By Minda Daughtry and Rhonda Gaster
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 endure 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 our climate and how much protection you can provide.
The time of year when daylight falls below 10 hours per day is known as the “Persephone Period” referencing the daughter of Demeter, Goddess of the Harvest in Greek mythology. According to the The US Naval Observatory that period for Sanford, NC for 2018-2019 is from November 30, 2018 through January 12, 2019. You can find information for U.S. cities or towns and other locations around the world as well as specify a year at http://aa.usno.navy.mil/data/docs/Dur_OneYear.php/. This charts information to use as a guide for when to sow for fall/winter harvest and when you need to protect crops for overwintering.
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 daylight falls below 10 hours per day, plant growth nearly stops. Overwintering crops should be at least 75% mature when this happens in order to harvest them the following year and closer to 100% mature if you want to harvest them during the winter. If you are growing from seed, you will need to check the maturity date on the packet, pull out your calendar and start backtracking to 75% of the maturity date to determine when to sow the seeds. If you are planting transplants, you will need to estimate how old the transplant is based on it’s size and species. All vegetable transplants have an ideal age/size that enable them to continue active growth in the field after transplanting and be somewhat resistant to environmental stress.
Most overwintering crops can be direct-sown provided the soil is near the ideal temperatures to germinate seeds. 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 watch the irrigation closely, as well so your plants don’t dry out.
Understanding the cold tolerance of vegetables is the key to knowing when your vegetables may need a little added protection. Add row covers or low tunnels so that your harvest carries into winter. Root vegetables perform 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 be damaged by heavy mulch, so consider the low tunnel covering approach. Tunnels can be made using pieces of sturdy, 9-gauge wire arched 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, because leaves touching the plastic or row cover can be damaged by frost more easily. Row covers come in different degrees of protection that can increase temperatures under the hoops by about 2°–8°F. These 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.
Harvest only in temperatures above freezing. Even frost-tolerant crops will be limp with frost damage if they are not allowed to warm up on the plant first. A frozen leaf can only recover while still on the plant, and if harvested while frozen, it will remain limp from frost damage.
Source: ncsu.edu