Growing in a greenhouse without any heat is sometimes risky, but with the right planning it can be done to extend your growing season by several weeks or even months. The article below is from a 1978 article in Mother Earth Magazine, but the lessons from decades ago can still be applied to our greenhouse in the 21st century….. 

We began our winter gardening in an unheated greenhouse almost by accident. A small seedling (or seed) got lost under a bench, and in early January, going by chance into the ice-cold building, we found a flourishing, lush, and sizable lettuce plant growing through a clump of dry leaves. It had survived, unwatered and untended, through several months of outside freezing, in a sheltered but chill corner of a cold glassed-in building.

If this could happen, uncared for and unbeknownst, why could not more lettuces, and other plants, survive, under better conditions, still without artificial heat? We were launched on an experimental period of greenhouse building and planting that has provided us with fresh green things through thirty winters of freezing and below-zero weather.

Without question, plant germination and growth is checked by cold weather, and only certain plants can survive. Very low temperatures will kill almost any growing thing eventually. But there is a wide margin, and our experiments pointed up the plants that will not be killed by low temperatures.

Almost all of our gardening experience has been in the North Temperate Zone, taking advantage of summer sunshine when we can get it, and taking cover as cold blasts from the north and east strip the foliage from trees and crumple down green crops in the garden. One of our chief aims in gardening is to find those plants that can remain succulent and edible throughout the coldest weather.

Read the full article here http://www.motherearthnews.com/Nature-Community/1978-01-01/Our-Sun-Heated-Greenhouse.aspx