Four-season gardening is a pastime that’s growing nationally, and you don’t have to live in a tropical area to do it.
Locally, the Mississippi Cooperative Extension Service has been experimenting with hoop houses, and Bill Evans has produced some great results.
That includes producing succulent, red tomatoes when there’s frost outside. This is not from a “hot house,” or heated greenhouse, but from plastic hoop houses outdoors.
Hoop houses, also called high tunnels, work by using layers of plastic to trap warmer daytime air inside and minimize heat loss from the system at night. They don’t have to be big or towering affairs. You can bend plastic pipes over metal rebar spikes pounded into the ground and covered by plastic.
For details, visithttp://www.dafvm.msstate.edu/landmarks/09/spring/25.pdf. Or, Google high tunnels and frames and look at the photos.
The key to winter growing is recognizing which plants will grow in different conditions.
For example, my wife Annette planted carrots in our cold frames, which are glass or plastic boxes outdoors that use sunlight during the winter to stay warm. Just make sure and vent the glass during the day. At night, keep the cold frames closed and they’ll retain heat.
Carrots are slow growers when the days are short (the hours of daylight makes a bigger difference than temperature for some plants); so we’ll – hopefully – have lots of carrots this spring.
Withstanding a freeze: Some plants will grow quite well even in bitterly cold conditions.
For example, last year, I was digging turnips through January and harvesting mustard greens. Some days, the plants would be bowed over and covered with a glaze of ice. Surely they were dead, I thought. But no, once the sun came over the trees, the ice would melt, and steaming in the sunlight, they would straighten up.
Kale for example, can survive cold down to about 20 degrees. Brussel sprouts, cabbages, radishes and beets – along with mustards and collards – can survive cold temps because they actually have antifreeze proteins that allow them to survive below freezing. But they have to get roots established and develop hardiness first.
Winter growing is limited by this fact: Greens require 28 degrees to grow. So, if you’ve got 10 weeks or so above 28 degrees, you can grow greens. (See: http://msucares.com/crops/comhort/greens.html.)
Beyond that, if you have a hoop house, high tunnel or cold frame, you can extend the season.
One trick we learned with our unheated greenhouse is to plant the greens, then when temps go down, cover them in Agribon. That way, they get the benefit of the trapped greenhouse heat, as well as protection from frost within the greenhouse.
This way, you can continue to grow down to about 22 degrees. Below that, you have to provide heating.
A good book on the subject is The Winter Harvest Handbook: Year Round Vegetable Production Using Deep Organic Techniques and Unheated Greenhouses by Eliot Coleman (Chelsea Green, 2009, $29.95).