It wouldn't take long...

...for the world to go "wild". You hear that Cockroaches would survive a nuclear holocaust. Chances are if vegetation was threatened Buddleia would survive and take over. We have three in the garden, two miniature - potted and one Alternifolia at the front.
These two, full-size, monsters appeared this year' and got to this size lurking behind/beneath the BBQ. Hard by was the Willow sapling. A potential forest, and these between fairly close set brick pavers. Imagine their chances in open ground.