I don’t use glue to hold down ground foam, except for larger shrubs, then it’s a drop of CA on the bush and place manually.
After the spackling compound over the plaster cloth is carved to suit me, I paint everything with Wal-mart arcylics mixed to simulate dry dirt, wet earth, dead grass, peak foliage, pine duff, and three colors for rock, dark, medium and highlight. Rock to grass borders are pretty abrupt, same for rock to dirt, except on talus beards below steep faces. Grass to dirt interfaces get smeared over several inches, to near dry brushing on the fringes.
Then I go back and start adding ground foam. For thick grass or weeds, I’ll use one of the greens, dab on say four inches by four inches, then one or two wet brushes worth of extra paint to re-wetten the earliest parts, then sprinkle grass or foam or random size sand, or what have you where it needs to go. If it’s real gnarly grass, weeds, thin, or on steep surfaces, I use one of the dirt browns instead, and sprinkle very lightly so I end up with patches. Ledges on vertical faces sometimes get soil colors to adhere greenery, and sometimes rock colors.
24 hours after application, I go over it with the dustbuster, being careful not to touch any surfaces with the snout. It usually doesn’t need touchup, but if it does, more paint and more foam till it’s right.
In nature, greenery rarely transitions suddenly to bare dirt, except at certain kinds of escarpments and washes. With paint, I find it a lot easier to blend a nice transition. Heavier foliage increases all the way up to mature forest, towards flat ground, water, lower altitudes, etc. Runs of trees and shrubs and grass often form vertical strips in the mountains, because the seeds follow the gravity/fall line each autumn.
Vertical or overhanging terrain rarely supports much growth. Ledges can, and sometimes this growth can hang over the edge in sort of a spillway pattern. Getting these right is easier done in laye
