How invasive plant species spread

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 the UK. 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
  • Unsuitable management i.e. strimming to create new plant fragments
  • Plant fragments transported by unwashed vehicle tyres, tools, machinery and footwear
  • 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 4 metres of the parent plant. Seeds can be transported by:

  • seeds transported by unwashed vehicle tyres, tools, machinery and footwear -
  • water - if the parent plant is close to a river or stream
  • 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 7 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
  • seeds transported by unwashed vehicle tyres, tools, machinery and 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 two to three years, so any control programme needs to continue for a at least this period, followed by a five year monitoring programme.

Further information

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