I'm trying to find explanations for a strain of beans (or any other food-producing plant) that have become large, maybe as large as most trees, and about as wide in girth, that overgrows countries into huge bean-filled forests.
I started by saying that a larger strain of plant had been selectively bred (Not GM, please! That's far too generic!) to be large as bush, strong, able to grow anywhere, and have enormous yield. I chose beans specifically because their produce can be used for both food and replanting.
I then imagined that, because of the plentiful seeds, these large plants began to grow in undesired places, and, by very unlucky natural selection after overuse of weed-killers, another strain of larger size and weedkiller resistance evolves.
... and that's when I hit a dead end.
I can't find an explanation for any further growth than much more than a bush. What other reasons might there be for accelerated growth of these plants?
Also, is there another, more suitable plant that could grow to huge sizes?
Note: I may have said earlier, but I really want to stay away from dull, generic explanations such as "Nuclear radiation" or "Genetic modification". I'd like to explain this in terms of natural laws such as evolution or survival of the fittest.