As someone who gardens in a cool climate (zone 5), I welcome the rest period that comes with a snowy winter. What I DON'T appreciate is not being able to grow my own food when the temperatures drop. I don't have a greenhouse and I'm not a skilled carpenter, but that doesn't mean I have to be left out in the cold.
Individual plant covers can be used to protect small plants like herbs, radishes, or lettuces. Place protection over your rosemary and you'll be able to harvest well past snowfall. Vented solar bells are modeled on Victorian bell jars. They're made from light weight plastic and their nifty top vents allow for easy temperature regulation. A larger, clear, plastic umbrella cloche can be placed over a cluster of carrots or a container full of kale.
Need something a little larger? You can make a cold frame quickly and easily with some bales of straw and an old window. No special tools (or carpentry skills) required.
If you're assembling this cold frame around existing plants, then obviously you have to put it where the plants are. If you're building it to accommodate seedlings or potted plants, choose a spot in full sun, out of the wind, and facing south. Don't set the frame on a lawn; it's the bare soil that needs to get warm, store heat, then radiate it back at night. If necessary, remove sod from the spot where you want to place your cold frame.
The cover should be angled -- high at the back, lower down front -- to maximize the amount of sunlight that comes in. A cover at a 35-45 degree angle allows the most light to enter the frame and warm the soil. The cover itself can be made of glass, plexiglass, or heavy plastic; whatever is most convenient.
Lay out your bales to form a rectangle, the interior dimensions of which are a few inches smaller than your cover will be. Outline the bales with a sprinkling of lime, then remove them. Dig a trench to accommodate the bottom of the bales. If your location is on level ground, the front of your trench will have to be dug deeper than the back to create the necessary slope. Make the trench deep enough to cover the bottom two or three inches of the bales, giving them a firm foundation.
Lay the transparent cover on top and voila! You now have a basic cold frame. Straw insulates pretty well, but if you're feeling extra-motivated, consider filling some large plastic milk jugs with water and placing them along the interior north wall of the frame. Yes, they take up valuable space, but they also absorb heat during the day and release it at night; it's a solar energy thing. Painting the jugs black makes them even more efficient.
Temperature control is essential; it's easy for a cold frame to get too hot when the sun is shining. A good general rule of thumb is that when outdoor temperatures are in the forties, prop the lid open four to six inches. If temperatures are in the fifties, take off the cover altogether. Be sure to close or replace it before the sun sets to trap the heat inside the frame. On a really cold night, throw a blanket or beach towel over the transparent cover for extra insulation.
A straw bale cold frame should last a few years, so come spring you can use it to get a head start on seed starting and hardening off. And when the bales start to decompose you can use the straw as mulch or add it to the compost pile. Bonus!
Just an hour's work and some inexpensive materials can make the difference between eating your own, fresh-picked spinach in November or buying your leafy greens at the grocery store. I know which one I'd rather do!
Ellen Zachos is the owner of Acme Plant Stuff (www.acmeplant.com), a garden design, installation, and maintenance company in NYC specializing in rooftop gardens and indoor plants. She is the author of numerous magazine articles and six books and also blogs at www.downanddirtygardening.com Ellen is a Harvard graduate and an instructor at the New York Botanical Garden; she lectures at garden shows and events across the country.