How invasive plants spread
Guidance to understand specific ways in which different invasive plants can spread and multiply, and how to stop them.
To remove invasive plants from your premises or to stop them from spreading, it helps to understand how new plants grow and spread. This will help you 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 decide what you actually need and could save you money.
How Japanese knotweed spreads
Japanese knotweed does not spread from seeds in Northern Ireland. It is spread when small pieces of the plant or rhizomes (underground root-like stems) are broken off. One piece of rhizome or plant the size of a fingernail can produce a new plant.
Pieces of plant or rhizome can be transported to a new location by:
- water - if the parent plant is close to a river or stream
- moving soil which contains them
- fly-tipping cut or pulled stems
Individual plants can cover several square metres of land, joined up below ground by an extensive rhizome network. Herbicide treatment can be a very effective way of controlling Japanese knotweed, but a lack of regrowth does not mean the underground rhizome is dead. If the soil is disturbed, knotweed often regrows.
How giant hogweed spreads
Giant hogweed produces large, umbrella-like flowers, each of which can produce up to 50,000 seeds. These seeds fall typically within four 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
- footwear
- moving soil which contains them
The seeds can remain dormant in the soil for 15 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. Any control programme needs to continue for several years, including checks for new growth. When managing giant hogweed it is important to maintain a healthy grass sward, either by using selective herbicides or by sowing grass mixes. A dense grass sward helps to prevent giant hogweed seeds from germinating.
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 sunlight, resulting in painful blisters which appear up to two days after contact and may reoccur for several years.
How Himalayan balsam spreads
Himalayan balsam plants can produce around 2,500 seeds each year. The seedpods open in such a way that the seeds are thrown up to seven metres away from the parent plant, helping the species to quickly spread. Seeds can also be transported by:
- water - if the parent plant is close to a river or stream
- tracked vehicles
- footwear
- moving soil which contains them
Even if you remove these plants, or treat them with herbicides and they die, several hundred seeds can be waiting in the ground below for the opportunity to take their place. The seeds can survive for several years, so any control programme needs to continue for a couple of years, including checks for new growth.