Chip, you know my history in MR. If I knew I was going to plant a whole bunch of trees, and had pretty much decided against foam, I don’t know anything else that would do as good a job at ancoring them. I used two rolls of plaster cloth here and there to cover gaps and to smooth grades between layers of foam when I became too impatient to continue to plane it down. Two rolls won’t break your bank, and your layout is much smaller.
So, bang for bang, buck for buck, foam is the best and easiest for placing and retaining quantities of trees. Take a thin phillips screwdriver or thin metal punch/pointed skewer, press into the foam to a depth that seems reasonable, and then press in tree trunk. One tree installed. No hot glue for me, no driling into thin air below a shell, no matter how thick (thanks a lot), no dust, no mixing or cleanup.
My trees all look great, and only lie askew where I have repeatedly pushed against them reaching beyond or over them to get at something else. Punch a new hole beside the old one, press in three trunk, and Bob’s your uncle. My telephone/telegraph poles are wedged into glued ballast, and they never stay longer than a couple of gentle whacks. Foam is much more forgiving.
-Crandell