Digging up invasive weeds and spraying regrowth with herbicide

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

To clear ground contaminated with giant hogweed, you may need to remove soil up to 4m away from the plants and to a depth of 1.5m. You will need to check for regrowth regularly. You should spray regrowth with herbicide before the plants flower.

The root system beneath a stand of japanese knotweed can be over 4m deep and could extend for a number of metres around the stand. If you are going to dig out the root system, you should use a specialist to help identify the root system as you dig.

You will need to check any cleared areas regularly for regrowth. You can spray regrowth with herbicide or pull up and burn any new shoots.

Himalayan balsam can throw its seeds as far as 6m from the parent plant, so removing contaminated soil is not an option.

However, as seeds remain viable in the soil for around three years, annual cutting, mowing or grazing or annual herbicide treatment during the spring growing season can be an effective control for this plant.

Never stockpile contaminated soil or plant material within 10m of a watercourse.

Collect any water you use for cleaning vehicles that are used in contaminated areas. If it is contaminated with seeds or plant material, you may not be able to 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. Speak to your environmental regulator to determine your best option.

Clearly mark out areas of your site that contain invasive weeds. Fence them off until you intend to clear them. Put the fence at least 3m away from the plants to contain any contaminated soil or roots.

Before you clear contaminated areas, fence off any land that is not contaminated with invasive weeds. This will reduce the spread of seeds or contaminated soil 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 the site, 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.

Look out for regrowth by roads and areas where vehicles have been parked or cleaned. Spray these roads and areas with herbicide if required.

If you are working between November and March in an area where invasive weeds are known to be present, then dead shoots from the previous year can help to identify infected areas. Even if there is no growth evident above ground seeds from giant hogweed and Himalayan balsam, and Japanese knotweed roots (rhizomes) will still be present.