How to Prepare a Cold Frame for Fall

cold frame
Photo by Ofer El-Hashahar

A cold frame is a means to give tender plants a gentler dormancy through the winter months. It will extend the life of a heartier plant to produce a longer growing season. They are also useful to start an early growing season by protecting the plants from the cold of early spring or late winter. The idea of a cold frame is similar to that of a greenhouse, but it works more to extend a growing season and sustain life over the winter than to have plants produce all year long.

Use recycled materials

A cold frame can be made by using recycled or scrap material; it does not need to be an expensive undertaking. Two options for the cover of the cold frame are glass and plastic. Glass is heavier and less likely to blow away during a winter storm; breakage is unlikely as long as it is carefully handled. Old window panes and windows from unused doors work as a recycled material for the top of the cold frame.  The window could be framed or have a handle installed to make it easier to move.

Any type of wood is acceptable for the sides of the cold frame, even mixtures of wood planks. The wood for the sides and the frame of the window pane should not be painted or finished with any product that will be toxic to a garden, such as lead based paint.

Determine the size of the cold frame

The size of the cold frame will be determined by the size of the framed window. Build a box based on the size of the window using scrap or fresh wood planks. They need to be well secured at the corners with nails or screws. Have four 2×2 blocks for the inside corners of the box.  Two blocks (at opposite corners) will be for the window frame to rest upon, these should be the same height as the box.  The other two should be the same height but installed one inch down from the edge of the box. This is for when more than one box is needed to cover the height of the plants, the boxes will stay secured to each other with these blocks.

Window position according to temperature

During the winter months, the placement of the window, either on, off, or open, will depend upon the weather. On warm sunny days where the temperature reaches above 40 degrees Fahrenheit, the window should be removed otherwise the plants may overheat. A 2×4 works well as a prop to open the window on not-too-cold days, above freezing but under 40 degrees Fahrenheit. The window should be propped open in a direction that the plants will not get direct wind.

Cold frames extend the life of plants, either in the beginning or end of the growing season. They are useful in cool springs and late starting summers to keep plants protected from frost and chills. Used properly, cold frames are a tool that every gardener will appreciate.