Out-of-control alien weed with colourful flowers
INDIAN BALSAM is a handsome plant with attractive foliage and colourful flowers that are in bloom at present.
It grows best along the sides of waterways and in damp waste ground close to water. Its flowers are generally pinkish ranging in colour from such an extremely pale pink that it is almost white to such an extremely dark pink that it is almost purplish.
Balsams, Impatiens in Latin, are a family of garden and wild plants, the best-known of which is the ever-popular, low-maintenance Busy Lizzy house-plant that flowers non-stop. Indian Balsam is another member of the group and is native to the Himalayan mountain range in India.
Its native habitat is cold and the soil is thin and nutrient-poor so the plant has evolved to be an annual with seeds lying dormant during the winter, germinating in springtime, producing lots of flowers in summer and vast numbers of seeds in autumn in the hope that some will survive the rigours of the Himalayan winter.
When plant collectors introduced the plant to Ireland as a garden flower during the nineteenth century, it took off in our mild climate and fertile soils and garden escapes and throw-outs are now spreading out of control smothering other vegetation as they march ever-onwards. It is regarded a ‘ high impact invasive alien weed’ that has no place in wild Ireland.
Growing up to two metres high, Indian Balsam is one of the tallest annual weeds we have in Ireland. It is so vigorous it can even take over, out-compete and kill a bed of Common Nettles. Each flower-head produces several hundred seeds. They are self-fertile and are contained in pods that open explosively scattering young in all directions, up to seven metres away from the parent plant.
Seeds are known to survive in water and to remain viable for up to two years so our waterways are major arteries along which the invasive alien is spreading. Elevated levels of water pollution further nourish the alien and assist in its spread. The fact that plants die in winter and leave the ground bare contributes to flooding.
How to control it is controversial and can be costly. Parties of balsam bashing volunteers have reported success by pulling and cutting shallow-rooted plants in early summer before they flower and set seed. Otherwise chemical control by trained personnel over several seasons is effective using contact weed killers that are inactivated on soil contact or have low persistence in the soil.