Invasive Species

Japanese Knotweed

alt=Rhododendron

Himalayan Balsam (Impatiens glandulifera)

An annual plant that appears in the early summer and grows rapidly to form a tall sappy stem with serrated leaves and pinkish flowers. The seed pods explode when touched and release shiny black seeds.

Himalayan Balsam escaped into the wild over a hundred years ago and is now a very common plant through much of Britain. It has undergone a dramatic increase in population in Snowdonia over the last twenty years. River banks are the most favoured habitat but any areas of damp and disturbed ground are readily colonised.

The tall vigorous growth outcompetes and displaces native herbs and grasses.

Control Strategy

Himalayan Balsam is now so widespread that eradication is currently not feasible. Control on sites with a particular conservation interest is worthwhile. Local control should also be undertaken where there are isolated colonies in otherwise uninfected areas, especially where they are near to water courses. Targeted control of this sort will slow down the spread of the plant.

Himalayan Balsam should not be grown in gardens.

Control

The plant is an annual. Seeds remain viable in the ground for only a year or two. The key to success is to break the cycle and prevent the plant from setting seed for at least three years. Various methods are possible:

  • Hand weeding  (before seed is set – typically early July in Snowdonia))
  • Cutting by scythe or strimmer (just before flowering starts – typically late June)

Herbicides

Grazing and trampling by cattle and sheep.

Bare soil provides ideal conditions for the seeds to germinate. Where Himalayan Balsam is present, excavation, ditch digging and ploughing should be avoided. Poorly targeted herbicide application can also worsen the problem by creating areas of bare ground

For further details see:

www.nerc-wallingford.ac.uk/research/

www.cornwall.gov.uk