Plantwatch: Pretty weeds and alien space invaders

Himalayan balsam is in bloom, its stunning helmet-shaped flowers giving off a heady fragrance unlike any other plant. It is also spectacular, standing up to over 10ft tall and growing in dense clusters. But this is a highly contentious plant that can spread rapidly and colonise damp ground, and with no natural enemies in the UK it has become a widespread weed.

Himalayan balsam was brought here in 1839 from the western Himalayas and made a magnificent garden plant. But it didn't take long before it escaped and began colonising the banks of waterways across the country. It grows at astonishing speed, up to 1.5 inches a day, and so tightly packed that it smothers most other plant life. And each plant produces up to 800 seeds that are flung into the air and can land up to 35ft away when the seedpods explode, helping spread the plants.