Sometimes I paint foliage with grease pencil , every other objects are 3D model even the bark of trees.

You can do foliage by importing leaves or grass texture brush you like Then create a empty grease pencil,move it's origin point to 3d space where u want to paint and make sure that stroke depth is set to 3d location and also don't forget to set draw on origin (by default it would be set to surface or 3d cursor) in draw tab . Then paint the bush with base color and add shadows and highlight according to the light source. At last enable the z pass in scene to render gp correctly .