By David Kennedy, Director of Leaf for Life, author of Eat Your Greens (2014) and 21st Century Greens (2011)
In order to photosynthesize and make food, plants need a minimum of both sunlight and warmth. Summer tends to have both. Most of the temperate zone gets about half of its total annual allotment of solar energy over 100 summer days.
Cold weather is very hard on the plants that produce our food. They can’t move to seek shelter the way animals do. Temperatures below the freezing point of water not only stop photosynthesis, they quickly kill many plants. When water within and between the plants cells freezes, sharp ice crystals form that can rupture the cell walls that protect the plant’s key functions.
{{ image(3118, {“class”: “flor round”, “width”: “300”, “height”: “200”}) }}And yet, some food can be grown in cold weather. How can kale, collards, mustard greens, Swiss chard, scallions, turnip greens, beet greens, lettuce, and various Asian cabbages and mustards continue producing food long after tomatoes, peppers and zucchinis have closed up shop for the winter? Cold-hardy plants use two different strategies for preventing fatal ice crystals from forming. The most common approach is to lower the freezing point of the water they hold with dissolved sugars and the amino acid, proline. This is like putting salt on icy roads to lower the temperature at which the water freezes. The less common but more amazing trick of cold-hardy crops is the use of specialized “antifreeze” proteins. These are molecules that bind to the surface of tiny ice crystals and prevent them from forming the large sharp crystals that rupture plant cells.
Almost all of the food crops that grow well in cold weather are greens. This is no coincidence. With all plants it is the green leaves that initially create the food. When there is adequate warmth and sunshine most plants can afford to send much of the food formed in their leaves to be saved in roots, as with sweet potatoes, or to make fruits and seeds. Both of these processes require a surplus of carbohydrates to be produced by the leaves, and that surplus is produced in the summer.
With leaf crops we eat the initial stage of production before the plant generates surplus food. Most people think of warm summer days as the time to garden, but leaf vegetables can be successfully grown in much cooler weather. Seeds should be started a bit before the first frost so they can get in some growth before the onset of very cold weather. Once established, the hardiest greens, like Siberian kale, can be grown right through the winter in most locations. A simple cold frame or floating row cover can provide some extra protection against the cold.
Your winter garden will grow very slowly, but it will grow. And the bright flavorful greens that you harvest through the snow are most welcome in the middle of winter. Packing more vitamins, minerals, and protective antioxidants per calorie than any other category of food, winter greens can help keep you healthy and fit till the first buds of spring. Cold weather greens are Slow Food at its best.
