The basic concept behind growing season extension is to cover plants. You want to put a blanket on your plants to keep them warm. It sounds simple and easy and it is.
At night in our cozy beds a blanket keeps us warm. Wool, cotton, or polyester, the blanket doesn't actually create higher temperatures. The blanket traps our body heat and radiates it back to our chilled skin. Our own body is what keeps us warm when we cover it with a blanket. In the garden a blanket keeps the plants warm in the same manner by trapping and returning the heat of the soil.
Plants aren't picky about the blanket material. It can be plastic, canvas, wood, or cloth. Of primary importance is that you cover plants before the temperatures approach or drop below freezing.
When you suspect or expect cold temperatures overnight, apply the blanket while the outside air temperature is still above freezing. This usually means during the day when the sun is shining. The waning daytime heat will couple with the radiating soil heat and create a warm air pocket that protects the plant during the cold temperatures.
Like a blanket on our own beds, the garden blanket should fully cover the garden bed that you want to keep warm. It should be large enough to ensure no leaves or plant parts are sticking out from the edges. Any part of the plant that is exposed to freezing temperatures may experience freeze damage. Your foot gets cold when you dangle it outside the covers; the same happens with dangling stems and leaves.
The last critical component of plant protection is to remove the blanket when temperatures warm up again. Once the sun comes up and temperatures rise above freezing, take the cover off. If you leave the blanket on, you run the risk of potential plant damage through excessive heating. You also want to give the soil more opportunity to warm up again.
You can continue the cycle of covering plants at night and removing the cover during the day for weeks. Until daytime temperatures drop to a point where the soil no longer absorbs heat, you can continue gardening.
Colorado State University conducted studies for extending the growing season in the spring, but the same systems can be used in the fall. They found that a simple cloth covering (a row cover) provided 2F to 4F degrees (1-2C) of protection. That means that the outside air temperature can fall to about 28F degrees (-2C) while the air around the plants stays above freezing. Plastic supported by a metal mesh frame provided 3-6F degrees (3C) of protection. It's important to note that the plastic completely covered the frame, allowing no exchange of air. This is enough protection to keep your plants alive for the first frost of the season and for a few weeks after.
For even more protection, a space blanket, or thermal blanket, is highly effective. The light-weight, metallic blankets reflect up to 99 percent of heat. Adding a space blanket on top of a plastic cover adds protection when the night temperatures drop below the 28F degree (-2C) threshold. In CSU trials, a space blanket added to a plastic-covered frame kept plants from freezing when the night temperature dropped to 0F degrees (-18C), following a sunny, spring day. It is critical to remove it during the day because you've also created an oven that can bake plants in the sun.
If you are really serious about extending the growing season you can take extra efforts to continue gardening when temperatures drop below the point where daytime temperatures can sustain this cycle. You can add a heat source to the plastic umbrella over your plants. Christmas tree lights are one solution, but they need to be the old-style lights, the big ones that heat your fingers when you touch them; new LED lights won't have the same result. The CSU trial showed that a string of C-9 lights draped from the metal frame, under the plastic sheet, added up to 18F degrees (10C) of frost protection. With a space blanket on top, that protection extended up to 30F degrees (16C). That means the outside temperatures can be bone-chilling cold and the plants are still enjoying temperatures above freezing.
Here are a few more thoughts about covering your plants to extend the growing season. Cloth covers work well, but will lose much of their heat retention ability when they get wet; avoid a cloth blanket or sheet if you expect rain or snow. Heavy tarps or wood sheets retain heat well but can crush plants underneath. Moist soil will retain more heat than dry; plants also prefer moist soil over dry so there's no reason to withhold normal water. Plastic covers can stay in place if you open the edges to avoid over-heating during the day; set up a plastic or wooden frame, drape plastic over it, and just open and close an end during the day-night cycle.
Cover your plants to get past the first frost in fall. Often you only need a few extra days to harvest the crop being threatened by cold. Keep it as simple as you need. One year I didn't see the frost forecast until the nightly news. I ran out as the sun was setting, threw a large tarp over the pumpkin plants, weighed down the edges with rocks, and went back in the house. That was the only crop that needed a little more time to grow. The next morning I pulled off the tarp to see the plant was alive and happy; a few of the leaves that stuck out from the edges were damaged by frost, but I gained enough time for the pumpkins to turn orange in the next week.
At some point in cold-winter regions, the growing season has to end. When you've picked the last zucchini or tomato, there's no reason to protect the plant any longer. Pull off the blankets and plastic and store them away until you need to repeat this process in spring with young plants. By blanketing your plants you can gain time and extend your growing season on both ends.