Here in central Alabama we have lots of red dirt and clay (from the iorn content) I usually find several 'clods' on a bank crumble them up and sift it thru a screen, mix it with 50/50 white glue and water and 'paint' it on. Dead blooms off of wild Hydrangea make excellent trees: pluck off and remaining flowers, coat the small branches with the glue and roll them in ground foam, I would also follow up with a spray of Aquanet hair spray. Bath powder makes good marble (limestone) dust if you have a quarry on your layout.
Dead Goldenrod blooms make excellent Georgia pine trees...we have had a couple of good frosts already so when the flowers are gone and just the 'fluf' of the seed pods are left, I'll pick them and give them a shot of dark green from a rattle can.
I have used lichens to simulate a thick forest canopy, you can coat them with ground foam, and you can seperate them into sprigs to make scrub brush and weeds.
David