Cold frames fall into the category of season extenders – an honor they share with hot beds and greenhouses. Usually unheated, they provide protection for tender seedlings and allow starting seed beds before all danger of frost is past. For areas that have a short growing season, this can be of great benefit. They are distinguished from hot beds by not using a manure compost or heat cables to warm them. A cold frame is usually a boxlike structure with glazing that is slanted toward the winter sun, creating a passive solar heat effect. Good insulation and opening and closing the cold frame are an essential part of this gardening process.

Building a cold frame

In its simplest format, a cold frame is an insulated box that is topped with a clear cover. The cover can be created by using old storm windows, specially cut and framed glass, old shower doors or even clear plastic stretched over a frame. The structure is not heated, and should not be considered as a substitute for a greenhouse or a hotbed – those are different structures, each of which has its own unique features. A cold frame works by using solar gain to warm the soil inside the insulated box.

A traditional cold frame structure is a box in which the back wall is about three feet tall, and the front wall is about six to twelve inches tall. The top of the back wall has a lip that enables fastening hinges to it to facilitate opening and closing the structure. This is an important feature because as the season advances, it can become quite warm inside the bed during the day, but it will still cool off somewhat when the sun goes down. The sides of the cold frame are shaped to support the lid and to give a good fit when the frame is closed. This helps to prevent drafts from chilling the young plants.

Cold frames can be hastily assembled using other materials. An easy, quick cold frame can be created by making a perimeter of straw bales and resting old storm windows across them. Any old window will do as glazing, but storm windows typically have aluminum frames, making them a little lighter and easier to handle than a conventional window. Concrete blocks or bricks can also be used, especially if the edges are lined with flakes of straw from a rectangular bale. A south facing slope can be dug out to create a mini earth berm for young plants.

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Bottom line key for creating a cold frame

It needs a clear top that faces southward and gets six or more hours of sunlight during the day. It needs sturdy sides that are braced against wind or animal damage. In colder climates, it is a good idea to insulate the back and ends to keep the warmth inside the cold frame after the sun goes down.

Preparing Soil for Planting in a Cold Frame

Soil preparation in a cold frame is like any other garden bed preparation. If you know that you will want a cold frame in the spring and you want to seed it directly, it is a good idea to prepare the ground in the fall while the weather is still warm. However, if you missed that step, you can still add “store dirt” (soil that is purchased in bags from the garden department of a supermarket or hardware) to your cold frame. It is a good idea to set the cold frame in place about two weeks before planting so the soil will warm up. If you are in doubt as to whether the earth in your cold frame is ready for plants, use a compost thermometer to check the soil temperature. If you do not have one, you can even use a meat thermometer. The important thing is for it to have a sensor that can be pushed into the soil to discover the temperature below the surface. When the soil temperature consistently registers about 60 degrees Fahrenheit, your cold frame is warm enough to receive seeds or plants.

Planting in a cold frame

Cold frames are not heated, unlike hot beds or greenhouses. Therefore, plants that will do best in a cold frame are those that are naturally tolerant of chilly temperatures. Kale, cabbage, turnips, snow peas and even some types of lettuce thrive in chilly spring temperatures. They can be started in pots or flats in the house, or sown directing into prepared soil. The cold frame helps make the best of early season sunlight and protects the young plants from falling temperatures at night. Cold tolerant vegetables can be sown directly in the soil, if desired.

A cold frame can also be used for hardening off plants that have been started indoors. The pots or flats can be carefully placed inside the cold frame after the sun has had a chance to warm it up in the morning. Exceptionally tender plants might need to be returned to the house or greenhouse at the end of the day. As the season warms and temperatures begin to stabilize, these plants can be left out longer. When the seasons stabilize, they can then be moved into the vegetable garden.

Maintaining a cold frame

Unless you are fortunate enough to be able to afford a premade cold frame with a cover that opens and closes thanks to a thermostatically controlled cover, your cold frame will need daily maintenance. Depending upon the weather, the cover might need to be opened or removed during the day to keep from overheating the plants inside. It will then need closed back up at night to prevent heat loss.

The plants inside the frame will receive less moisture from rainfall than plants that are uncovered. Therefore, they will need to be judiciously watered. This is probably best done with a watering can or a slow-release plant bulb waterer to prevent shocking the young plants with cold water.

As with any garden plant growth, they will need to be checked for mold, fungus or insect pests – who also appreciate the warmer environment inside your cold frame.

Using your Cold Frame as Protection

Finally, cold frames can be used as shade beds for plants that are sensitive to excess sunlight or insect pests. When the weather has warmed to the point that glazing is no longer needed, refit or exchange the cover for one that is covered in a fine mesh netting. If added shading is needed, the bed can be draped with a loosely woven cloth. When the weather at the end of the growing season begins to threaten your garden with frost, you can bring back the glazing to protect those last plants and allow them time to mature.

Cold frames are an easy and often inexpensive way to extend your garden’s growing season. Once again, remember that they are not heated, and therefore should not be considered a substitute for a greenhouse or hotbed. However, they are less expensive and somewhat easier to maintain than either of these season extenders.