Although Himalayan balsam is an attractive plant, it has rapidly become one of the UK’s most widespread invasive weed species, colonising river banks, waste land, damp woodlands, roadways and railways. It reaches well over head height, and is a major weed problem.
Introduced as a garden ornamental in the mid-19th century, it now successfully competes with native plant species for space, light, nutrients and pollinators, and excludes other plant growth, thereby reducing native biodiversity. As an annual, Himalayan balsam dies back in the winter, and where the plant grows in riparian systems this can leave river banks bare of vegetation and liable to erosion. Dead plant material can also enter the river, increasing the risk of flooding.
Himalayan balsam is Europe’s tallest annual plant; commonly attaining a height of 2m and can even reach 3m at maturity in deciduous woodland. It has erect, hollow green stems with a reddish tinge. Himalayan balsam is an attractive plant with erect, hollow green stems with a reddish tinge. Branching occurs up the stem and the branches are arranged in whorls of 3. Leaves are lance shaped, always longer than they are wide and have serrate margins.
Its flowers are variable in colour from purple-pink and occasionally almost white and are produced from June through to October, long after most native annual plant species have matured. Seeds are large and black in colour at maturity, and produced in capsules, which open explosively when ripe; hence, the other common names for this plant are ‘Touch-me-not’ and ‘Jumping Jack’.
See an identification guide.