A thick application of green flock over the walls and ground once you've painted up the rubble should do well for representing moss, mold, another assorted low level plant life which would be the first things to overrun an abandoned building. Then adding static grass in to open areas where the ground has been broken up will provide more detail. Maybe real small leafty aquatic plants for a kudzu-like appearance, though you have to do something to make it look less blatantly fake and plastic. You don't even necessarily need that many trees to do most of the smaller terrain pieces, and could be more sparing with them (also saves cash). Really depends on just how long you want your city to have been abandoned, and also what the climate is like. A typical habitable world would require somewhere around a decade or more before the trees really start getting good-sized. Less time for a death world-styled environment. If you're going with something like that ruined temple look then obviously you'll want the completely overgrown look, in which case the city won't be much more than rubble. I personally like the "abandoned for a few years" look where the city still has a definite urban look, but nature is just starting to get a real foothold in reclaiming it. I think that's the best area to aim for, to really get the most milage out of both feels (urban and wilderness) that you're playing with.
Just my opinion though. Great project, its making me think of having a go at making a building like that.