I apply a coat of latex earth-colored paint over the foam and lightweight spackling. I used to try throwing ground cover on while the paint was wet to get a base layer of foliage. But, I found that I wouldn't get complete coverage without applying more adhesive, so I don't even bother with that step anymore.
I now allow the paint to dry and then come back with the scenic materials of choice. The steps that I follow are: (1) apply the foliage/ballast, (2) spray or dribble on a wetting agent (water with a few drops of dish detergent added or full-strength 70% isopropyl alcohol), and (3) dribble on the adhesive. I've also used Woodland Scenics' Scenic Cement successfully, but diluted white glue is much more inexpensive.
By the way, I did not have success with matte medium the last time I used it. It had "snots" (lumps) in the mix and it caused a sheen on the track I ballasted. Besides, matte medium can be very expensive.
Hope this helps,
DFF