Invasive Weeds
Invasive weeds are plant species that take over habitats, decreasing their wildlife value and eradicating native plant species. If allowed to grow unchecked, they can destroy the characteristic Chilterns natural landscapes.
In Chesham, three plants are known to be invading the area:
Japanese Knotweed
This highly invasive plant forms dense stands with hollow stems that can reach 3 metres tall. Its leaves are large, green and heart-shaped. Towards the end of August, clusters of cream flowers can be seen. For assistance in identifying this plant, you can download a leaflet from the Non Native Species Secretariat.
Japanese knotweed is causing huge problems across the country, displacing native vegetation and causing structural damage to properties. It is against the law to plant, or otherwise cause to grow, Japanese knotweed in the wild. If you allow knotweed to spread from your land into the wild, you are breaking the law.
Responsibility for knotweed control lies with the landowner or tenant of the land. Knotweed has to be controlled carefully as even tiny amounts of cut stem, crown or underground stem can re-grow and help spread this weed. Knotweed must not be strimmed, flailed or mown, do not chip knotweed material or add knotweed to compost, as it will cause it to spread. The herbicide glyphosate is an effective way to control knotweed, but if you intend to use a herbicide in or near water, you need to obtain the approval of the Environment Agency first. It is also important to follow manufacturer’s instructions regarding protective clothing and safe herbicide use. For further advice on controlling knotweed, contact the Environment Agency on 03708 506 506 (Mon-Fri, 8am-6pm).
Chesham Town Council is working with the Chilterns Conservation Board to map this weed. If you have seen these plants growing in Chesham, please contact [email protected]. We are aware of the large stand of knotweed near Canon’s Mill along Waterside.
A large infestation of Japanese knotweed had been growing on Town Council land at The Moor in Waterside. In 2010, the Council initiated a successful three-year control programme with a specialist contractor to wipe out this stand using the technique of glyphosate stem injection. This method was chosen as it has minimal environmental impact for the surrounding land. Further treatment was necessary in 2023, owing to the spread of knotweed from a neighbouring property.
Himalayan Balsam
Himalayan balsam rapidly takes over riverbanks and areas of damp ground. It can grow up to 3 metres tall, with pinky-red stems and spear-shaped, dark green leaves with serrated edges. Flowers are pink and slipper-shaped and appear from June to October. To find out more about this species, visit the Non Native Species Secretariat website.
This weed has already caused major problems on other chalk streams in the UK, such as the Dorset Frome. It suppresses the growth of native plants and leaves bare river banks in autumn after the balsam has died back, which are then liable to erosion. In 2010, the legal status of this plant changed, to reflect the serious problem that it is causing. It is now against the law to plant or otherwise cause this plant to grow in the wild. Therefore, if you are growing Himalayan balsam in your garden and you do not prevent it from spreading into the wild, you are breaking the law.
Himalayan balsam has been found growing at several points along the Chess over the years and was spreading rapidly. Chesham Environmental Group volunteers have done amazing work over the last decade to wipe out large stands by hand pulling the weeds. It is important to deal with Himalayan balsam in Chesham, as this is where the Chess starts, in order to prevent further infestations developing downstream. Get in touch with the Environmental Group if you would like to find out more, or get involved.
It is important to kill this plant before it flowers, by hand-pulling, cutting, mowing, strimming or by herbicides. If the plant is handled when it is flowering, you are more likely to help it to spread as the seed heads explode showering the area with seeds.
Orange Balsam
Quite similar in appearance to Himalayan balsam, the plant produces orange flowers between July and September. You can view images of this plant by searching for Orange Balsam on the Non Native Species Secretariat website.
Chesham Town Council is working with the Chilterns Conservation Board to map this weed. If you have seen this plant growing in Chesham, please contact [email protected].
Invasive Animals
Citrus Longhorn Beetle
DEFRA is warning gardeners to be on the look-out for this beetle, originally from South East Asia. The beetle has been found in nurseries and private gardens in the UK and is a pest of a range of trees and shrubs, especially maples. However, the Citrus Longhorn Beetle also attacks beech, birch, hawthorn, hazel, horse chestnut, plane, poplar, oak and willow. Visit the Non Native Species Secretariat website for more information.
Harlequin Ladybird
The most invasive ladybird in the world, the Harlequin Ladybird, has arrived in Chesham. Originally from East Asia, this ladybird was accidentally introduced to the UK in 2004 and has been spotted in Chesham.
The Harlequin has a big appetite, eating a wide range of foods, including butterflies, moths and other ladybirds – which means that the Harlequin could endanger native insects.
Mink
The spread of American Mink in the River Chess catchment contributed to a dramatic population crash of the native water vole, as mink are voracious predators. The Chilterns Chalk Streams Project and BBOWT implemented a successful water vole recovery project, which included trapping and controlling mink on the river. Mink are still being monitored for along the length of the river to ensure that a population does not re-establish. People are urged to be vigilant for the presence of mink and to report any sightings to BBOWT via their web site.
