Treating invasive non-native species (INNS)

What are non-native invasive species?

Over 2,000 plants and animals have been introduced to Britain from all over the world by people. These are known as non-native species. Most are harmless, but around 10-15% spread and become invasive non-native species. 

Invasive non-native species have become an issue in the UK due to direct or indirect introduction by humans. These species are introduced from other parts of the world and can quickly harm wildlife and the environment, are costly to the economy and can even impact on our health and way of life.

Why are they a problem? 

Impacts on the environment

Invasive non-native species are capable of causing extinctions of native plants and animals, reducing biodiversity, altering habitats and even increase the risk of flooding. 

Impacts on our health and way of life

In the UK invasive non-native species threaten the survival of native wildlife and damage our natural ecosystems by preying on or out-competing other plants and animals, disrupting habitats and ecosystems, and spreading harmful diseases.

Impacts on the economy 

Invasive species cost the British economy close to £2 billion a year. Costs include damage to buildings and infrastructure, interference with the production of food and materials, losses to other activities such as tourism and navigation, and high management costs for established invasive species. Japanese knotweed alone is estimated to cost the GB economy around £250 million per year.

The most problematic invasive species in Wirral are: 

  • Himalayan balsam
  • Giant hogweed 
  • Japanese knotweed

Himalayan balsam

Himalayan balsam with pink flowers

Where would you find it living?

Found in riverbanks, waste land and damp woodlands but can also invade gardens and allotments.

What are the impacts?

  • each plant can produce up to 800 seeds. These are dispersed widely as the ripe seedpods shoot their seeds up to 7m (22ft) away
  • Himalayan balsam can completely take over river banks and woodland, crowding our native species preventing them from growing
  • it can also increase the chances of soil erosion, danger of landslides and risks of flooding

How do you remove Himalayan balsam?

pink balsam weeds

Hand-pulling or cutting Himalayan balsam plants well in advance of its flowering and as long as it is cut down below the last node is an effective method, especially for small infestations. Ensure that you remove the entire plant, including the roots, to prevent regrowth. Wear appropriate protective clothing, including gloves, to avoid contact with the sap.

When should the removal process take place?

Schedule removal efforts before the plant reaches the seed-setting stage. This is typically in early summer, before the seed pods mature and explode, spreading the seeds. By preventing seed production, you can limit the plant's ability to reproduce and spread.

How do you dispose of Himalayan balsam?

Collect and bag the pulled or cut Himalayan balsam plants for proper disposal. Do not compost them, as the seeds can remain viable and germinate.

Do you need to monitor the treatment of Himalayan balsam?

Yes. Himalayan balsam has a high seed production rate and can quickly re-establish itself if seeds remain in the soil. Continuously monitor the treated areas and promptly remove any regrowth. Multiple years of treatment may be necessary to exhaust the seed bank and eradicate the plant.

How do you report a sighting of Himalayan balsam on council land?

Please report to with a photograph and specific location.

Giant hogweed


Where would you find it living?

It is generally found near riverbanks and in damp meadows, and it can also be found on waste ground and roadsides.

What are the impacts?

It is highly invasive. Each plant can produce up to 50,000 seeds which can survive for up to 15 years. Contact with any part of this plant must be avoided as even minute amounts of sap can cause blistering of the skin following exposure to sunlight. 

Other negative impacts include outcompeting native flora, river bank erosion and increase in flood risk. Can cause delays and additional costs on development sites where the plant must be removed as controlled waste in order to comply with legislation.

How is giant hogweed treated?

Treatment of hogweed on council land is usually carried out in June and July when it is in full growth. Qualified staff use a spray containing Glyphosate, with a follow up treatment in September if needed. After 7 to 10 days the plants will wilt and die off.

giant hogweed green leaves

Can you prevent the spread of giant hogweed?

In some cases, cutting or mowing the plants before they produce seeds can help reduce their spread. However, this method should only be conducted by professionals wearing appropriate protective gear, as sap can still be present and cause harm.

What should you do if you have giant hogweed growing on your property?

If you have giant hogweed on your property, it is recommended that you hire a professional exterminator to remove it. The plant will be removed safely and as few seeds as possible will be spread.

How do you dispose of giant hogweed?

Giant hogweed is a controlled waste so it can only be disposed of in licensed landfill sites with the required documentation.

How do you report a sighting of giant hogweed on council land? Please report to with a photograph and specific location.

What to do if see giant hogweed growing by a road or in a park

Report giant hogweed to the council

What to do if the hogweed is growing on private property such as a golf course or residential area

If giant hogweed is located on land or property in Wirral the council will inform the land owner or tenant.

Japanese knotweed

Where would I find it living?

Found in riverbanks, waste grounds and roadside verges, beds and borders where it prevents native species from growing.

Japanese knotwood close up

What are the impacts?

This invasive plant thrives on disturbance.

  • the smallest piece can grow and spread by natural means and human activity
  • it can soon overrun riverbanks, railway embankments, road verges, gardens and hedgerows
  • it is a threat to the survival of native plants, insects and animals
  • it exploits week areas in structures if left unchecked, causing damage to hard surfaces, block drains and worsen cracks in building infrastructure
  • it can sometimes grow from seed too, this means that if Japanese knotweed is growing in a neighbouring garden it could likely come into your garden, either by its spreading roots or by seed

What is the removal process of Japanese knotweed?

green leaves of Japanese knotwood under shade

Cutting and digging are not effective methods for controlling established Japanese knotweed and take many years to have any effect. Japanese knotweed is best controlled by the application of a suitable herbicide. Glyphosate-based herbicides are commonly used to treat Japanese knotweed.

Read our policy on treatment of Japanese knotweed

Does Japanese knotweed need continuous monitoring?

Yes, Japanese knotweed can remain dormant underground for several years, so consistent monitoring is crucial even after treatment. Regular inspections should be conducted to identify and promptly address any regrowth or new infestations.

How do you report a sighting of Japanese knotweed on council land?

Please report to with a photograph and specific location.

How you can help

Wirral Council are keen to raise awareness that invasive non-native species are one of the top drivers for global biodiversity loss. The management of invasive species is a shared problem that requires cooperation and support of the general public:

  1. stop the spread: keep your plants in your garden, don’t plant or allow them to grow in the wild
  2. know what you grow: choose the right plants for your garden, pond and water features
  3. compost with care: dispose of your unwanted plants, roots, weeds and seeds responsibly
  4. do your research: look up plants before you buy them to make sure they are suitable for your needs, easy to dispose of, and won’t be invasive

Read comprehensive advice on how to deal with invasive non-native weeds on GOV.UK

How to report sightings on council land

The council is only required to undertake treatment of Invasive Non-Native Species where it is present on council owned land. If you are concerned about any possible Invasive Non-Native Species on Wirral Council land, please report it to with a photograph and specific location.