Cold Frames Provide Gardening Benefits

Cold frames are easy to construct, require little space, are relatively inexpensive and may be used for many years after construction. A cold frame is nothing more than a rectangular bottomless box, usually made of wood. Normally, they are built only 12 to 18 inches high in front, and about 6 inches higher in back. They can be built in any convenient width or length.

The top is covered with a frame of some transparent material, usually glass or clear plastic. Used window sashes sometimes are utilized when available. Because there is no need for heat in a cold frame, they may be used in any part of the yard or garden without regard to the proximity of heat sources. Some gardeners make portable cold frames that may be taken away in the spring if space is limited and needed for another use in summer. Portable or temporary cold frames may be placed directly around tender plants in beds.

Small rooted cuttings or plants that are very young benefit from this protection as well as do less hardy varieties of the chrysanthemum. A small herb garden, containing such plants as parsley or chives, may be covered with a temporary cold frame to keep the plants growing and usable for long into the winter. Such temporary cold frames may be better insulated by piling soil around the sides after they have been set into place. Most cold frames, however, are not built as movable or temporary structures. They are built and used in one location. Plants such as chrysanthemums are lifted and placed in them for winter protection.

More plants can thereby be overwintered since they can be placed close together. Potted hyacinth or tulip bulbs for winter forcing may be put in them to receive cool temperatures they require before flowering. Potted bulbs can be placed pot-to-pot in the cold frame now, and covered with straw or mulch. They may then be brought indoors in January for flowering in February. Some garden plants other than chrysantemum benefit from winter protection in a cold frame. These include perennials that have been started from seeds in the late summer or fall, such as pansy, delphinium, Iceland poppy, primula, dianthus or Oriental poppy.

A cold frame needs to be ventilated on sunny days by raising the cover. During the coldest parts of the winter, the cold frame should be kept closed, and when the cold frame is used for winter protection of plants, the top should be covered with mats, old carpets or straw to provide shade on sunny days. This maintains a more uniform temperature in the frame by preventing heat buildup on sunny days, as well as rapid heat loss after sunset. Bright light is not required during that time.

In spring, as the plants kept there for winter protection are removed, the cold frame may be used to harden young plants that have been started indoors for planting into the garden. Seedlings of hardy plants such as cabbage, broccoli, snapdragon or petunia may be moved into it first. Tender plants, such as tomato or marigold, should not be moved into a cold frame until temperatures outdoors are fairly mild. The top may be gradually raised and removed to acclimate the plants to outdoor conditions.

Building & Using Cold Frames by Charles Siegchrist.