Himalayan Balsam (Impatiens glandulifera) has a bad rep among ecologists but it wasn't born to be bad; I think it might just be misunderstood.
 It does indeed hail from the Himalayas and was introduced to British gardens by the Victorians who loved finding exotic specimens. I. glandulifera (also known as Giant Balsam) is a very fast growing annual, up to 10 feet tall. It seeds itself by exploding pods propelling seeds up to 10 or 12 yards away. Therein lies the perceived problem: it quickly jumped the garden fence into the countryside where it has naturalised and can be prolific. It favours water courses and spreads along them; the seed dispersal is no doubt aided by the current.
 Ecologists fear that it displaces native vegetation and conservation groups often include a day of "balsam bashing" in their calendar of events. But I like this triffid of a plant. Drifts of it really are very pretty and it grows through dense vegetation like bracken where other wildflowers cannot. It flowers late in the summer providing a timely hit of nectar for the bumblebees.
 The photos above were taken last week in Dorset of a stand which has been there for as long as I can remember. It doesn't seem to be destroying the local habitat. It certainly has spread along the River Lym from this colony but seems to coexist quite happily with the native ferns, Ramsons, Bluebells et al. Where the river flows through Lyme Regis the Balsams grow alongside other "foreign" garden escapes like Buddleia (Chinese), Red Valerian (Mediterranean) and Mexican Daisy. The combination makes for a very attractive display.