How invasive weeds spread

To remove invasive weeds from your premises or to stop them from spreading, it helps to understand how new plants grow. This will help you to decide what action to take.

If you employ a contractor to do the work for you, you should understand what they intend to do and why. This could help you to decide what you actually need and could save you money.

Japanese knotweed does not spread from seeds. It is spread when small pieces of the plant or its roots (rhizomes) are broken off. One piece of root the size of a fingernail can produce a new plant.

Pieces of plant or roots can be transported to a new location by:

• the wind, particularly along roads and railways
• water, if the parent plant is close to a river or stream
• moving contaminated soil.

Individual plants can cover several square metres of land, joined up below ground by an extensive network of roots. Because of this, many herbicides and other chemicals will not kill off this plant. Even if the plant seems to be dying back at the surface, below ground the root network may be intact and able to produce more new growth soon after treatment.

Giant hogweed produces large, umbrella-like flowers, each of which can produce up to 50,000 seeds.

The seeds are heavy and fall within a few metres of the parent plant. Seeds can be transported by:

• vehicles, particularly along roads and railways
• water, if the parent plant is close to a river or stream
• moving contaminated soil.

The seeds can remain dormant in the soil for several years. Even if you treat the plants with herbicides and they die, several thousand seeds are waiting in the ground below for the opportunity to take their place.

In addition, giant hogweed contains sap that is released when the plant is cut or by brushing against the plant. Contact with the sap causes skin to become sensitive to sun-light, resulting in painful blisters which appear up to two days after contact and may last for several years.

Himalayan balsam plants can produce around 2,500 seeds each year. The seedpods open in such a way that the seeds are thrown several metres away from the parent plant, helping the species to spread. Seeds can also be transported by:

• water, if the parent plant is close to a river or stream
• moving contaminated soil.

Even if you remove these plants, or treat them with herbicides and they die, several hundreds of seeds are waiting in the ground below for the opportunity to take their place.