In article ,
Martin Brown wrote:

Himalayan balsam is becoming quite a problem in North Yorkshire on
stream and river banks wiping out almost everything else and spreading
rapidly. Only Japanese knotweed out competes it. Nettles and rosebay
willow herb both die out after a few years leaving a monoculture.

I have never seen it do that, and I have been looking out for it for
several decades now. I believe you that it is a (very) localised
problem, but it assuredly isn't a widespread one. Every apparent
monoculture of it I have looked at has turned out to be, on closer
inspection, merely that it dominates the top layer and there is a
wide range of other species underneath it.


Regards,
Nick Maclaren.