A cold frame will give you greater control over the weather by creating nearly ideal conditions where and when your plants need them most. A cold frame will extend your growing season, and allow you to grow plants suited to a warmer climate. You can think of a cold frame as a passive solar collector that captures the warmth and light of the sun.
Cold frames are most commonly used in spring for starting seedlings, but they can be used for much more. Fall crops will benefit from a moist, shaded cold frame during the heat of summer. The same cold frame can be used for forcing spring bulbs during the winter.
Cold Frames Defined
A cold frame is a rectangular structure with a glass top. Most face south and have a slanting roof that enables snow and water to slide off rather than accumulate. The angle of the slope ranges from 35 to 55 degrees, as this range captures the most sunlight during summer and fall conditions. A white interior is used to reflect sunlight onto the plants, and blocks or poles are used to prop the box open for air circulation.
Choose a site for your cold frame that will maximize exposure to the sun. It should face south, but southeast or southwest are the best alternatives if a site with a true south exposure is unavailable. Choose a site that receives full sun from mid-morning to mid-afternoon during the winter and spring. Prepare the site to be as level as possible and so that it drains well. Protect your cold frame during the winter by siting it with a fence, hedge, or building on its north side.
Choose Between Permanent and Portable Cold Frames
A permanent cold frame is dug into the ground or built on the surface. A cold frame above ground provides less protection from frost than one that has been dug into the ground, but both offer better protection from frost than a portable frame, and will last longer, as they are sturdier.
A portable cold frame is essentially a box without a bottom, with a clear cover. They are less sturdily-built than permanent cold frames and are therefore less insulated, resulting in a wider fluctuation in temperatures. There are many types of portable cold frames that are available commercially, and many of them are collapsible for storage.
Choosing Between a Cold Frame and a Hot Bed
The only difference between a cold frame and a hot bed is that the hot bed contains an artificial source of heat, such as hot compost or a heating coil. This reliable source of heat allows you to use them earlier in the season and they maintain ideal conditions for starting the widest variety of seeds.
Create your own hot bed by digging a pit at least one foot deep, to hold the heating cable. Spread a layer of gravel at the base of your pit, and then a two inch layer of vermiculite. Lay the cable in long loops on top of the vermiculite. Don’t cross the cable, and keep the loops at least 8 inches apart and 3 inches from the edge of the frame. Spread a one inch layer of sand over the cable, and then a layer of screen or hardware cloth to protect it from digging tools. Lay 4 to 6 inches of coarse sand over the wire, and place your cold frame above this.
Bright sun on a closed, insulated and unventilated cold frame can generate temperatures over 100 degrees. Place a thermometer in a shady spot within the cold frame to help you determine when to ventilate it.
With a newly-established cold frame, be sure to check soil conditions frequently so that you do not under- or over-water your plants, especially in warm weather. The majority of plants require soil to be moist but not soggy. Check your soil at a depth of 1 to 1 ½ inches. Avoid watering on cold and cloudy days, and water early in the day to discourage fungal growth. Cold water will shock plants and slow growth, so use water that is roughly the same temperature as the soil inside the cold frame.
Pests and Disease
Insects, pests and mold will thrive just as well as your plants in your cold frame or hot bed, during mild weather. Check regularly for signs of disease and pests. Proper ventilation and generous spacing of plants can lessen the occurrence of disease. If problems persist, remove your plants and sterilize your cold frame. Pour boiling water on the gravel or soil at the base of your cold frame, or leave the lid shut to allow heat to build during the summer.
Spring Uses for a Cold Frame
Harden seedlings already started indoors by moving them into your cold frame one or two weeks before transplanting them. Shade them at first and protect them from burning with burlap or by painting the glass lid with a mixture of clay soil and water. Acclimate them by opening the vent for a longer period each day.
Germinate the seeds of cold-tolerant vegetables, perennials and annuals in your cold frame. Sow the seeds in pots or directly in the soil of your cold frame 2 months before the last spring frost date, or even earlier in a hot bed.
Summer Uses for a Cold Frame
Remove the glass top of your cold frame and replace it with screen. Use wood lath to shade fall crops of heat-sensitive vegetables such as lettuce. Use the cold frame for rooting cuttings.
Fall and Winter Uses for a Cold Frame
Cold treat seeds by planting them in your cold frame for spring germination. Sow the seeds just before the ground freezes. Use your cold frame for fall crops of lettuce and spinach. Store herbs, perennials and container plants in your cold frame for protection over the winter. Force spring bulbs in your cold frame.
All-New Encyclopedia of Organic Gardening: The Indispensable Resource for Every Gardner, Fern Marshall Bradley and Barbara W Ellis, Editors