What Is Japanese knotweed ? (Fallopia japonica) 


Japanese knotweed was introduced into Britain in the 19th century as an ornamental plant. Over time it has become widespread in a range of habitats, including roadsides, riverbanks and derelict buildings. It outcompetes native plants and animals and its shoots can cause extensive damage to roads, paths and underground services.

The plant produces green/red shoots in early spring which can reach a height of 3 metres before dying back in autumn. Its stems can range between 2-3m in height; stems are green with red or purple specks and form dense cane-like clumps. The plant's shoots and leaves, which can be up to 120mm long, can cause very dense coverage of colonised areas. The plant spreads underground by means of rhizomes (a root-like underground stem). Rhizome fragments can produce new plants. However, Japanese Knotweed cannot spread by seed.

If you have Japanese knotweed on your land you may be causing a private nuisance to surrounding properties and treatment must be undertaken.


Can it be cut down?


Cutting it down may only work in a small and defined area and will require several years of persistent cutting to be sure of removal.  It may not stop the underground rhizome from spreading. The stems do not regenerate if cut and piled up, but small sections of the stem base/rhizome can root again and must be disposed of carefully. 


Will burning it down kill it?


Burning knotweed down will not kill it, it will only destroy the above growing part of the plant, it will regrow shortly afterwards and may encourage the plant to spread.


Can I pull/dig it out?


Digging out is possible on a small scale but unless done thoroughly it cannot be guaranteed to be successful. Additionally the root/rhizome fragments and associated soil are classed as 'controlled waste' so cannot be removed from the site, except to licensed disposal locations.

If digging out is required, it is recommended that this is carried out by a licensed contractor to dispose of all the soil and plant matter that is removed. Even when the plant stops growing back, any soil excavated from the area is likely to contain dormant rhizome and must be disposed of as described within the knotweed code of practice.

Digging out (and improper disposal) has been one of the major causes of Japanese knotweed spread.

Rather than digging out, it is recommended that the plant is treated with herbicide, cut off above ground once dead and left in situ until dry.  The dead stems can then be burnt onsite in accordance with the Environment Agency guidelines Exemption D7 – Burning Waste in the Open or removed from site for incineration.


Herbicide Treatment


The most effective method of control is by herbicide application close to the flowering stage in late summer or autumn.  All above-ground portions of the plant need to be controlled repeatedly for several years in order to weaken and kill the entire patch. Picking the right herbicide is essential, as it must travel through the plant and into the root system below. Glyphosate is the best active ingredient in herbicide for use on Japanese knotweed as it is ’systemic’; it penetrates through the whole plant and travels to the roots. 

Herbicide can be applied using a foliar spray which will be absorbed by the leaves and taken down to the rhizome or directly injected into the stems.  Both methods have advantages and disadvantages


Foliar Spray


With this method, a solution of between 2% - 5% Glyphosate is applied as a coarse spray evenly over the whole plant.  However, Glyphosate is a non-selective herbicide and therefore kills, or has a detrimental effect on all surrounding plants including grass.


Stem Injection


This method targets only the knotweed as 3ml of Glyphosate is injected directly into the stem, between the second and third leaf node.  Only stems at least 12mm in diameter can be treated.  This method is particularly useful where surrounding planting is to be preserved, is generally more effective than spraying and is not effected by the weather.


Management Plan


Following the Japanese knotweed code of practice produced by The Environment Agency, a management plan should be drawn up.  This will detail the extent of the problem on the site, all procedures to be followed and which method(s) of eradication will be employed over the projected treatment cycle which could be up to 3 years.  In the case of mature infestation, the treatment period may be longer.


Free Quotation


Let Davidia provide you with a free quotation to carry out all necessary works to rid your garden or development of Japanese Knotweed.  All operatives are fully qualified to deal with this problem having been trained to the appropriate standard by the NPTC.