when the plant stops growing back, any soil removed from the
area is likely to have dormant rhizome and must be disposed of as described within
the knotweed code of practice.
Site trials have
shown that combining digging and herbicide treatment is more effective in
controlling Japanese knotweed than just applying herbicides to the plant.  This is because the digging breaks up the
rhizome, which stimulates leaf production, making the plant more vulnerable to
herbicide treatment.
Physical control
Cutting can be used to reduce underground biomass. Treating
fully-grown stems in late summer provides the most effective herbicide control.
If it is not possible to treat the mature stems (eg because of spray-drift),
the stems can be cut in the spring and the re-growth treated in late summer.
This reduces the height of the canes. The plant should be cut at the base of
the stem. Cutting methods that produce fragments, such as flailing, should be
avoided as just a small part of the stem can produce a new plant.
Studies have shown
that with four cuts a year the plant loses vigour and underground biomass. The
first cut should be carried out when the first shoots appear and the last cut
should be done when the plant before it dies back in the autumn (September or
October). Annual cutting will be required. Cut stems should be thoroughly dried
before they are burnt or taken to landfill.
Whenever Japanese knotweed
is moved from a site precaution must be taken to ensure that it doesn't spread
to other areas. It will also need to be disposed of properly
Further advice
If you would like further advice on Japanese knotweed please
contact us on our contact page.