Yes, holes in bottom of styrofoam - ya do not want *any* water 'pooling' anywhere - ever
Any deciduous tree (like elm) does not require much of anything when 'asleep'.
A bit confused on humidity tray..you mean for in-ground/buried-pot things?? If so, not at all. Any evergreen slows waaaaay down when its really cold, and just a tad when its 'moderately cold, no humidity tray ever needed outside in winter - just be sure to not overwater the evergreen(s), sometimes only needed *maybe* once a month (just check once a week
maybe - even if just to say "hi-how are ya?" to the trees, LOL).
On further thought, you may be better off
removing the entire bottom of the 'foam' box - you *will* get warmth from Mother Earth into the box this way. In Winter, Earth *will* warm things upon it (think igloo here - does not get below freezing in there, and ground is actually warmer than the air (right??)) Even permafrost above Arctic Circle only goes so far down!
The issue you are combatting is the cold that would be going into the pot
from above-ground sides and especially from above - cold air 'drops/sinks', remember? You will also not have to worry about water pooling and/or freezing within pot(s) this way. Make sense?
In a way, I feel you are kinda overthinking this a bit, but I fully understand your 'nervousness'. Trust Mother Earth here - she's been keeping roots warm a loooong time, 'eh?!
A survivalist's first rule of staying 'warm/un-frozen' when stranded in ~Arctic conditions? Get underground! So....just get the roots/soil underground & covered, my friend - and the juniper is practically freeze-proof on above-ground growths, fwiw. Think of the stuff that lives on bare rock way up high on mountain-tops and gets VERY old while doing so - Mother Earth is there for 'em!
One more thing: thank
you for being 'out there' taking care of power when all of us gripe, whine, and moan endlessly when lights go out from ice-storms or such. You do not have the option of 'going underground', LOL!! Seriously, *I* appreciate ya, and all of your cohorts - please pass it on, OK?
HTH,
Alex
