Japanese knotweed, giant hogweed and other invasive plants

Clearing the leaves and stems of Japanese knotweed or giant hogweed that are above ground and then removing soil contaminated with roots, rhizomes (underground root-like stems) and seeds can provide faster results than just spraying with herbicide.

Try to minimise the amount of waste you generate that contains invasive plants, or their seeds and rhizomes. Any waste you do produce should be treated on site where possible.

Any waste that is taken off site must be taken by a licensed waste carrier and must go to a suitably authorised landfill site.

Plant material can be buried deep on the site where it is produced, as long as it does not interfere with the water table - see burying invasive plant material on site.

You should not remove soil from river banks, as this can cause water pollution. If you are planning to carry out work near a river you should contact the Northern Ireland Environment Agency (NIEA).

If your site floods, the seeds will be spread further across the site, so you will need to manage a larger area.

Digging up giant hogweed

To clear ground contaminated with giant hogweed, you may need to remove soil up to four metres away from the plants and to a depth of 0.5 metres. You will need to check for regrowth regularly. You should spray regrowth with the herbicide glyphosate before the plants flower.

Digging up Japanese knotweed

The rhizome system beneath a stand of Japanese knotweed can be over four metres deep and could extend for at least seven metres around the stand. If you are going to dig out the rhizome system you will need to remove all of the plant material. You should use the rhizome identification guide in the knotweed code of practice, or ask a specialist, to help you identify the plant material.

You will need to check any cleared areas regularly for regrowth. You can spray any regrowth with herbicide.

Digging up Himalayan balsam

To clear ground contaminated with Himalayan balsam, you may need to remove soil up to six metres from the parent plant and to a depth of 0.5 metres. You should not remove soil while the seed pods are present. You will need to check for regrowth regularly. You should pull by hand or strim regrowth before the plants flower.

However, as seeds remain viable in the soil for several years, annual cutting, mowing or grazing or annual herbicide treatment during the spring growing season can be an effective control for this plant. You must also carry out follow up checks for late germinating seeds.

What you must do when digging up invasive plants

Never stockpile contaminated soil or plant material within ten metres of a watercourse, and it should not be stockpiled within seven metres of your site boundary.

Collect any water you use for cleaning vehicles that are used in contaminated areas. If it is contaminated with seeds or plant material, you must not discharge it to a watercourse. You could treat the water by passing it through a settlement tank to remove any soil before passing it through a very fine mesh sieve to remove seeds or plant material. Settlement alone may not be adequate because seeds and plant material float.

You may be able to deposit material sieved from water used for vehicle washing in a controlled area on your land and monitor it for regrowth. You should speak to the NIEA to determine your best option.

See preventing water pollution and discharging trade effluent.

Good practice tips when digging up invasive plants

• You should clearly mark out any areas of your land that contain invasive plants. Fence them off until you intend to clear them. Put the fence at least seven metres away from the plants to contain any contaminated soil or roots.
• When you clear contaminated areas, take care to ensure contaminated soil, seeds and plant material are not spread to unaffected areas.
• Limit the use of tracked machinery where possible. Seeds and plant material can get caught in the tracks and moved around the area.
• If you are developing your land, consider creating a haul road using a strong geotextile overlain with hardcore as a base for vehicles to travel on.
• Cover all lorries, dumpers or haulage vehicles carrying contaminated soil or plant material.
• Thoroughly clean tracked machinery when it leaves contaminated areas of the site. Do this within a designated area that is as close as possible to the contaminated area on which the machinery has been working. Always carry out a visual inspection of wheel arches and tracks before the vehicle leaves the site.
• Look out for regrowth by roads and areas where vehicles have been parked or cleaned. Spray any regrowth with herbicide if required.
• If you are working between November and March in an area where invasive plants are known to be present, look for dead canes from the previous year to identify infected areas. Even if there is no growth evident above ground, seeds from giant hogweed and Himalayan balsam, and Japanese knotweed rhizome will still be present. Try to inspect a development site for evidence of invasive species before it is cleared.