With the cold weather we’ve had lately it’s hard to imagine that anything could be growing in the unheated high tunnels around our region. While some growers do let their tunnels rest over the winter, others keep them in production, growing crops of cold hardy winter greens – how do they do it?

The first step is to use a full-sized high tunnel. You might think that a smaller tunnel would be easier to keep warm but in fact, the opposite is true. The large volume of air in a high tunnel acts as a buffer, warming up quickly on a sunny day and cooling down more slowly than the outside air at night.

Growers do need to pull covers over their plants every night and on cloudy days, whenever the sun isn’t out. These layers vary between single and double layers of rowcover, the spunbound fabric home gardeners use outdoors, and sometimes a sheet of plastic on top of that on the really cold nights.

Only the most cold-hardy crops will get through the winter without additional heat, but spinach, kale and various mustard greens do fine. Lettuce and chard are less hardy but do well in all but the coldest, darkest months of December, January and early February. By mid-February the days become long enough to really make a difference to winter crops in a tunnel.

Some growers may supply a small amount of heat, either through tubing in the soil much like household radiant floor heat, or with short term heat sources during the coldest weather such as a wood stove or a propane heater.

Another way growers in Northern New York produce winter crops is to plant two sets of crops. One set is planted in late August for harvesting all fall into December. The second set is planted in late September or early October, but will not be harvested until mid- to late February when the days are longer and new growth begins. This second set of crops grows enough in the fall to become established, and is then covered with a couple of layers of rowcover and left alone for the deepest part of winter. This crop is held in a sort of dormant, cold storage until February. Then, once the days lengthen and the sun becomes stronger, they are uncovered on the sunny days to resume growth and can be harvested from late February through spring.

This process of growing throughout the winter is far more complicated than I can fully explain here, but this overview gives you an idea of how it’s done. An excellent resource on this topic is The Winter Harvest Handbook by Eliot Coleman.

Locally-grown winter greens are available at the Winter Farmers Market in Plattsburgh, the first and third Saturdays at the Plattsburgh City Gym. For more information about the market and products available visit http://www.plattsburghfarmersmarket.com/. If you do not live near the Plattsburgh area, ask your local Extension office for winter markets in your area.

Photo: Greens growing in a high tunnel in Willsboro.