There are a few species that you need to be aware of along our waterways. We actively manage these on a regular basis with volunteers where possible (Himalayan balsam) or through stem injection in to the more noxious plants (Giant hogweed) and invasive plants (Japanese knotweed).
Giant hogweed
You can help us in our eradication of this plant by letting us know if you spot it. There is a similar native plant called Hogweed which does not cause the same problems and therefore we do not need to treat it in the same way. You do not have to be wary of it. Use this ID sheet to help you spot the difference.
This short video explains some of the dangers of Giant hogweed and why it is such a big problem along river systems.
If you are unfortunate enough to get some of the sap on your skin, don’t panic, cover the infected area immediately to keep out of sunlight, wash the area thoroughly with lots of soapy water and if any irritation is experienced seek medical attention.
Do not let this plant prevent you from enjoying our waterways. As long as you are aware of the plant and do not allow the sap on you it will not be a problem.
Himalayan balsam
Other Names: Touch-me-not; Policeman’s Helmet; Indian Balsam; Jumping Jack
Himalayan Balsam is a plant native to the Western Himalayas. It was introduced deliberately in 1839 by Victorian plant hunters (owing to its very attractive flowers).
It is now found throughout Britain, particularly along riverbanks and on damp ground. The plant is very fast growing, reaching up to 3m in height and is reputed to be the tallest annual plant currently found in the UK. The seed-pods are ‘dehiscent’, i.e. when they’re mature, they explode when touched, scattering the seed up to 7m away! The seeds can remain viable for up to 2 years and can be dispersed by water, animals and on the soles of shoes.
Why is it a problem?
Each plant can produce up to 800 seeds and a dense stand can produce 30,000 seeds per m2. It has previously spread through the UK at a rate of 645km2per year. Its popularity with bumblebees may lead to other (native) flowers being overlooked. This could potentially have a detrimental impact on our flora long-term. Dense stands of Balsam can shade out other flora beneath, thereby suppressing their growth and reducing the diversity of wild flowers in the immediate area. Balsam is an annual plant which can displace other soil-binding, perennial plants. When it dies back in autumn, bare areas of soil left behind may be more prone to erosion.
How you can help
Throughout the summer we run a number of events to remove this plant. This culminates in the 3 Rivers Clean Up. This is an annual event, where volunteers from all over south-east London and beyond help to pull up the Balsam by its shallow roots. This manual removal eliminates the need for chemical treatment and allows surrounding wildflowers to be left unaffected. The plants are pulled before the seeds are produced (to avoid further spread) and the stems can then be composted nearby.
You can follow the work they and others are doing at their Twitter feed.
Pulling Himalayan Balsam in late season
The video below shows how to remove Himalayan Balsam late in the season, in cases where it is flowering and been allowed to set seed. The plant is popular with bee keepers as a late source of nectar, but advice from the British Beekeepers Association it should only be kept in gardens and cut before it sets seed.
Tom Morgan from Wiltshire Wildlife Trust advises exactly how to remove the plant and prevent seed dispersal.
Other invasive species
For more information on the invasive species in London, you can visit the London Invasive Species Initiate (LISI) website. The London Invasive Species Initiative is a subgroup of the London Biodiversity Partnership, and is hosted by Greenspace Information for Greater London. LISI brings together organisations from a range of sectors to deliver practical action to prevent, control and eradicate invasive non-native species in London. LISI encourages co-ordination and partnership working to prevent, reduce and eliminate the impacts of invasive non-native species across Greater London.
They also have a current list of species of concern and read the London Invasive Species Plan. You can also read their list of species of concern.
Useful ID Sheets for invasive species
Chinese Mitten crab (Eriocheir sinensis)
Floating Pennywort (Hydrocotyle ranunculoides)
Giant Hogweed (Heracleum mantegazzianum)
Himalayan Balsam (Impatiens glandulifera)
Japanese Knotweed (Fallopia japonica)
Parrots Feather (Myriophyllum aquaticum)
Rivers and People 