Sooo... Your post got me intrigued enough about the exact snow mechanics to stare at trees for the last 30min, LOL.
(Kinda wanted to do an similar area in one of my zoos too

)
I'd already figured out from my Antarctica zoo, the snow on the whole map thaws for a couple months, every few years (now I know why.)
Going from that baseline for the exact temperatures... Set in all biomes with a spruce and a -20°C cooler.
-1°C gives you full map snow.

5°C area of effect from cooler only with full frost/snow effects.

16°C frost/snow effects drop to slightly but still heavy coverage.

20°C frost/snow effects drop to a more frost, very little snow effect.

25°C frost/snow effects drop to looking purely like a heavy frost.

30°C frost effects drop to a medium frost.

35°C frost effects drop to a light frost.

39°C frost effects disappear.
BUT, here's the kicker that makes it impossible to have frosty trees in a cold map like the taiga where it rarely gets above 30°C... The frost mechanics won't last more than about a month in game time.
So in conclusion, it's only possible to keep the frost/snow effects on your scenery items if you make it snow once a month... No regular snow, and the frost melts away. Aaaannnd, before anybody asks, if you let it snow then cover it with a roof, the frost disappears instantly, so no doming it over
