Cold Frames

Cold frames allow gardeners to extend the growing season.

A cold frame is a walled box with a clear roof. It can be raised above the soil level, sunken, or sit on top of the ground. The transparent roof allows sunlight in and keeps heat from escaping. The result: a warm, sunny, productive growing space!

The microclimate created within a cold frame protects against frost damage and it allows seedlings to be started earlier and winter crops to be grown later. Many cold frames are temporary arrangements, but there are also permanent cold frames. Two things to keep in mind when starting with cold frames: it will take a couple of weeks for your cold frame to warm the soil; and, more plants die in a cold frame from heat and drought than from cold. Consider yourself warned!

So, what, exactly, are cold frames?

The Lost Gardens of Helgian, in Cornwall, England, feature formal cold frames (Daderot) Public Domain

Cold frames, greenhouse, hotbeds and hoophouses

Hoophouses are large tunnels made with plastic sheeting laid over bent rod frames. Hoophouses, used predominantly in commercial agriculture, are heated, humidified, vented, and irrigated.  Greenhouses also tend to be large, and they are heated. Cold frames are small and, well, they are cold. Traditionally, cold frames were built against the southern wall of greenhouses (in northern latitudes; the other way around in southern latitudes). These cold frames were used to harden off seedlings started in the greenhouse, before moving them to the garden proper. Hotbeds, or hotboxes, are the same size as cold frames, but they generate or retain a lot more heat. In each case, a more nurturing microclimate is being created, providing growers with the ability to grow more food, flowers, and ornamentals over a longer period of time.

Old glass windows are commonly used to make DIY cold frames (Clark Hamilton) CC BY 2.0

Cold frame design

You can certainly buy cold frame kits, but this innovation is easy to make for yourself. There are tons of free online instructions and designs available. [Personally, I enjoy the variety found at Instructables.] The next time you see someone getting their windows replaced, grab the old one(s). These old windows act as the roof to your cold frame. All you have to do is build a 2-foot tall frame for the sides. Just be sure to use wood that is rot resistant, such as redwood or cedar, and that it does not contain any toxic chemicals, paint, or stain.

Ideally, your frame will tilt the glass toward the winter sun, allowing the space to collect as much heat and light as possible. You will want to make sure that the angled roof directs rain away from your home’s foundation, if that is where you put your cold frame. If you do not have access to old windows, you can also use plastic sheeting, clear plastic panels, or row cover material. When building a cold frame, be sure that you can prop the cover up, or remove it, during warmer weather, or you may cook your plants.

Crops suited to cold frames

Being relatively low to the ground, cold frames are suited to low-growing, cool season crops. This includes lettuce, spinach, scallions, radishes, turnips, rutabagas, parsley, cilantro, chard, beets, and daikon, just to name a few. You certainly won’t bring sunflowers to harvest in a cold frame, but you can use one to nurture their seedlings, giving them an early start.

Cold frames and dormancy

Unless you have a greenhouse, or bring them indoors, some tender perennials simply do not look their best during the winter months. Some may not be able to survive at all. You can use a cold frame to protect these plants from harsh winter weather. They will still go dormant, but it will be a gentle dormancy, rather than a life threatening struggle to survive. This makes them better able to thrive when spring arrives. To overwinter dormant plants in a cold frame, use these steps:

1. Cut the plant back as much as possible, depending on the species, before the first frost.
2. If the plants are not already in containers, dig them up and put them in pots.
3. Fill the cold frame with potted plants, stuffing the spaces between pots with mulch, straw, soil, or dry leaves.
4. Top the pots with mulch.
5. Water well, at first, then taper off as temperatures drop.

Heat management in cold frames

The whole point of a cold frame is to protect plants from weather they might not otherwise be able to handle, especially damage from frost. However, problems can arise when a warm day occurs in the middle of winter. This can trigger plants into thinking it is time to start growing again, even if it’s January. On days when temperatures are 35°F to 45°F, open the cold frame part way. If temperatures reach 45°F to 50°F, open it completely. [Just be sure to close it again before you go to bed!]

Seedling support

Just as cool weather crops can be grown deeper into winter, seedlings can get as much as a 6 week head start when started indoors or in a greenhouse, and then hardened off in a cold frame. Seedlings can be planted directly in the soil and protected with a portable cold frame, or potted seedlings can be held in a permanent cold frame until they are strong enough (and temperatures outside are warm enough) to be given their place in the soil. Seeds will need to be watered regularly. Once they germinate, be sure to vent the cold frame frequently to avoid damping off disease of seedlings.

Most pests are kept out of a well-designed cold frame. The same characteristics will also prevent pollinators from doing what they do, so don’t expect to be able to grow melons or squash in your cold frame unless you hand-pollinate the flowers.