Species checklist - plants

Himalayan balsam by Chris WoodHimalayan balsam by Chris Wood

Japanese knotweed
Fallopia japonica

Perennial plant, typically about 2m tall, with large roughly triangular leaves. The stem is bambo-like in appearance and can grow up to 10cm a day. It produces white flowers in September/October. Usually found in sunny urban areas, river banks and waste ground.

Now widely recognised as the most invasive plant in Britain, Japanese knotweed can grow through concrete, get up to 13ft high and chokes off all other plant life. It can regenerate from just a small amount of root or stem and is very difficult to control.

The Environment Agency's website has some useful pages for understanding the management requirements of this species and other difficult INNS.


Himalayan Balsam
Impatiens glandulifera

Also known as Indian balsam. It is an annual herb with reddish-translucent hollow stems which can grow over 2.5m tall. It has hairless oval leaves, up to 18 cm long and 7 cm wide and deep purplish-pink to white flowers with a short spur, helmeted upper petal and strong balsam smell. Usually found in moist and semi-shaded damp places, mainly on riversides by slow-moving watercourses. Seeds float, spreading downstream and can remain viable in the soil for several years.

Himalayan balsam can rapidly colonise bare ground (up to 20 metres in all directions per season) and dense stands of it can out-compete native species. It is an annual plant with shallow roots and when it dies back in winter can leave river banks bare and more prone to erosion, leading to habitat damage and potentially increasing the likelihood of flooding.

Have a look at our handy leaflet on the management of this species.


Giant hogweed
Heracleum mantegazzianum

Member of the cow-parsley family. Has flowering stems typically 2-3 m (up to 5m) high bearing umbels of flowers up to 80 cm in diameter. The basal leaves are often 1m or more in size. Usually found by lowland rivers and on waste land.

Giant hogweed became a problem when it was found to be highly poisonous. It contains a toxic sap which causes skin to blister after exposure to sunlight. Affected skin may remain sensitive for several years. The plant is also a vigorous competitor, producing almost pure stands which exclude native vegetation. Please do not attempt to control this plant yourself as serious personal injury could result.


New Zealand Pygmyweed
Crassula helmsii

Perennial with yellowish-green succulent leaves less than 2cm long and solitary white or pale pink flowers. It grows in ponds, lakes, reservoirs, canals, ditches and damp mud. It may cover small ponds to a depth of 0.5m (blocking out light) or cover the margins and bed of larger deeper waterbodies over many square metres. New Zealand pygmyweed can prevent use of the waterways and may cause extensive declines in native plants.


Floating Pennywort
Hydrocotyle ranunculoides

This has fleshy stems with roundish bluntly toothed leaves, which creep through other vegetation, over soil or over the surface of the water. Greenish flowers are without petals. It's usually found in still or slow-flowing water in lakes, ponds, streams, ditches and canals. Floating pennywort grows to such an extent it precludes growth of other aquatic plants, obstructs movements of animals and boats, preventing navigation and recreational use of watercourses.


Rhododendron ponticum

Rhododendron is a densely branched evergreen shrub, growing to 5m. Flowers, in clusters of 10-15, range in colour from violet to purple. Its found on acidic soils in moorland, woods, screes, rocky banks, derelict gardens and streamsides. Its dense thickets shade out indigenous plants, preventing tree regeneration and obliterating the ground vegetation on moors. It is poisonous to livestock and cannot be controlled by grazing. It is host to the disease organisms Phytophthora ramorum and P. kernoviae, which attack oak and beech.