Control of Knotweed
Control of Japanese Knotweed relies on the death of the extensive rhizome system, which usually takes a number of years. So far in Devon there are no records that the plant has produced seed so treatment of the existing plants is the key to the problem in the county.
Methods of spread
In the UK Japanese Knotweed has not yet been recorded as producing viable seeds, however, hybrid crosses can produce viable seed. So far all Japanese Knotweed plants recorded in the UK are female and all male plants have been shown to be hybrids. Fertile hybrids will add a whole new dimension to the control of the spread of Japanese Knotweed which currently only reproduces vegetatively.
The plant does have an extraordinary ability to spread vegetatively from crown, stem and rhizome (underground root). Even tiny amounts of cut stem, crown or rhizome are capable of producing a new plant. Controlling spread is therefore dependent on preventing the spread of stem, crown or rhizome.
Rhizomes grow rapidly underground and are responsible for the spread of the plant on site. They produce long white shoots at the apices of the rhizome which send up shoots to the surface.
If the rhizome is cut it will produce a shoot. Digging or other disturbance is known to increase stem density. If soil contaminated with rhizome is moved to another part of the site or to another site it will regrow and cause spread. Rhizome is particularly resistant to dehydration and freezing. As little as 10 mm or 0.7 gm of rhizome can regenerate into a new plant.
Crown and stems are also capable of regenerating and even small fragments of cut crown or stem are capable of regenerating and becoming a new plant. If the plants are cut back it is important to dispose of the crown and stems properly. Once stems are thoroughly dried they are unable to regenerate.
It is important that an effective Japanese Knotweed management programme is established including, where possible, herbicide treatment otherwise the plant will inevitably spread. All cut or pulled stems of Japanese Knotweed should be treated with extreme care as they can potentially re-sprout and cause spread. They should be kept on site, or disposed of in a licensed landfill site that can carry out deep burial (by prior arrangement) to prevent spread onto other sites. Treatment of colonies on riverbanks should be treated as soon as possible because bank erosion can lead to plant material breaking off and dispersing downstream,
As stems, crowns and rhizomes readily regenerate, they must be allowed to dry out thoroughly after they have been pulled or cut, this can be helped by putting the material onto a plastic sheet, or raised wooden platform, rather than on the ground. Regular checks should be made to ensure that this material is not contaminating watercourses or other sites, or developing roots. Thoroughly burning plant material on site after cutting and drying, where current by-laws allow, can be an effective means of disposal provided that the waste is burnt on site and not removed to other land.
Ensure that machinery, tools and work clothes are free of fragments of Knotweed before leaving the site. Tracked vehicles, off-road tyres, tools and even work boots can harbour fragments of Knotweed and could potentially cause spread to another site. These items should be thoroughly cleaned before leaving a Knotweed contaminated site.
Japanese Knotweed material is regarded as ‘controlled waste’ and not disposing of it properly would be an offence under the Environmental Protection Act, 1990. Allowing the spread of Japanese Knotweed into the wild is also an offence under the Wildlife and Countryside Act 1981.
For further information see:
Code of Practice
- Environment Agency – Prevent Japanese Knotweed from spreading
- Environment Agency – Prevent harmful weeds and non-native plants from spreading
Methods of Control
NB This information is not designed to be specific but to raise awareness of what you need to consider and be aware of before using herbicides. The Devon Knotweed Forum provides this information in good faith and cannot be held responsible for any problems arising from the interpretation and use of this advice.
The use of herbicides is often the most effective option for the control of Japanese Knotweed but there are issues that need to be considered first.
The main issue is whether the stand is near water or not. Consent is required from the Environment Agency if herbicides are to be sprayed near water and only two products are approved (see below). It is advisable to contact 08708 506 506 if you are considering using herbicides near a watercourse and they will be able to advise you on your options.
There are health and safety issues and those administering herbicide need to be properly trained and hold a Certificate of Competence. Herbicides should only be used in accordance with the advice on the label otherwise it is considered an offence. It is also an offence to use products that are not approved. Check that the herbicide formulation is approved for the intended use. Furthermore there is an obligation to avoid the spread of herbicide to non-target areas so spraying needs to be carried out in dry weather (no rain for 6 hours, preferably 24), spraying should also not be carried out in wind speeds above Force 2 on the Beaufort Scale to avoid spray drift.
Japanese Knotweed can become very tall which makes it difficult to apply herbicides. The plants should ideally be sprayed in the spring when they are about 1m tall or after cutting when they have re-grown to this height. There are various methods of application including tractor-mounted spraying for large areas; knapsack spraying for small areas; lance sprayer for tall stands or for stands in inaccessible places (such as steep slopes and river banks); controlled droplet application; injection method for small stands and weed wiper or herbicide gloves for direct application onto leaves of specific plants.
Dense stands of Japanese Knotweed can be treated with a glyphosate-based herbicide, such as ‘Roundup Pro Biactive’. Glyphosate is a systemic herbicide which acts by blocking a plant’s enzyme system. The herbicide is absorbed through growing leaves and stems where it is translocated throughout the plant and root network. It kills virtually all annual and perennial weeds including grasses. Glyphosate is quickly broken down in soil or sediment and is harmless to animal life. Not all formulations containing Glyphosate are approved for use in or near watercourses under the Control of Pesticides Regulations 1986.
If the Japanese Knotweed is sparsely distributed, use a 2,4-D amine preparation such as Dormone, which is specific to broadleaved plants and will not harm the grasses. It may take two or three years to completely kill the entire plant. More effective control can be achieved if Japanese Knotweed is cut or sprayed in early summer, then sprayed again in late summer, just before the winter dieback.
More targeted methods of applying herbicides are being developed for sites where it is important to protect the native flora. This includes using a weed wiper to apply the herbicide directly to the leaves of the plant rather than spraying or injecting herbicide directly into the plant. This ensures that only target plants are treated.
The plants should be treated between March and October when they are in the growing phase. Plants can take up to 6 weeks to show signs of die-back. There are two main groups of herbicides and the one you choose will depend on the site. The main recommended herbicides are:
For environmentally sensitive habitats and those near water
For habitats not near water
* Not all herbicides containing these active ingredients are suitable for use in or near water. Use of herbicides in or near water requires consultation with the Environment Agency.
There are detailed fact files on these products in The Japanese Knotweed Manual by Lois Child and Max Wade. You can also telephone Environment Agency’s Ecological Appraisal Team for Devon on 08708 506 506 for further information and advice on aquatic situations. Specialist contractors can also advise.
For further information see:
Cutting, Mowing or Pulling
Regular pulling will, after a number of years, eventually exhaust the rhizome and kill the plant. This is only an effective method of control if it is carried out continually over a number of years and is only effective on small or newly established stands. Cutting and mowing can be used to prevent spread, to reduce vigour and to reduce underground biomass and are useful to use before applying herbicide but not as the sole control method. With all these methods the main problem is the safe disposal of the cut or pulled stems to prevent spread.
It is important that all cut or pulled stems of Japanese Knotweed are kept on site, or disposed of in a licensed landfill site that can carry out deep burial (by prior arrangement). Burning of fresh growing material has little effect on the plant.
Cutting – can be carried out with loppers, cutters, hooks, scythes, bladed cutters such as those with a metal circular blade or strimmers with a metal blade. Do not use bladed cutting equipment such as brush cutters or strimmers near a watercourse (even ditches) as fragments of Knotweed could enter the water and spread downstream. Cut material should be collected up, then dried out and burnt on site or removed and taken to a licensed landfill site. Ensure workers use protective clothing and face visor and they clean equipment before leaving the site and to avoid leaving fragments of Knotweed spread around the site.
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 is at its most luxuriant in late summer but before it dies back in the autumn (September/October). Annual cutting will be required.
Cutting can also be used to reduce plant height before chemical treatment. Allow a re-growth of 2-3 ft before spraying to be most effective.
You should monitor the extent of the Knotweed stand to make sure it is not spreading sideways as there is evidence that cutting can cause the rhizome to spread laterally.
Regular (fortnightly) mowing in parks, gardens and on road verges is a good way of controlling the growth of Japanese Knotweed, however, if mowing is stopped then the plant can take over the site again. It is not known how long the rhizomes persist in the soil. Avoid using a flail mower as this may result in small, viable fragments of stem regenerating in neighbouring, non-infested areas. It is preferable to use a mower with a collecting box for Knotweed areas. Mowings should be collected up and left on site. Compost should be checked for re-growth.
Pulling as a method of eradication is only really useful when treating small or new infestations where only a few stems have established. It is an excellent method to use on sites with native or sensitive species growing and where the use of herbicides is undesirable (e.g. on wildlife sites or riverbanks) as it specifically targets Knotweed plants. Care should, however, be taken to avoid trampling valuable flora in the vicinity.
Stems should be pulled regularly when they reach full height. They should be pulled near the base to include some rhizome. Control of a small infestation could be achieved in 3 years but this method requires regular, sustained treatment to work.
You must ensure that pulled stems are disposed of correctly to avoid spread.
Japanese Knotweed is used as animal fodder in the Far East and here in Britain it is known that cattle, sheep, horses, donkeys and goats graze the plant. Animals prefer the young shoots as they emerge in the spring and after about June the stems become rather woody. Grazing may reduce shoot densities and height but will not eradicate it. Although grazing can help reduce the spread into uninfested areas it is not a method of control. Dead stems should be cut back in winter as these can deter grazing in the spring. Continued grazing will ensure the supply of new shoots throughout the growing season.
Grazing is therefore not an eradication tool but is helpful in suppressing the plant and reducing spread.
There is scope for the introduction of insects or fungi from the original native areas of Japanese Knotweed that could help manage the plant but this control method would have to undergo extensive trials and will not be available for some years.
Summary of non-chemical control methods
|Removal of dead stems||Cutting with loppers, cutter, hook or scythe||Winter||Annually|
|Preparation of site prior to chemical treatment||Cutting with loppers, cutter, hook or scythe. Allow 2-3 ft of re-growth before spraying||March - October||Annually|
|Preventing invasion of an adjacent stand of Japanese Knotweed into amenity grassland||Mowing or grazing||Throughout the growing season (March – October)||Mow every 2 weeks. Allow livestock to graze throughout the growing season|
|Reducing the vigour of the plant||Cutting or mowing||March - October||Four times a year|
|Removing individual stems of Japanese Knotweed in mixed vegetation||Pulling||All year||As new shoots appear|
|Reducing plant height prior to chemical treatment||Cutting||March – August|
Allow regrowth to attain a height of 0.5 – 1.0 m (2-3 ft) before application of herbicide
(from The Japanese Knotweed Manual)
Note: Digging of established Japanese Knotweed stands will lead to a significant increase in stem density. The cut rhizome is capable of regenerating, even small fragments are capable of producing a new plant.
For further information see Knotweed Do’s and Don’ts
Methods of Knotweed Control – Case Studies
Below is a list of case studies underlining some of the different approaches to Japanese knotweed control. These programmes have been undertaken in Devon by members of the Devon Knotweed Forum.
- Knotweed control in gardens – Bicton College Gardens:
Bicton College has extensive grounds and gardens which are open to the public. Within the grounds there are three stands of Japanese knotweed which are probably the remains of deliberate planting in the past when it was regarded as an attractive exotic species.
Control of the Japanese knotweed has been focused on one particular stand which was threatening to move across a ha-ha wall into adjacent farmland. If this had been allowed to progress it may have also damaged the historic wall itself. Because the knotweed was located away from other desirable plants the stand was sprayed with Glyphosate. The plant was first sprayed when it had grown to between 1.2 and 1.5 metres tall, then the treatment was repeated ten days later. The stand was then left for the rest of the growing season until the leaves were about to turn brown. At this stage the plant was then sprayed again, with a repeat spraying ten days later.
The stand has now been reduced to a few individual stems which will continue to be treated in subsequent years. As the stand has been sufficiently reduced, stem injection becomes a more attractive action.
- Community involvement in knotweed control – Lee, North Devon:
Japanese knotweed removal can benefit from a concerted effort from local landowners meaning community projects can be very successful. The project in Lee has treated 23 sites in the area involving 18 different landowners and demonstrates the community principal.
The project for Lee and Lincombe originated with a public meeting identifying the spread of knotweed as a significant problem. A steering committee was established and the Environment Agency was consulted. As knotweed treatment by professional contractors can be very costly the committee decided that they would organise a treatment programme themselves. A training programme was established to promote skills within the local community which resulted in six people gaining the necessary qualifications to undertake chemical treatment work. Gaining the trust of local landowners was also vital as it allowed for widespread take-up of the programme. Free knotweed treatment was offered but many made a donation to augment funding from North Devon District Council and North Devon AONB. These sources paid for training, equipment and the chemical needed to undertake the work.
Depending on the nature of the Japanese knotweed stand, a variety of control methods have been implemented including cutting and burning, spot and knapsack spraying, weed wiping and stem injection. Some key lessons have been learnt:
- Engage with the whole community to develop trust.
- Involve and communicate with the community throughout the programme.
- Persuade landowners to sign agreements committing them to the knotweed initiative as this provides certainty when planning how to progress the programme.
- Engage with the Environment Agency
- Multi-agency knotweed control – The Exmoor Knotweed Control Project
The Exmoor Knotweed Control Project is a collaboration between the Environment Agency, English Nature, The National Trust and Exmoor National Park Authority, supported by both Devon and Somerset Highways Authorities. The overall aim of the project which began in 2000 is to control and eventually eradicate knotweed (Japanese, Himalayan, giant, and associated hybrid species) from within the National Park and its associated river catchments. There are currently over 680 known knotweed sites on Exmoor, covering a total area of over 8000m². The plant is found in a range of locations and habitats, including riverbanks, hedges, roadside verges, and gardens.
The details of known knotweed sites are reported, recorded and mapped. The landowner is then contacted to determine if they would like the site to be treated as part of the Project. The site is surveyed and where the knotweed is growing on or near a watercourse, the Environment Agency is contacted on behalf of the landowner to obtain consent to carry out treatment with herbicides. Summer survey work initially concentrated on the Lyn and Heddon river catchments, but over the last few years it has been extended to cover the whole of the National Park.
Currently the most effective method of control is repeated spraying with herbicides over a number of years, which gradually reduces the vigour of the plant. With funding from the project partners, a qualified contractor is employed to carry out a programme of herbicide spraying. Many sites have now greatly reduced in size and vigour but are still treated every year to prevent re-growth or spread
If you have Japanese Knotweed don’t forget to send records to Devon Biodiversity Records Centre using the on-line recording form. Please note the Devon Biodiversity Records Centre is not responsible for controlling Japanese Knotweed.