Well today is below zero for the first time this fall, December 9th, 2011.
The greenhouse is heated with a wood air tight furnace with forced air draft, temperature of water at 110 , shuts off the blower until the temp drops 5 degrees.
The water circulates into the greenhouse, first through a heat exchanger to heat the potting room, then into the finpipe under the potted plants.
A temperature probe in a pot of soil, regulates the opening and closing of a valve, so the plants don't get too hot. Soil temp never gets up to 60 in the winter. Plants like it cool, I get chilled as the air temp is much cooler.
Today is forecast for sunny and warming up. I can then work out there keeping the plants happy.
The barrel is growing a potatoe or two and last week I put Leeks in there. They should like the cool weather and produce some edible plants in a few weeks.
Meanwhile I anticipate more veggies from the cool realms of the winter greenhouse.
My favorite at this time is Swiss Chard which is 2 seasons old. I continue to harvest when I need some greens. I expect it to bolt one day, but so far so good. These plants are from the seed mix called Bright Lights.