Advice
RHS Help & Advice
Himalayan balsam control
Himalayan balsam (Impatiens glandulifera) is a relative of the busy Lizzie and is known by a wide variety of common names, including Indian balsam, jumping jack and policeman's helmet. It is a tall, robust, annual producing clusters of purplish pink (or rarely white) helmet-shaped flowers. These are followed by seed pods that open explosively when ripe, shooting their seeds up to 7m (22ft) away. Each plant can produce up to 800 seeds.
Introduced to the UK in 1839, it is now naturalised, especially on riverbanks and increasingly in waste places and has become a problematical weed. Himalayan balsam tolerates low light levels and, in turn, tends to shade out other vegetation, impoverishing habitats. It is sometimes seen in gardens, either uninvited or grown deliberately, but care must be taken to ensure that it does not escape into the wild.
Cultural control
The main method of control, and usually the most appropriate, is pulling or cutting plants before they flower and set seed. Conservation authorities regularly organise ‘balsam bashing’ work parties to clear the weed from marshland and riverbanks.
Chemical control
Before using weedkillers alongside waterways it is necessary to contact the Environment Agency (see telephone directory for your local office). It can advise on suitably qualified contractors, as can the National Association of Agricultural and Amenity Contractors (Tel: 01733 362920).
Himalayan balsam can be controlled with a weedkiller based on glyphosate, such as Roundup or Tumbleweed. Glyphosate is a non-selective, systemic weedkiller that is applied to the foliage. It is inactivated on contact with the soil, so there is no risk of damage to the roots of nearby ornamentals, but care must be taken that the spray doesn't drift onto their foliage. Glyphosate is most effective when weed growth is vigorous. This usually occurs at flowering stage but before die-back begins; with most weeds, this is not earlier than mid-summer. Alternatively, the non-selective contact weedkiller Weedol 2 (contains diquat) can be applied before flowering.
Take care when applying either herbicides near ornamental plants. Cover them with plastic sheeting while spraying, and only remove it once the spray has dried on the weed foliage.
It may take a couple of seasons to obtain good control due to the germination of more weed seedlings.
Leigh Hunt