"How do you grow food in the winter?"
We grow all of our crops in soil, without supplemental light. Two of our 34x96 high tunnels are unheated, and one is heated to 28 degrees. There are four main principles to follow when growing food in the winter in northern climates. They are light, timing, double covers, and variety selection. By paying attention to these factors and principles, we are able to extend the harvest season on many crops and extend the growing season on others.
Every latitude reaches a point in the year when day length is fewer than 10 hours. This period of prolonged darkness for us at 48 degrees north is between Halloween (October 28th) and Valentine's Day (February 13th). Since most cold-hardy crops are more sensitive to day length than actual low temperatures, we know that as day length begins to increase in February, our crops will respond with increased growth even though the outdoor temperatures are typically as cold as they get all winter.
All of our unheated winter crops are grown in high tunnels. These provide two main functions. They keep the weather (snow, wind, rain, sleet etc.) off the crops, and they also allow us to apply double covers. We erect low support structures over our rows of crops and draw a thin blanket of spun bonded polypropylene (the material that long johns are typically made of) over them at night, and remove it during the day to allow more light transmission. These "low tunnels" provide slightly warmer micro climates that mean the difference between high quality re-growth and low quality stagnant plants. They also explain why we use the term "high tunnel" for our structures; the double cover system consists of low tunnels and a high tunnel.
The growth rate of all annual vegetable crops is much slower in the fall, winter and spring than it is in summer. There are many crops that simply won't survive temperatures below freezing. Those that will, take much longer to grow than the "days to maturity" listed in the seed catalogs once temperatures dip below freezing and day length shortens. A 30 day crop such as spinach might take as long as 90 days to mature when seeded on November 1. We take prolonged maturation rates into account when creating our crop plan to ensure we have a variety or crops to harvest at any given time from November through June.
Many seed catalogs are extremely helpful in providing information on which varieties have been trialed in cold weather and are good winter selections. They are a good place to start. We trial numerous varieties on our own each season to try and make new discoveries of winter hardy varieties. As mentioned earlier, many crops just can't survive frosts, such as tomatoes, certain varieties of lettuce, cucumbers, and basil. We don't try to grow those. We gravitate towards those varieties that can survive frosts on their own, and then we provide them a little extra protection with low and high tunnels to get more growth and higher quality. The crops which we heat minimally can survive in an unheated environment, but we've found that their quality and re-growth are great enough to justify the extra energy use.