Taken from Eclectic Gardener @ http://www.teachinggarden.org/
Written by Alan Hickman

It may have been a couple of weeks before the winter solstice and the official season opener, but when the temperature outside the greenhouse dropped to minus thirteen, last Sunday night, that was the real beginning of winter.

By their very nature, whether they be hobby or commercial, greenhouses are energy hogs. When the sun shines, even on the coldest days, they cleverly allow in the high-frequency radiation and then trap the low-frequency infrared re-radiation to heat the enclosed space. But, when the sun doesn't shine - and in Canada there is a lot of night and a lot of winter - keeping a greenhouse green is an uphill battle.

Thirty five years ago, the 12x14 foot greenhouse that I built over my back door was a cut-down section of one of the Dale ranges which once covered many acres at the north end of Brampton. The oil crisis of the early 1970's put the last nail in the coffin of the great flower growing industry in the town and, when the property was cleared for 'redevelopment', the buildings were sold off to anyone who would take them down.

For $30.00 I got a 30 x 30 foot section of greenhouse: all the glass, wood, steel, vents, and openers, and even the hosepipes, sprinklers, thermostats and flower pots. Those were the days when I could drag it all home behind a VW Beetle.

The first version of the structure used the original square glass panes, overlapped like roof shingles, and the heating was just an extension of the house furnace hot-air duct.

When the inside temperature dropped below five degrees, a booster fan sucked more warm air out of the house to keep the frost at bay.

The system worked well enough and the overwintering palnts were self selecting. Anything that couldn't withstand a few degrees of frost on the coldest days didn't come back the next year.

After a few years, the whole glass area was covered in a double layer of heavy-duty clear plastic sheeting. The plastic sealed up most of the air leaks and a small fan blew air between the layers to form an insulating balloon. That really made a difference and the number of freezing days dropped dramatically.

On the downside, the roof vents couldn't be opened so, to prevent overheating the hanging plants, a fan had to be installed to suck hot air out on sunny days. As well, the plastic lasted only so long and the whole wrapping job had to be redone every few years.

The latest upgrade is only four years old. All the plastic came off, the half-century-and-more year-old glass came out, and in went rigid panels of thermal Lucite.

The roof-vents opened and shut for the first time in a couple of decades and, when I finally gave-up painting the clapboard on the house and had the wood covered with vinyl, the new siding pulled the residence and the greenhouse into one unified whole. Really quite spiffy!

The original thermostat and blower still keep the plants from freezing down to minus five or ten degrees, depending on the wind. About another ten degrees can be bought with a 1Kw electric heater and, on the nights that have been below minus fifteen, a big keosene heater has yet to be defeated.

In late afternoon, when snow is falling from leaden clouds, the passing view into the greenhouse, where scarlet pelargoniums and purple fuschias spill over the side of hanging baskets, lifts the sagging spirit like little else can.

On the benches, thirty-odd pots of agapanthus doze through the winter like a patch of lush thick grass and pelargonium cuttings from last year's borders are just opening their first flowers.

No matter how big a greenhouse is, there is never enough room so the hanging baskets of ferns that lined the pergola all summer are now dangling off the front of the bench like a green fringe.

A shelf is home to a dozen or more clay pots filled with an assortment of cacti that have accumulated and grown bigger over the years. Cacti and succulents seem to thrive in the chilly air and draughty conditions - or at least these ones do - and, in a couple of months, a big rather ugly epiphyllum will burst into a veritable firestorm of huge red trumpet flowers.

The contents of the greenhouse are green, but what about the other green? What about all the carbon dioxide from burning gas and oil and coal to keep everything warm?

Well, the best I can offer is that last Sunday and Monday-and most of the night between - the gas furnace was turned off, the stove in the basement was fired up, and the house and greenhouse were kept cosy burning wood.

At least on the coldest days of the year the operation is maybe close to carbon neutral.

Day -8 wc-11 mainly cloudy with trace of flurries Winds NW24km/h P.O.P.40%
Night -18 wc -28 mostly cloudy Winds WNW20km/h P.O.P.20%