Indian balsam, policeman
Impatiens glandulifera Royle (I. roylei)
Himalayan balsam is an introduced annual naturalised along riverbanks and ditches. It prefers moist soils but will grow anywhere. Himalayan balsam grows up to 3 m tall and is reputed to be the tallest annual plant found in the UK. A native of the Western Himalaya, it was introduced in 1839 and is now recorded throughout Britain. It grows rapidly, spreads easily, out-competes other vegetation and readily colonises new areas. Himalayan balsam is said to be relatively shade tolerant. When the plants die down in winter they leave large bare areas that are sensitive to erosion.
Himalayan balsam flowers from June to October. It is pollinated by bumble-bees. Seeds are set from August to October. There are 4-16 seeds per pod and each plant can produce 800 seeds.
The seeds have a chilling requirement for germination to occur. The entire seed population germinates synchronously in spring to form a dense stand.
The seeds can remain viable for up to 2 years but Himalayan balsam does not form a persistent seedbank in soil.
The seedpods are dehiscent and explode when touched or shaken. The seeds are expelled up to 7 m from the parent plant. The seed is transported by water but can also be carried in mud by animals and man. Himalayan balsam has spread at the rate of 645 km²per year in the UK.
The plant is shallow rooted and is easily pulled up. Control is by grazing and by cutting or pulling before seeding. Grazing by cattle and sheep should begin in mid-April and continue through the growing season. Repeated mowing will prevent it over-shading other vegetation. Plants should be cut to ground level by the end of June and before the plant flowers. Earlier cutting results in rapid regrowth of new stems that will flowers and set seed. Cutting above the lowest leaves stimulates the axillary buds to regrow.
An extensive stand of Himalayan balsam may reduce species richness by 25%. Controlling the weed leaves bare areas of soil that are subject to erosion, particularly steeply sloping land along riverbanks. Other invasive non-native species may benefit from clearance of the weed. It is therefore important after clearance to encourage the native vegetation to regenerate or to plant up the area with appropriate species.
Updated October 2007.