Japanese knotweed, giant hogweed and other invasive plants

Taking plant material and soil containing plant material away for disposal off-site uses valuable landfill capacity and increases the likelihood of the spread of invasive plants. Another option is to bury this soil and plant material on your own land because, without sunlight, plants cannot survive and seeds will not germinate.

However, this material will need to remain buried for several years to ensure that it will not grow again. Giant hogweed seeds can be viable for up to 15 years and Japanese knotweed rhizome (underground root-like stems) is believed to survive for 20 years.

What you must do

Soil and plant material containing Japanese knotweed may need to be buried five metres below ground level. You should place a barrier membrane on top of the material and fill the hole with clean soil. Alternatively, the knotweed code of practice describes how you can bury it less deeply.

Soil containing Himalayan balsam and giant hogweed seeds should be buried at least one metre below ground level.

You must not bury anything other than plant material and soil containing invasive plants that have originated on site.

You must make sure that deep burial does not interfere with the groundwater level.

Buried soil and plant material that have been treated with a herbicide that does not break down in the environment could cause groundwater pollution. If you intend to bury treated material, you should treat it with glyphosate herbicide only. Check with the Northern Ireland Environment Agency (NIEA).

Herbicides that do not break down in the environment are described as persistent. Those that do break down are described as biodegradable or non-persistent. The herbicide packaging or safety data sheet will state whether it is persistent or non-persistent.

Soil contaminated with some persistent herbicides will be classed as hazardous and so will need to be disposed of as hazardous waste - see dealing with hazardous waste.

You must follow the guidelines for spraying plants with herbicide and digging up plants - see spraying invasive plants with herbicide and digging up invasive plants.

Good practice for burying invasive plants

You should bury the material in an area where it is not likely to be disturbed. You should keep records of the quantity of material that you have buried and a map showing the location of the burial pit and its depth. Use signs to mark the burial pit and keep heavy-tracked machinery off the area.

You should not bury materials deeply within seven metres of an adjacent landowner's site.