However, the
"mulching" process still confuses me.

Mulch doesn't stop the ground from freezing; it prevents the ground from *thawing*. The greatest stress on both trees and pots is irregular weather when the ground continually freezes and thaws. Mulch acts as a buffer. Snow will do the same thing, if yo
u are lucky enough (nyah nyah Jim) to live where you get a continuous blanket of snow. Both mulch and snow will buffer the ground from going too much BELOW freezing, because a sudden severe cold snap can kill inadequately protected roots.

A real cold-frame acts on the same principle: the trees are placed in a hole deep enough that the ground stays frozen, but does not get much colder than freezing, or much above freezing.

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