Identifying invasive plants

It is important that you can identify invasive plants on your premises. This will allow you to manage and deal with them in the most appropriate way.

Identifying invasive plants on a site early lets developers assess and cost options for destroying, disposing of and managing them.

Managing land infested by invasive plants in a timely and appropriate way can avoid:

  • excessive cost
  • potential prosecution and compensation claims
  • physical damage to buildings and hard surfaces
  • harm to the environment.

Identifying Japanese knotweed

Photo credit: © GBNNSS

Japanese knotweed begins to grow in early spring and can grow in any type of soil, no matter how poor. It does not produce viable seeds in the UK, but instead spreads through rhizome (underground root-like stem) fragments and cut stems.

Japanese knotweed:

  • can grow as much as 20 centimetres per day, and can reach a height of 1.5 metres by May and 3 metres by June
  • produces fleshy red tinged shoots when it first breaks through the ground
  • has large, heart or spade-shaped green leaves
  • has leaves arranged in a zig-zag pattern along the stem
  • has a hollow stem, like bamboo
  • can form dense clumps that can be several metres deep
  • produces clusters of cream flowers towards the end of July
  • dies back between September and November, leaving brown stems

Invasive Species Northern Ireland: Japanese knotweed

GB Non-Native Species: Japanese knotweed

SEPA: On site management of Japanese Knotweed and associated contaminated soil

Identifying giant hogweed

Photo credit: © GBNNSS

You should take great care when identifying giant hogweed. Contact with the plant, particularly the sap, can lead to severe blistering and scarring.

Giant hogweed closely resembles native cow parsley or hogweed but can grow very much larger.

Giant hogweed:

  • can take four years to reach its full height of up to 5m
  • has a reddish purple stem with fine spines that make it appear furry - like a stinging nettle
  • has hollow stems
  • has spotted leaf stalks
  • has leaves up to 1.5 metres wide
  • flowers in June and July on an umbrella shaped flowerhead made up of hundreds of small white flowers
  • has flower heads that are usually 80cm’ centimetres wide - each flower head is capable of producing 50,000 seeds
  • has seeds that can stay in the soil for several years before they develop.

Invasive species Northern Ireland: Giant Hogweed

GB Non-Native Species: Giant Hogweed

Identifying Himalayan balsam

Photo credit: © GBNNSS

Himalayan balsam is often found on river banks.

Each plant lasts for one year and dies at the end of the growing season. Himalayan balsam:

  • can grow up to 2 metres in height
  • has reddish coloured stems
  • has dark green, lance-shaped leaves with serrated edges
  • flowers from June to October
  • has brightly coloured flowers that are 2.5-4cm long and usually in variable shades from purple to pale pink
  • can produce around 2,500 seeds per plant each year
  • has explosive seed pods that can throw seeds over 6 metres away from the plant.

Invasive species Northern Ireland: Himalayan balsam

GB Non-Native Species: Himalayan balsam

Identifying other invasive plants

Other species of invasive plants in the UK include:

  • floating pennywort
  • parrot's feather
  • creeping water primrose
  • New Zealand pigmyweed (also known as Australian swamp stonecrop)
  • curly waterweed
  • nuttall's waterweed
  • water fern (also known as fairy fern).

Further information

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