Japanese knotweed, giant hogweed and other 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

Japanese knotweed begins to grow in early spring and can grow in any type of soil, no matter how poor. It can grow as much as 20 centimetres per day, and can reach a height of 1.5 metres by May and three metres by June. It does not produce viable seeds in the United Kingdom, but instead spreads through rhizome (underground root-like stem) fragments and cut stems. Japanese knotweed:

• 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

Identifying giant hogweed

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. It can take four years to reach its full height of three to five metres and flower. Giant hogweed:

• 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
• has flower heads that are usually 50 centimetres wide - each flower head is capable of producing 50,000 seeds every year
• has seeds that can stay in the soil for several years before they develop

Identifying Himalayan balsam

Himalayan balsam is often found on river banks, growing up to two metres in height. Each plant lasts for one year and dies at the end of the growing season. Himalayan balsam:

• has reddish coloured stems
• has dark green, lance-shaped leaves with jagged edges
• flowers from June to October
• has large, brightly coloured flowers that are 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

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
• Canadian pondweed
• water fern (also known as fairy fern)