Grass. It's a thing I always struggle with, usually it's a layered process. I start with some basic color blocks, shades of green/brown. Over that I'll paint with rough textured brushes in lighter or darker shades, to get a sense of where the light hits the grass or the lay of the land. After that I'll go in with a simpel round brush (very small/thin)) and scratch in individual strands/leaves (what's it called?) and lumps of grass. I usually pepper that with some dots/spots of colour where flower beds grow.
That's sort of how I work. There are some great foliage brushes that can give a nice base for undergrowth. Those are great If you want to speed things up.