We have 3 large high tunnels and one smaller tunnel at Fiddlehead Farm. The larger ones are 30′ X 96′ and the smaller is 14′ X 100′. None of our tunnels have a heat source and everything is grown in the ground. There is very little growth during the middle of the winter season so we need to get our tunnels planted in such as way that the plants are mature by the beginning/middle of November.
The tunnels get warm during the day but at night, they quickly cool down until they are equal to the outside temperature. To deal with this, we cover the plants with a very thin fabric at night. It gets approximately one layer of fabric for every ten degrees below freezing. So when its predicted to be 14 degrees at night, we put on two layers of fabric. In the picture above, you can see Keenan taking the fabric off of the planted beds. During the day, the fabric is taken off of the beds and the sun heats up the ground. In the afternoon, the fabric it put back onto the plants to keep the heat that is radiating out of the ground, close to the plants. This year we had some tomato seedling volunteers sprout in the winter beds (we grow tomatoes, among other things in the high tunnels during the summer). Even after more than a month of nights that had frosts outside and a few nights that dipped down into the teens, the tomato volunteers were still alive. Some had a little frost damage on them, but it had not gotten cold enough to kill them during any of those chilly nights with the agricultural fabric covering the bed.
The tomato seedlings did eventually succumb to the cold, we can’t grow warm weather crops during the winter without heat (and supplemental light). But we can grow a lot of greens, and some other vegetables as well. A partial list would include: lettuce, arugula, spinach, kale, mizuna, radishes, hakurei turnips, scallions, bok choy, chinese cabbage, dandelion greens, fennel, escarole, radicchio, swiss chard, and collard greens.