Invasive Alien Species in Northern Ireland

Impatiens glandulifera, Himalayan Balsam

Impatiens glandulifera
© Ian Dodkins
Impatiens glandulifera
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Impatiens glandulifera Royle
Himalayan balsam is now a very common and locally abundant species of river banks and lake sides. It has spread widely in Northern Ireland over the past seven decades, and has become a local nuisance which has a damaging effect on native vegetation.
Himalayan balsam is an attractive plant found growing in dense stands on the banks of rivers, canals or on lake or pond margins. Individual plants can reach 2m in height, have translucent, fleshy stems, large oval pointed leaves with obvious teeth around their edges, each tooth carrying a small globular 'gland', and produce large numbers of flowers of variable colour, but usually some shade of purple which are followed by ' seed pods' about 25mm long. When mature and dry, the slightest touch causes these fruits to split open explosively, flinging the seeds a considerable distance from the parent plant. Each plant produces about 2,500 seeds which fall to the ground at a density of between 5000 - 6000 seeds per square metre. It is an annual species.
Another English name coined for this plant is policeman's helmet a good description of the shape of the flower which resembles that of an English police helmet. It is also called Indian balsam.
Country of origin
The plant is native to the Himalayas
Location in Ireland
The species is now found throughout Northern Ireland and the Republic of Ireland in suitable situations.
Life cycle
An annual species, seeds germinating in spring, flowering in June - August, producing seed in August October. The plants die completely from October onwards.
Wildlife and habitat impacts
There appears to be no direct deleterious impact on animal life. However, recent research by two botanists in Germany has shown that it competes for pollinators such as bumblebees with the native riverbank species, and so reduces seed set in these other plants. Its success in this is in part due to a very high rate of sugar (nectar) production for instance about 47 times greater than the great willowherb (Epilobium hirsutum) and about 23 times greater than purple loosestrife (Lythrum salicaria). The plant has colonised many rivers on the European continent and seed set in marsh woundwort (Stachys palustris) is reduced by some 25% where it grows mixed with Himalayan balsam plants as compared to pure patches.
Human impacts
None known.
Key vectors
There are no special vectors for long-distance dispersal, although dispersal by water is probable. Local dispersal is by seed from existing colonies.
What you can do as an individual
None recommended.
Management measures
Mechanical control, by repeated cutting or mowing, is an effective control, but plants can regrow if the lower parts are left intact. Regular grazing also suppresses this species.
Control by herbicides is effective for detailed advice on this, see the Centre for Aquatic Plant Management web site (Information Sheet 3: Himalayan Balsam). Herbicide should be sprayed before flowering.
Further information
Centre for Aquatic Plant Management, CEH, Wallingford - advice on mechanical and chemical control
Royal Botanic Gardens Kew; information sheet T4 on invasive species
See the Invasive Species Ireland web site for further information -
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum