Winter survival depends on more than just cold. Excess dryness, alternating high and low temperatures, and saturated soil will all kill plants that might have survived simple cold. Here is a simple checklist for winter survival:
Good soil preparation is key–plenty of compost will encourage good growth that is also hardy, as well as promoting both good drainage and water retention. Since water logging and drying are two major causes of winterkill, this is a simple way to give your garden its best chance. Make sure there is plenty of potash, which gives your plants hardiness and disease-resistance. (greensand, ashes, or seaweed are good sources) Seaweed increases cold hardiness and disease resistance by supplying several other substances besides potash, and is a valuable supplement for winter gardens, either as mulch, as a soil amendment, or as foliar spray. Kelp meal is a type of seaweed available in some garden stores that can be incorporated into your soil. Maxi-crop is more concentrated, and is water-soluble: easy to use when watering, or as a foliar spray.
Stable soil temperatures can be more important than how cold it gets. Plants that are hardy in cold or even frozen soil will be killed by alternating freeze-and-thaw conditions. Plants “wake up” and come out of dormancy during thaws, only to be hit unprepared by the next cold snap. Freezes will tear roots as the ground heaves, and then again when it thaws. Worse yet, the top layer of ice or frozen soil can thaw into a puddle that can’t drain, blocked by the frozen layers below. This rots the crown of the plant, from which both roots and tops spring, and the plant dies. The best way to address this problem is by mulching well, and doing it about the time of the first frost, so that soil stabilizes at a cool but not extreme temperature. If freezing and thawing is a big problem in your climate, consider shading your winter crops so that they don’t thaw every time the morning sun hits them. In a climate with frequent thaws, the north-facing slope in shade will loose fewer hardy plants than the south-facing sunny slope, because the plants on the north slope will stay safely dormant. Sometimes a cold frame or hoop house will help a lot because it can keep the soil from getting too wet and thus avoid rot, It will also, of course, mitigate cold snaps. However, it will also heat up much more during sunny spells than the outdoors will, so take care to avoid overheating.
Plant protection can be layered, just like your clothes. Combine different layers to increase your protections till further. Generally speaking, each layer you add will give you a half of a climate zone additional warmth—zone 7A to Zone 7B for example.
Mulch would be the first layer of protection. Be sure you don’t mulch over the crown of the plant, only around it. And be alert for an increase in mice and slugs, which can live in mulch.
Fleece, also called floating row cover, and often trademarked as Agribon or Reemay, requires no support (it “floats” on the leaves) and gives enough protection for spring and fall frosts. It admits enough light that it can be left in place. Fleece is a good choice for someone who doesn’t want to have to do the venting or watering that greenhouses and other covers may need, but it can be crushed by snow and gives little shelter from excessive wind or rain.
The next level of protection, and the most versatile, is a cold frame. This is a wood box with no bottom and a slanting glass or hard-plastic top. It is placed over the planting area, protects from wind and extreme temperatures, and can be propped open to vent on sunny days. It is easy to open the hinged lid and harvest food. Snow on the rigid lid can be left to provide insulation in very cold weather, or swept off with a broom. You can glue foam insulation on the box for extreme climates, and add plastic or fleece for cold spells.
A plastic row cover or tunnel will extend your season and go together cheaply and easily. You can even get them ready-made with built-in wire supports. Or, flexible pvc pipe works well. For a really sturdy row cover that will resist wind, rain, and snow loads, use longer supports than if you were going straight across the bed from side to side, and place them in a row of X’s (as seen from above), tying them where they cross. Cover with clear plastic, weighted with rocks soil, lumber, or pipe. In heavy snow areas, sturdy endposts and ridgepole of metal pipe or lumber can support the conventional wire, plastic, or sticks that hold the tunnel’s shape.
A hoop house is this like a row cover/tunnel, built tall and wide enough to stand and walk around in. They can be purchased as kits or made from pvc pipe and clear plastic sheeting. Plans are also available for “Gothic” arched greenhouses made from lumber rather than plastic pipe. Paradise Lot shows one of these.
You can “stack” protection as weather gets colder–fleece inside a coldframe or greenhouse for example. An extra sheet of clear plastic thrown over any of these covers will increase their effectiveness, especially if there are spacers to keep the layers from touching.
It is surprisingly easy to make a the most effective of all winter growing situations–a hotbed: dig a hole 3’x4’ and 2’ deep. Fill it with manure or garden waste like grass clippings and straw. Then cover with 8-12” of dirt, sow seeds, and cover with a cold frame or plastic tent. The composting process will generate heat over a long period, enabling salad crops to grow all winter. This works well for spinach, lettuce, and the hardy greens, and was the main source of salad crops in Europe before the era of cheap oil. In extreme climates, the hotbeds can be sited inside of a greenhouse.
Plans for row covers, cold frames, greenhouses, and root cellars can be found in Four-Season Harvest, by Elliot Coleman, or check your public library.