If you live in hardiness zones 1 through 6 where winters have freezing temperatures, your only alternative to grow plants in winter is inside the house or a greenhouse. Other zones with milder winter temperatures have the alternative of using a cold frame. Carole Ottesen, in her article Cool Cold Frames, "Country Gardens" Early Spring 2010, explains that a cold frame has raised sides to protect the plants and a glass top to keep out the chill. It is used to extend the growing season later into winter and allows earlier spring planting.
Most herbs are warm-weather plants, such as basil, rosemary, sage, marjoram, oregano and thyme. Those herbs do better in pots inside the house on a sunny windowsill. The window should get six hours of sunlight. Turn the pots so the herbs don't get lopsided by bending towards the light source.
Herbs that like cooler temperatures, such as parsley, dill and coriander, will do just as well inside in pots.
If your kitchen has a sunny window, keep the pots in the kitchen and snip the herbs to as needed.
Lettuces grow nicely in a cold frame. Place the frame against the house, a building wall or stone fence so the sun's warmth is absorbed during the day and released at night. Grow the lettuces in flats or 6-inch pots. Leaf lettuces work better than head lettuces. Snip off leaves to harvest.
Plant greens such as chard, kale, mustard greens and spinach directly in the soil in the cold frame. Add 6 inches of potting soil and 3 inches of compost over the garden dirt to improve it. Cover the soil with clear plastic directly on the ground until the seeds sprout. The plastic traps heat and moisture. Harvest leaves at any stage they're big enough to pick. Snip them off with scissors, and the plant will continue to grow.
Prop open the glass top of the cold frame to release heat on warmer days.
Tomatoes, Cucumbers and Peppers
Charlie Ryrie, in his book "The Country Garden," says that tomatoes, cucumbers and peppers may successfully be grown inside or in a greenhouse. These warm-season vegetables require six to eight hours of sunlight, so place them in a sunny window. Start in 4-inch pots as seedlings, then transplant to 1-gallon-size containers with adequate drainage. Feed every time you water with a water-soluble fertilizer diluted to 1/4 strength. In other words, if the directions say use 2 tablespoons per gallon of water, only use 1/2 tablespoon. Pollinate the flowers with a soft brush, dusting each flower. Or use a commercial blossom-set spray.
Broccoli, Brussels Sprouts and Other Cabbage Family Members
While these are cool-season vegetables and prefer lower temperatures, most get too tall to easily grow outside in a cold frame. They also have a longer time to maturity, which means a hard freeze may hit before you can harvest them. If winters are mild and it seldom gets below freezing, start the crops as seedlings in August and transplant to the garden in October for a December crop.