Invasive weeds: Guidance for the control of invasive weeds in or near fresh water

Himalayan balsam

Fact file

Himalayan or Indian balsam (Impatiens glandulifera) is a native of the western Himalayas. Introduced to Britain in 1839, it escaped from gardens and rapidly colonised river banks and areas of damp ground. It is the tallest annual plant in Britain, growing up to 3m high. The characteristic purplish-pink slipper-shaped flowers appear in June. When the seed pods mature, they explode when touched, scattering the seed up to 7m away. Seeds are also spread by water and they may remain viable for up to two years.

Himalayan balsam plants grow in dense stands that suppress the growth of native grasses and other flora. In autumn the plants die back, leaving the  banks bare of vegetation, and therefore liable to erosion.

The stems are pinkish-red, hollow and jointed, often with some branching. Leaves and side branches originate from stem joints. The stem is sappy and brittle. The shiny dark green leaves are lance-shaped, have serrated edges, a dark red midrib and can be up to 150mm long. They grow on the stem in whorls of three. Purplish-pink flowers, held on long stalks, appear from June to October. The white, brown or black seeds are produced from July to October and are 4-7mm in diameter. There are between 4 and 16 seeds per pod.

Source: Biological Records Centre

Control

Control measures should aim to prevent flowering, and are best carried out before June for maximum effectiveness.

Chemical control near water can be carried out with herbicides containing glyphosate or 2,4-D amine. Glyphosate will kill all plants, but 2,4-D amine will kill only broad-leaved weeds; either should be used when the plant is actively growing in early spring for best effect.

Cutting, mowing or strimming on a regular basis for about three years will be effective and may even eradicate the plant from isolated sites.

Non-chemical control

Cutting

Cut at ground level using a scythe, machete, flail or strimmer before the flowering stage in June. Cutting earlier than this will promote  greater seed production from plants that regrow. Cutting should be repeated annually until no more growth occurs.

Digging

Shallow-rooted plants can be pulled up very easily and disposed of by burning or composting, unless seeds are present.

Grazing

Grazing by cattle and sheep is effective from April throughout the growing season. It should be continued until no new growth occurs.

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Chemical control

Glyphosate

2,4-D amine

In general

Treatment with a weed wipe in mixed stands, or by foliar spray in dense stands, before flowering. If all plants are controlled, then spraying programmes should only be required for two to three years.

Treat during early spring at the rosette stage for effective control.

The plant must not be allowed to flower and set seed. Reproduction is mainly by seed, and if control can be achieved before seeds are set, eradication is possible in two to three years.

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