You will find that Project Dogwaffle has many types of brushes. One such category is the group particle brushes, and it contains 4 sub-categories:

1. Particle brushes, the original Optipuss (optical octopuss) going back to version 1.6 (!)
2. Bristle brushes
3. Orbicles (orbiting 'swarms of bees' particles)
4. Foliage brushes

For nature painting, especially when adding grass, weeds, bushes and trees, you'll want to explore the Particles and the Foliages.

Have a look at the many tutorials from PD Particles

Below are some tutorials that can be useful whether you use PD Particles, PD Artist or PD Howler.

Painting on landscapes - part 1: the initial background mountains and foreground hills

Painting on landscapes - part 2: focus on sky with clouds

Painting on landscapes - part 3: Foliage brushes for distant trees and bushes