You can use Vegetation Studio’s awesome runtime prefab spawner to spawn the flammable objects to the root of the bushes/grass/trees. You cannot animate the shaders using this method due to the optimization of the instancer, but you will get Flame VFX.
Full instructions how to set this up in Usage->Use Vegetation Studio.If you have Pro Version of Vegetation studio you can have more advanced and better flames with shader animation due to the fact that you can mask objects. Full instructions in Usage->Use Vegetation Studio Pro.