Part of the story may be migrants taking their native plants with them out of nostalgia.
That may be true for the Americas and perhaps Australia, but intuitively not for Eurasia. We have botanical gardens, though. It's actually part of their mission to popularize biodiversity, and what better evidence to show for the effort than to have an introduced species become widely cultivated? Horse chestnuts have no place in Kyiv's native flora, but they became a symbol of the city. ('Oh, the horse chestnuts suffer from leaf miners! Why doesn't the Institute of zoology DO something?' 'Stop planting the friggin' trees.' 'You, sir, are not a patriot.')
I think we need a discussion thread for the californian drought going on. I would like to compile information in the main post and would like help compiling it. If we really are proud to be effective altruists then this is an area we should really figure out.
Any one have any good ideas on how we can help?