Freeze/Thaw conditions are plain tough for dirt trapping, but I've come to realize it is part of the game too and periodic & routine set maintenance is key to keep things going each night.
I've tried coverhulls, dry dirt, waxed dirt, coal slag, etc. and all have worked to some degree or other, but plain fact is that if there is any moisture in the ground or air at night, a light coating of frost is OK for one night, maybe two, but beyond that the crust will be too much to allow the trap to work properly when fired.
Waxed dirt so far I've found is the best at fending off moisture...but it takes some work to put it up in quantity needed for a decent line. Coverhulls are nice and light to carry, but can be expensive and they too will crust over after a frost or two, or even a misty or light rain.
I've even gone back to plain dry dirt with a mixture of table salt or calcium blended in, and that too will freeze over but it is "dirt" cheap and not nearly as labor-intensive to put up. I store mine in clean, gallon milk jugs, as this is a handy container to tote around and I can stash them near set locations in brush piles, etc. so I always have a supply close by. Of course, pick up the jugs after season is over...but so far that has been the game plan I use for canine trapping. I go out couple weeks ahead of season and start stashing jugs wherever I plan on making sets. Its real handy after freeze up to rebuild or remake a set when the dry dirt is only a few paces away stashed in a brushpile or weedy fencerow. I think key for me to battle these tough freeze/thaw conditions is some hard preseason work and planning ahead with stashing supplies where I need it most under these conditions. The capped jugs keep the dry dirt/salt (or calcium) mixtures dry and weather-proof all season long for me, and by stashing them ahead of time, the extra work involved with carrying the stuff, remaking sets, etc. is cut down substantially.
I also use a piece of fiberglass insulation under my trap pans, and this seems to help keep dirt from compacting under the pan, and seems to help it fall more freely when fired. This insulation too will get damp after a few nights of freeze/thaw conditions, so be sure to change that as necessary when performing your set maintenance routines. Test fire sets once and awhile if you are not sure how they will work. Take a stick and poke down about as hard as a canine would on that pan. But, keep in mind that during the day the "thaw" conditions may be present and the trap may fire fine. At night if that set crusts over or the dirt has sufficient moisture content to freeze solidly, the same effort to fire that trap during the day may not be enough at night under the "freeze" conditions. If I have a set that has not been fired or worked for a few nights like this and under these conditions...I try to test fire those right away in the morning when the "freeze" conditions are still there. Its about as close to duplicating the animal firing motions and set conditions as I can duplicate during the season. I rebuild the set either way, but at that point at least I know I am going into that night's activity with fresh, dry dirt and a potentially more "able" set trap awaiting the next canine that comes by.
Jim-NE