No matter what your climate, these plant shelters expand your gardening options
by Adrianna Vargo
A few years ago, while driving with my family through the countryside of Bulgaria, I noticed how strikingly similar the landscape was to that of central Virginia. I also noticed that nearly every home had a garden plot, a grape arbor, and a cold frame of some sort. It made me realize how ancient and universal the practice of using cold frames is.
A cold frame is a simple structure that utilizes solar energy and insulation to create a microclimate within your garden. For those of you who have harvested and eaten a salad of fresh greens in February or have flowers blooming well past frost, you know the attraction of using cold frames. You also know how easy they are to make and use. Although we have a greenhouse on our farm, space is always limited, so we rely heavily, especially in spring and fall, on our cold frames to overwinter plants, extend the growing season, start seeds, and harden off plants.
For whatever purpose you want to use a cold frame, you need to keep in mind a few basic factors. First, some plants fare better in cold frames than others, with low-growing, cool-season plants being the best suited. Second, the type of cold frame you use dictates how much protection you can offer your plants. In all cases, the main conditions you need to monitor and control are temperature, sunlight, moisture, and wind exposure.
Consider Yourself A Zone Warmer
If you love to use tender or tropical plants in your garden but don’t know what to do with them come fall, a cold frame provides a simple solution. It is not a greenhouse: You won’t be able to keep your plants growing lushly through the winter months. But you can provide plants with the right conditions for a gentle dormancy, and they will be eager to resume growth come spring. I keep tender plants in a sunken cold frame, as it provides the best and most consistent insulation. Although my garden is in USDA Hardiness Zone 7, I can reliably overwinter plants that are hardy to Zones 8 or 9—and occasionally Zone 10 if we have a mild winter.
To keep a tender plant in a cold frame, cut it back as much as possible before the first fall frost. If the plant is not already in a container, lift and sink the root mass into a large plastic pot, with plenty of room for soil to insulate it. Pack the cold frame tightly with pots, and add leaves or mulch over the soil surface and into any significant air gaps. Make sure all the pots are well watered.
Once you have your tender plants stored in a cold frame, you will need to control the weather conditions to maintain a healthy dormancy. Keep the soil moderately moist but not wet; the plants won’t need much water over the winter and will rot if overwatered. They should not receive much sunlight, either, because this will encourage active growth. I like to use a white plastic cover on my cold frame as it limits the amount of light penetration and moderates spikes in daytime temperatures.
Temperature fluctuations can be harmful to dormant plants and should be minimized by venting the cold frame. As a general rule, if it’s 35°F to 40°F and sunny, you should open the frame partway, while at 45°F to 50°F, you may need to open it up entirely. Using a sunken cold frame or siting your aboveground frame near a windbreak offers much-needed protection from exposure. This is especially important for those plants that may retain some foliage in winter.
Pay attention to temperature
While heat and humidity are important for germinating seeds, excessive heat (above 90°F) can damage fragile seedlings. A min/ max thermometer hung on an inside wall of the cold frame is a great way to monitor temperature fluctuations.
Give seedlings an early start
Whether you are starting seeds in flats or sowing them directly into the soil, a portable cold frame provides the opportunity to get your plants going a few weeks early, and it eliminates the transplanting shock that many plants face because they will be better acclimated from the outset. If you are seeding in the early spring or fall, focus on cool-season plants, as they tend to have lower temperature thresholds for germination. Keep in mind that seedlings are more susceptible to extreme weather conditions than established plants. If you are sowing directly into a portable cold frame, have it in place two weeks prior to seeding to warm the soil for germination. Whatever method you are using to start your seeds, make sure to keep the seedbed evenly moist. Once seedlings have germinated, the cold frame should be vented more frequently to discourage damping off by increasing air circulation.
If you start your seeds in a greenhouse or indoors under lights, you can start them a good six weeks earlier than usual and transplant them to a cold frame you’ve placed in your garden. It helps to have the cold frame in place at least two weeks prior to transplanting to warm the soil. Again, you will need to pay attention to the degree of sunlight, moisture, temperature, and wind. The frame also provides a windbreak while the plants are still small.
Because you’re encouraging active growth, you will want to use a transparent cover of plastic or glass. The soil will dry out more quickly inside the cold frame than outside, so be sure to keep the soil moist, especially while the plants are acclimating to their new site. Keep in mind that more plants die of excessive heat and drought in cold frames than from cold damage. Proper ventilation is particularly important for cool-season plants. If you have established transplants, vent the frame when the outside temperature is 40°F or higher. If your plants are closer to the seedling stage, you may want to wait until the outside temperatures are 45°F to 50°F before venting.
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* tagged in › Cold Frame, Gardening, Green House, Winter Gardening