Himalayan Balsam

Himalayan Balsam is continuing to expand its range, now occurring throughout the UK, much of temperate Europe and the USA, where it has a northward bias.

The genus Impatiens means impatient in the sense of hasty, referring to the explosive mechanism by which ripe seeds are hurled from the plant, to enlarge the colony or be carried away by water to fresh ground. In the case of Himalayan Balsam, seeds may be thrown 2 metres. As it is an annual, it has no other reproductive method other than seed, and thus prevention of seeding is the key to control.

Himalayan Balsam (Impatiens glandulifera) was introduced to the UK by a remarkable man, John Forbes Royle. Royle was born in India in 1799 but educated in Scotland, developing an interest in botany. He qualified as an assistant surgeon, afterwards joining the Bengal Army in 1819. He remained in India until 1831. Whilst there, he was appointed Superintendent of the East India Company's two hospitals at Saharunpur, and also Curator of the botanical gardens. He stocked these with many economically important and medicinal plants, also organising plant hunting expeditions to the mountains of Kashmir for specimens for the gardens and the herbarium collection. Amongst the plants he sent to the Horticultural Society of London, was the Himalayan Balsam, for many years known as Impatiens Roylei. The date of introduction, usually given, is 1839, though it is possible that it was earlier, during the time Royle lived in India.

Himalayan balsam is Britain's tallest growing annual plant, often reaching 3 metres. It prefers moist environments, and thus has mainly colonised waterside areas in towns or countryside. It also grows quite happily away from water, but as the soils become drier, so the stature of the plant is reduced. There are dense colonies in dry gardens on the top of the Cotswolds, achieving about 1 metre; and on Surrey sand, mature flowering individuals no more than 30cm high. It is remarkable that the largest plants are able to develop in so short a time. The flowers are mainly pollinated by bumblebees, also by wasps. The copious nectar also attracts other insects including the Hummingbird Hawk Moth. The seeds which are shed from July until October, need winter chill to break dormancy. They then germinate over a short period, often during April, and grow prodigiously to be able to set seed in so short a time. The main vector of spread is flowing water, into which the seeds are flung by the explosive seed capsules. Each plant has a great capacity for seed production with some individuals producing 2,500 seeds in a lifetime. Seed viability rarely exceeds two years, and no persistent seedbank is formed.

Himalayan Balsam is a stout annual herb that grows to 3 metres.

Stems are simple or somewhat branched; hollow, glabrous, fleshy, green; tinged red or purple, especially at the prominent nodes. Stem bases are very thick, up to 8cm diameter; often fringed with fleshy adventitious roots at the lowest nodes.

Leaves are opposite, in whorls of 3 to 4, lanceolate to lanceolate-elliptic with serrated margins. Leaf size is from 5-23 x 1.5-7 cm.

Flowers are abundant, fragrant, varying in colour from white to dark purple red, with their interior bearing yellow spots. Each flower is up to 4cm, in racemes of 3 or more; long pedunculate. The lower sepal is pouch-like, with a spur. The upper petal, a broad two lobed standard. Fruit, a pendent glabrous green seed pod, with elastic valves; 20-30 x 4-8mm, containing up to 16 seeds.