2 years max apparently. But there are tonnes of them/plant so you need to be very aggressive/thorough with your weeding.
@Lowri is there a conservation group or similar near you who might be up for descending mob-handed on the problem?
||Cover them with plastic sheeting while spraying, and only remove it once the spray has dried on the weed foliage. It may take a couple of seasons to obtain good control of Himalayan balsam, as additional weed seedlings germinate after the parent plants are killed off.|
||I'm told the seeds make a very fine oil, if you can find a sensible way to harvest a decent quantity.|
||They're annual so I don't see the real point of spraying. Keep pulling, do not let them flower. Perhaps a plastic sheet over the area would kill a big patch -but let them germinate first, else the existing seeds will just wait until the plastic comes off|
Keep ducks. Every growing thing dies under a duck. They stink though. Our new phrase for anything gross is 'ducksgusting'
Is there anything that eats it? My alpacas escape into the garden to eat bindweed. I'm delighted.