There are two purposes for fall sowing: to harvest in late fall or winter, or harvest in spring from plants that can overwinter. "Overwintering" means just what it sounds like, cold hardy plants sown in fall can stay in the ground over winter, due to their ability to survive harsh winter weather and still produce a useable crop. Some of the hardiest crops like carrots and spinach can survive very low temperatures (-20°F with mulch), but others need protection below 25°F. Choose varieties based on your climate and how much protection you can provide.
First, know when your daylight falls below 10 hours a day. A few good resources are http://aa.usno.navy.mil/data/docs/Dur_OneYear.php and http://www.timeanddate.com/sun/. Because plant growth is relative to day length and temperature, and because both are reduced in fall/winter, crops need more days to mature than in spring and summer. Use the chart below as a guide for when to sow for fall/winter harvest and when you need to protect crops for overwintering. You may find you want to adjust the number of weeks in your area due to warm soils. It can be helpful to use this record keeping chart, too. Crops should be a least 75% mature once your daylight falls below 10 hours per day for best survival.
Fun fact: The time of year when daylight falls below 10 hours a day is known as the "Persephone period", a term coined by gardening author Elliot Coleman in reference to the Greek mythology story of Persephone. Persephone lived with her husband, Hades, in the underworld during the winter months, which caused her mother, the goddess Demeter, much grief, so she ceased all plant growth during this time.
Most crops can be direct-sown provided the soil is near the ideal temperatures to germinate seeds. See our Soil Temperature for Best Germination Chart for guidance on soil temperature and use a soil thermometer to check. Lettuce and spinach will not germinate in soil temperatures above 80°F and 85°F respectively, which may mean starting them indoors. Remember, seeds sown in warm weather may need to be watered twice daily in order to maintain consistent moisture.
Snow is very insulating and, for less sensitive crops like carrots, piling snow on them can be a free, easy solution, provided it doesn't create a prolonged period of soggy soil when it melts. Root vegetables fare well if 4 or more inches of mulch are applied before a hard freeze to help keep them warm. Above-ground crops, like lettuce, may need a "tunnel" covering, which can be made using pieces of sturdy, 9-gauge wire from a hardware store. Arch the wire over the bed and then cover the hoops with clear plastic or row covers (i.e., special, permeable, gauzy material) when temperatures fall below freezing. Keep the arches above the ultimate height of the plants, as leaves touching the plastic or row cover can be damaged by frost more easily. Row covers come in different degrees of protection (increasing temperatures under the hoops by about 2°-8°F), can be double-layered to increased insulation; or plastic can be applied on top of row cover material. However, the more protective the row cover, the less light it lets in. Because of that, it is best to remove covers on days with temperatures above freezing. A cold frame can also be constructed for added protection (increasing by about 10°-20°F), and an early start in spring; read more in our Cold frame blog.
Harvest only in temperatures above freezing. A frozen leaf can only recover while still on the plant, and if harvested while frozen, it will remain limp from frost damage.
Growing frost tolerant vegetables is a great way to extend your harvest, and keep the fresh vegetables coming!