Guidance and Responsibility
What is the Government doing?
Government departments treat knotweed on the land within their jurisdiction but it can become re-infested from upstream. The whole of the Auldyn has been treated, as a catchment. DEFA advises both government and private landowners on the most effective methods of controlling knotweed. Successful control of this plant on the Isle of Man can only be achieved by the co-operation of all landowners. If you see any Japanese knotweed growing on central or local government owned land please report this to the relevant authority.
Whose responsibility is it to control knotweed?
It is the responsibility of the landowner or tenants of the land affected to treat and control Japanese knotweed.
How can you control Japanese knotweed?
Cultural (non-chemical) treatments
It is very difficult to remove large and long established infestations without the use of herbicides. Cutting every month during the growing season will eventually weaken the plant. However, it could take many years to fully eradicate the weed. Hand pulling is more effective than cutting, as it removes the crown and part of the rhizome. However the pulled stems and rhizome can easily take root and spread so the correct disposal of this material is important. Pulling can be used for small colonies in environmentally sensitive areas, such as alongside streams, and control should be achieved in 3 years. Stems should be pulled in June and July and laid on an impermeable surface until they are dry and brown and can be burnt. Shading out the knotweed using plastic sheeting with the addition of a mulch or bark chipping has been tried, but is less effective than chemical treatment. Digging out roots can be effective and quick but can be very expensive as even small pieces must be removed so large volumes of soil may need sifting. Livestock will eat it.
Chemical control by herbicides
Chemical control is the most successful treatment as it kills the extensive rhizome system, but even this takes several years to fully eradicate the plant. The most commonly used herbicide is glyphosate. Another herbicide that could be used is 2, 4-D Amine, which is more selective and will not affect grass. However, only glyphosate based herbicides are approved for use in or near watercourses, provided an appropriate preparation of the chemical is used. Care should be taken when applying glyphosate near trees and shrubs, as any spray drift may cause severe damage or kill the affected plant. Please read all product labels before use and always follow the manufacturer's instructions on safe use.
How to treat
Spraying is the most effective treatment but take care to avoid drift and damage to non-target plants such as neighbouring plants, shrubs and lawns. It is best to spray when there is only a very light wind and when the weather is likely to be dry for 24 hours afterwards. Take care on paved, waterlogged and steeply sloping areas to ensure that the herbicide run-off does not contaminate a watercourse. On paved or porous surfaces like gravel, care should be taken to ensure herbicide run-off does not seep through onto surface roots, as uptake may lead to the death of trees.
Weed wipers or impregnated weed gloves can be used for foliar application by applying the herbicide directly to the leaves.
Stem injection of herbicide into the lower part of mature stems using specialised equipment is the recommended solution for use in sensitive habitats such as around watercourses or around rare or protected plants because it is a more precise application method which reduces the risk of damage to nearby non-target species and habitats. Stem injection is more time consuming and may require assistance from a specialist but can be a very effective treatment method in small, targeted, areas.
It is important to follow the manufacturer’s instructions closely when using herbicides and to wear appropriate protective clothing.
When to treat
Best advice suggests threeapplications through the year – spring, mid-summer and late autumn. Spring application allows the herbicide to be applied when the plant is only 1m tall and there is plenty of leaf to absorb the chemical. By summer the plant could well be over three metres tall, making safe application to the leaves impossible. It may be necessary, therefore, to cut back the established growth (ensuring, safe disposal) before spraying, and again spraying the re-growth in late autumn when the plant is channelling nutrients back to the roots.
Year 1: For optimum effect, spray the plants with herbicide in April, June and September.
Year 2: Spray the plants as in year 1. Make sure that the affected area is marked out so that any small remaining plants can be found the following year.
Year 3: The growths are very tiny - a few centimetres in height - but they must be treated thoroughly again or the plant will re-grow.
Year 4 and onwards: Check the area for any re-growth and repeat treatment as necessary.
Application of herbicides near to streams and watercourses
Herbicides, which are not approved for use in or near watercourses, should not be applied within 5 metres of a watercourse (Code of Good Agricultural Practice for the Protection of Water).
Disposal of Japanese knotweed material
Japanese knotweed waste including the stems, leaves, rhizomes and crowns must be disposed of responsibly to prevent spread of this plant to new areas and to avoid committing an offence.
On-site burning is usually the most appropriate, but do not cause a nuisance to your neighbours or danger to road users. Consider the wind strength and direction before burning. Burn thoroughly.
- DO NOT bring to the amenity sites with your other garden waste
- DO NOT compost the stems or other parts of the plant
- DO NOT allow plant material to contaminate watercourses or other habitats
- DO NOT cut, strim or flail knotweed as this can spread cuttings
A Certificate of Competence is required for chemical treatments to commercial, agricultural and horticultural land. For further advice contact the DEFA Agriculture Team via email at email@example.com
Guidance for property developers
It is important to identify whether Japanese knotweed is present as it can grow through a metre of concrete and tarmac causing extensive structural damage and resulting in high costs. Recognition of stems, shoots and leaves should be possible with reference to the descriptions and photographs in this fact sheet. However, determining whether there are rhizome or crown fragments in the soil can be difficult. Look out for the carrot-like orange-red core of the rhizome and the hard brown crown from which the shoots grow.
If Japanese knotweed is found on the site:
- Cordon off the area where the knotweed is situated to prevent machinery accidentally spreading the material. Do not remove the knotweed material to another site unless sanctioned for legal disposal
- Do not disturb the soil at or in the vicinity of knotweed, unless for control purposes
- Treat the knotweed with an appropriate herbicide before work commences on the area. This can be done at any time when there are leaves on the plant
- Dispose of knotweed leaves and stems by drying and burning, on site if possible. Otherwise knotweed contaminated green waste and soil will need to be buried to a depth of at least 5 metres. The potentially viable knotweed material should then be covered with a geo-textile layer or a heavy gauge polythene sheet prior to infilling
- Remember that one herbicide treatment will not usually be sufficient and the knotweed is likely to re-grow for several years. Annual checks should be undertaken and repeat herbicide applications are likely to be necessary
This leaflet should provide all the information required to control the plant. However, further information and advice can be obtained from:
Japanese knotweed identification and control advice: DEFA Ecosystem Policy Team
Telephone: +44 1624 651577
Advice on other aspects of watercourse management: DEFA Inland Fisheries
Telephone: +44 1624 685587