Hi Nanoid,
I think something like this is likely to use a whole bunch of combined techniques to get what you are looking for.
Lets say you wanted to overun the school with vines, moss, grasses etc.
You could start with the “Hero” assets, large vines with thorns and and leaves. rig these in your chosen 3D app and animate the bones moving along a path so the vines creep across your scene.
For further away vines you could us an animated opacity mask to slowly reveal them.
add a few particle systems to this where the sprites themselves are not moving but simply grow over a few seconds.
You could also add a flipbook/sprite sheet to this to show buds opening, leaves changing colour etc.
Again use masks to add decay and moss etc to walls and maybe lerp between the 2 texture versions.
There maybe better ways of doing this using the foliage system but it’s certainly not something i’ve come across in working with it.
Hope this helps.
D