Chilly nights and frosty mornings would normally signal the end of the outdoor growing season for most home gardeners, but this does not have to be the case. Using some low-tech solutions, you can harvest the goodness that springs forth from the ground well into the winter months and get an earlier start the following season. By using high tunnels, cold frames or in-ground growing methods, fresh greens and crunchy root vegetables are as close as your own backyard.
High tunnels, also known as hoop houses, Quonset tunnels, cold frames, poly-houses or any combination of any of these terms, are structures comprised of arched or gothic-style hoops, usually made of metal or PVC and covered with a synthetic, translucent material. The difference between a high tunnel and a greenhouse is that high tunnels are generally not permanently installed and planting is done right in the ground.
Some high tunnels have supplemental heating and lighting sources, but most rely on the light energy from the sun. Like greenhouses, sunlight enters the tunnel and not all of it escapes as light. Some of that energy builds up as heat, which is what allows the tunnel to house and grow crops several weeks beyond what could be produced outside. Tunnel sizes vary greatly depending on the purpose, the scale of production and the physical limitations of the space. High tunnels are often custom-made to fit a particular space, but are generally 10-30-ft. wide and 20-90-ft. long.
To successfully use a high tunnel and achieve maximum yields, you must plant crops that tolerate colder climates. Even though your tunnel may be 40°F warmer inside than out on a sunny February day, it will not be able to support heat-loving crops such as tomatoes, eggplants and zucchini. It will, however, offer a safe haven for your root vegetables like carrots, radishes and turnips, and for greens such as lettuce, Swiss chard, kale and spinach. A double layer of covering, with either spacer blocks or an inflating fan, will increase the insulation value of your tunnel. Adding low row covers to your crops within the high tunnel will also achieve the same effect.
For those lacking the space or means to construct a high tunnel, a cold frame may be just the answer for harvesting hardy vegetables for your larder. Cold frames differ from high tunnels in scale, although they work the same in principle. They are usually no more than 3-ft. high and are often installed on the south-facing side of a building to take advantage of as much available daylight as possible, as well as the heat reflecting off the building and a certain amount of ambient heat from the building it is next to. Like high tunnels and greenhouses, cold frames trap some light energy as heat, keeping the soil inside several degrees warmer than the soil outside. Like high tunnels, only cold-hardy plants will thrive at this time of year.
Cold frames can be built in the middle of an existing garden or in their own area. One inexpensive option is to build a wall out of straw bales and cover the top with translucent plastic sheeting. Like high tunnels, cold frames can be constructed to fit any space, but unlike high tunnels, they can often be made entirely of free or re-purposed materials, such as old windows and doors.
Constructing gardens below the frost line to take advantage of geo-thermal heat is another option, although quite a bit of excavation work is often involved. To figure out where to put your in-ground garden, observe where the winter sun falls and consult your local county extension service to determine where the frost line is in your area. Once you are armed with this knowledge, you can begin digging. The excavated material can be used to berm and build up as you go. Reserve a portion of the topsoil to go back into the excavated area, as you may be down into clay or subsoil, which lacks the nutrients and tilth of healthy topsoil.
In-ground gardens are generally sited on a south-facing slope. Once you have achieved an appropriate depth, amend your soil as you would in your above-ground garden. Plan on adding copious amounts of compost or other nutrients as there may be little or no organic matter at this level.
Once you are satisfied with the depth and width of your in-ground garden, find a suitable translucent or relatively clear cover to let in as much light as possible. Keep in mind that air circulation will be a challenge underground, so you will need to ventilate as often as possible and a cumbersome cover will make this a daunting task. Using a cover with a hinged opening or the ability to prop it up is a suitable solution for this dilemma. Animals will likely be intrigued by this type of garden. Plan for this by making sure there are no wide openings in your cover and consider a mesh covering over your crops or over the top of your structure.
If you do not succeed at first with your in-ground garden, try different crops and play around with planting times, but again, do not expect warm-season crops to thrive during colder months. Like high tunnels and cold frames, in-ground gardens will help you enjoy cold-tolerant crops for several extra weeks or months longer than usual, but they will not allow you to completely overcome your hardiness zone.
There are a few other options for those wishing to cheat Old Man Winter out of some of his due. Using stone borders and embedding stones within your planting beds can raise the soil temperature by a few degrees. As the stone absorbs sunlight, it is able to radiate a portion of it into the soil. This will only benefit those plants directly adjacent to the stones, but for small beds, this can have a positive impact. The darker the stone, the more heat it is able to collect and disperse.
A similar approach is using reflective materials for mulch, such as light-colored plastic, marble chips or lids from metal cans. Again, these techniques work better on a small scale. You can reflect light and catch snails and slugs at the same time if you use a lid from a glass jar and place a small amount of beer in it. The snails and slugs are attracted to the beer instead of your plants’ leaves. These reflective traps can be set daily and the inebriated invertebrates can be hand-picked out.
Finally, a surefire way to extend the growing season, which can be done by anyone with a windowsill or grow lights, is to grow as much of your own food indoors as possible. Even growing a pot of basil in your kitchen or some greens on a windowsill can help you get one step closer to food independence and make the time of year irrelevant to the food you grow.
Whatever method is right for your particular situation, remember that with all of these growing methods, you will have to account for irrigation, as none of these growing areas will receive the direct benefits of a rainfall.