Cold Frames

By Lori Rose

Winter is here. Frosty mornings have already killed our heat-loving garden plants like tomatoes and basil. But that doesn't mean we can't continue to garden. One of the easiest ways to extend the growing season is with a cold frame. Cold frames are easy to construct and manage, and allow us to grow some of the hardier vegetables for an additional month or two in winter. They also allow us to start growing a month or two earlier in spring.

A cold frame is simply an enclosed gardening area with a clear top to shelter plants, let in sunlight and warm the soil inside. A cold frame is like a small greenhouse without heat. It can be large or small; portable, removable or permanent. Cold frames can be purchased in a wide variety of sizes, materials and prices, with or without accessories like thermostat-controlled ventilators. But they are so easy to build, it is worth building one to start off. If you find you love cold frame gardening, you can trade up next year.

The ideal place for a cold frame is a sunny, well-drained, south-facing spot against a house or other heated building. If the location does not face south, just be sure it gets at least six hours of sunlight per day. If your site is not ideal, here are some helpful tips.

If the site does not have good drainage, standing water can freeze and kill the plants. To ensure good drainage in a cold frame, dig out the top three inches of soil and put a layer of coarse gravel in the hole. If you plan to grow in the ground instead of in pots or flats, amend the soil with some compost before shoveling it back into the hole on top of the gravel.

If the site is not against a heated building, insulate the walls of the cold frame with old blankets, foam insulation boards, bubble wrap, straw or leaves. You can even use a clear milk jug with the bottom cut out to cover individual plants. Remove and replace the cap to control humidity.

In order to reach all the plants easily, and make the cold frame worthwhile, it should be larger than two feet by four feet, but no larger than four feet by six feet. The cover must be transparent to let in as much sunlight as possible. Try an old window, an old shower door, a sheet of clear plexiglass or thick plastic sheeting tacked to a frame. These can be attached to the cold frame with hinges to keep them in place, especially if you use plastic sheeting in a windy part of the yard.

Build the sides of the cold frame with hay bales, bricks, cinder blocks or wood. When using cinderblocks, stand them so the holes are up and down and cover the tops of the holes. Just about any kind of wood can be used to construct a cold frame, but avoid pressure treated wood as it contains toxic chemicals like arsenic. Use galvanized nails or screws to hold the wood together.

Whatever material is used to make the cold frame, be sure to angle it by building the back of the frame four to six inches higher than the front so rain and melting snow can easily run off the top.

Try painting the outside of the cold frame black to absorb heat and the inside white to reflect sunlight around the plants.

If there are already raised beds in the yard, constructing a simple tent-like top over one of them will create a custom cold frame right where you already garden.

For ideas on constructing your own cold frame, there are photos and basic instructions on twenty different types of cold frames at www.epicgardening.com/cold-frame-plans. Maybe use the top of one plan and the bottom of another to build exactly what is needed for your own yard.

As with regular-season gardening, successful extended-season gardening depends upon paying close attention to the plants and their surrounding conditions. With that in mind, here are a few cold frame specific do's and don'ts.

Keep the temperature inside the cold frame as close to sixty degrees as possible for plants that normally grow in spring and fall. Put a thermometer inside where it can be seen easily. The way to regulate the temperature inside the cold frame is by lifting or propping the lid open. The goal is to let excess warmth escape. When it is warmer than forty degrees outdoors, open the lid six inches until late afternoon. Close the lid to trap heat inside for the night. 

On frigid nights, the plants inside the cold frame may need some extra protection so they don't freeze. Most heat escapes through the glass or plastic, so cover the it with blankets, straw or newspaper to insulate the cold frame. Remove the insulation as early as possible in the morning to maximize sunlight.

Water plants in cold frames with water that is at least as warm as the soil. It should be warm, not hot. Don't use cold water as it can chill the plants and cool the soil, reducing the effectiveness of the cold frame.

So what are the best vegetables to grow in a cold frame in the fall? Just about any fast growing salad ingredients like spinach, radishes, baby or round carrots, bak-choy and arugula. Any variety of lettuce, green or red, whether mesclun, leaf lettuce or heading lettuce. Many varieties of Asian greens such as the peppery mizuna. How wonderful to have fresh salads from your own garden on the holiday table.

But cold frames are not just for extending the fall growing season. Use them to start the spring growing season a month or two early. Grow the same salad ingredients as in fall and be the first in the neighborhood to have garden fresh salads. Start cool-temperature spring vegetables like cauliflower, celery and Swiss chard in late winter. Start heat-loving summer vegetables like tomatoes, cucumbers and melons in early spring. Use the cold frame to acclimate seedlings grown indoors to outdoor conditions. Start flats of flowers like sweet alyssum, snapdragons or marigolds to get those first spring bouquets a little earlier than usual.

Still prefer to purchase a cold frame instead of building one? From basic, manual cold frames to mansion-worthy greenhouses, here are some websites with high quality merchandise and reasonable prices to get your shopping spree started. Gardener’s Supply Company (www.gardeners.com) has a number of cold frames in a variety of sizes and prices, to sit on the ground or on top of raised beds. Williams Sonoma (www.williams-sonoma.com) has an affordable cold frame that conveniently sits on the ground or on top of their VegTrug, a well-designed portable raised planter. And of course there is always Amazon (amazon.com) where you can find cold frames in any size and material to fit your needs and budget.

Whether you grow plants in pots, flats, or right in the ground, a cold frame can keep them cozy longer into the winter and allow you to start gardening again earlier in the spring.

Lori Rose, the Midnight Gardener, is a Temple University Certified Master Home Gardener and member of GardenComm: Garden Communicators International. She has gardened since childhood, and has been writing about gardening for over twenty years.