How come that kid isn't carrying a flat or a couple potted plants?

Efficiency!

... or at least pulling a sled full of plants. ;)
Nice setup Ian. Are you going to stick with single wall? Do you have a minimum low are you trying to maintain?

Actually he did help for a while. But not very long. I'll probably stick with a single wall at least for these two houses. I wouldn't want everything to start growing too fast; it will create even more work that I can't keep on top of. And that's no joke. I'm only heating to about 32 and not even worrying about it if it drops a few degrees below that for a few hours. But not to 13 - that's a problem.

Still lots of snow out there.

If it were me the roots of the kind of stuff you grow would never be allowed to freeze in small pots.

Well that really wasn't on purpose. I think there will be a few losses, but most plants that I brought in before it dropped below about 25F ought to be fine. Those Callistemons sat out all winter in 2008-9 in little pots, but protection from snow cover helped them.

Our snow is more than half melted now.

Killing temperatures of immature roots are pretty high, even with northern hardy nursery items like Japanese maples. If you lose the immature roots it may not show in the top, except perhaps for slower top growth until the lost roots are replaced. I encounter lots of potted stock that has partly dead roots, in addition to damage from careless or ineffective wintering over there is also the problem of roots cooking in small pots during summer (this latter problem is why stock in small containers with roots good on one side and dead on the other is so prevalent; the frequency of nursery plants infected with water molds will also account for partly dead or small roots systems on many specimens).