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 that, late summer/fall-sowing of cold-hardy/frost-tolerant plants that can 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 (check out this U. S. Navy resource and customize it to your location: Duration of Daylight/Darkness Table). 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. When your daylight falls below 10 hours per day, plant growth nearly stops, so you'll want your overwintering crops to be at least 75% mature when this happens if you want to harvest them the following year and closer to 100% mature if you want to harvest them during the winter.
Use this chart as a guide for when to sow for fall/winter harvest and when you need to protect crops for overwintering. It can also be helpful to print the chart and make notes on this chart, customizing it for your garden.
Most overwintering crops can be direct-sown provided the soil is near the ideal temperatures to germinate seeds. See our Soil Temperature for Higher Germination for guidance on soil temperature and use a soil thermometer to check. If the soil is too warm for some crops, like lettuce and spinach, you may have to start them indoors. Since it can still be hot in the late summer/early fall, it's best to keep up on the watering, as well.
Mulch. Root vegetables fare well if 4 or more inches of mulch are applied before a hard freeze to help keep them warm. Snow can be a free, easy mulch solution for these crops.
Row covers. Above-ground crops, like lettuce, may be smashed by heavy mulch, so "tunnel" covering may be better. Tunnels 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 when temperatures fall below freezing. Form the arches above the mature height of the plants, as leaves touching the plastic or row cover can be damaged by frost more easily. Made out of special, permeable, gauzy material, row covers come in different degrees of protection (increasing temperatures under the hoops by about 2°–8°F), and 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. You can also choose to use sheets or blankets, but because they generally do not let a lot of light or moisture in, be sure to uncover plants whenever the temperatures allow.
A cold frame can also be constructed for added protection (increasing by about 10°–20°F), and an early start in spring.
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!