What to Grow
Cool cole crops. Kale (really delicious after it's exposed to frost) is a dependable crop anywhere. Many varieties are reliably hardy down to zero or even below with a good snow cover. Collards are equally hardy. In milder areas (Zones 7 and 8), broccoli lasts through winter with the added bonus of no cabbageworms. It comes back to life as soon as the days start getting longer. Try one of the sprouting broccolis, which are bred to be started in the fall for early spring production.
Tough greens. Endive, escarole, radicchio, spinach, and many Asian greens stand up well to cold. These include a whole range of mustards, mizuna, and shungiku (edible chrysanthemum greens). Chard often survives winter freezes well. Lettuce and parsley take quite a bit of frost and, when covered with snow, often last to spring. Spinach planted in fall produces some tender leaves before winter shuts it down. Mulch it well, and it starts growing again when temperatures warm up in spring.
Deep roots. Leeks, carrots, parsnips, beets, and other root crops are protected from the cold by the soil they are growing in. You can sow a second crop of most of these in midsummer or leave some of your spring-grown crop in place, using the garden as a storeroom and harvesting through the winter. Covering the plants with about 8 inches of straw or other loose mulch when hard freezes arrive protects the roots and makes them easier to dig up when the ground is solid.
Crops for fall and winter production have the same needs as spring and summer vegetables: good drainage and fertile organic soil. But because the essential microbes in your soil are less active when the earth is cold, add compost or other organic material as well as organic fertilizer whenever you plant vegetables for a second harvest. It is especially important to renew soil that has just produced another crop. And practice patience. Shorter days and cooler nights slow down growing, adding time to the maturing process.
The most dependable way to extend your season is to set up a tunnel made from plastic tubing bent over into hoops covered with a sheet of clear plastic. This offers more protection than floating row covers and is most useful for really cold-hardy greens such as kale, chard, or mustard greens. Don't seal it too tightly; the sun can fry your plants even in midwinter.
If you have no plastic tunnel set up and no snow cover, protect your plants from a hard freeze by throwing a tarp over the bed temporarily. This also works during occasional ice storms.
Keep a lookout for plants that have been heaved by freeze/thaw cycles and settle them back into the ground.
You already understand the satisfaction that comes from growing and eating your own vegetables. Double that thrill when you harvest even a few vegetables in the cold and dark of fall and winter. Now that's a gardening challenge worth taking on.