We visited the Berkshire Botanical Garden, near Stockbridge in (Western) Massachusetts this fall. (That IS Norman Rockwell Country!) That area gets pretty cold in winters and they were moving some tender plants into a greenhouse. The greenhouse is set into a hill facing more or less south and somewhat sheltered from the north. The whole back side of it was lined with 50 or 60 gallon water drums. They were black and probably did a pretty good job of soaking up the sun's heat.

I know of a couple of killinuts with greenhouses. One does some really neat things with temperate zone killies and if we're ever in the Maryland area again, we've got to ask if we can stop by.

The 120-160 gallons of water holding containers, seated upon styrofoam (old fish boxes never die) in my room are mostly heated by submersible heaters to just a couple degrees warmer than the room. There are two other heat sources in there which (I surely hope) supply most of the warmth. The thermal mass of the water barrels has been useful in blunting the temperature drops in the room when a cold front rolls through though.

Most of the tanks have plants, though some need help. About ten pots are hanging or on the floor situated in "Baker's Square" pie tins - to limit overflows. Many of the foliage plants are spider plants descended from one rescued pot at my folk's house when we had to move my Dad out or Spathes that have gone AWOL from tanks. A rabbit's foot fern, which was a fishroom housewarming present 27 years ago, hangs on in there too.

While a couple of parties maintain that those pots are on the fish-room floor to trip passers-by or suspended, ready to clobber them in the head, I contend that when it is so bleak outside, any live green indoors (within limits) is a good thing. :)