Under Glass

For many gardeners, the season is over after the first frost. Our part of Ohio just is too cold to do much except clean up and tend a few houseplants. Yet, there is a whole world of gardening that can be accomplished over the winter if you have a cold frame. I am often surprised by how little many gardeners know about this simple approach to winter gardening.

Cold frames, at their simplest, are structures that hold glass or plastic over the growing area. The solar energy during the day heats the soil and air trapped inside the frame. The air provides insulation, which prevents the heat from escaping during the night. By using some types of cold frames, you can increase the soil temperature enough to grow all season long. Others are simply to keep the frost off of dormant plants, allowing the gardener to grow plants that would suffer if left in the open.

The name comes from one type of cold frame. The namesake structure resembles a raised bed. A simple four sided “frame” is built to hold glass above an area of ground. However, that is not the only type of cold frame. They can range from small glass garden bells (cloche) placed over a single plant to large row covers.

Cold frames are sometimes used to grow vegetables all year long. If you are growing a kitchen or an herb garden, this is a great use of your garden space. This application is often built at the site where your vegetable garden is the rest of the year. But you can also use the window wells of a house with basement windows. Window well covers are fairly inexpensive. They will provide a good protection against the loss of heat from your home, but also give you a small space where you can tuck a tender potted plant to provide sunlight and winter shelter.

For people training bonsai trees, cold frames are exceedingly useful. As trees, the bonsai need to be allowed to go dormant. Yet, because of the pot, they are very susceptible to cold weather. If you have a cold frame, you can shield the plant from harsh winds and much colder roots than it would have if left in the ground.

In the nineteenth century, glass was expensive and plastic was yet to be invented. The very wealthy invested in glass structures to help them extend the growing season, grow tropical plants, or protect tender perennials that needed cold temperatures for dormancy but could not withstand the harsh winds of winter. Conservatories, greenhouses, and cold frames were all ideas that developed during that time. Sadly, many modern gardeners feel these options are too expensive, too difficult, or too large to fit into their existing gardens.

But cold frames are so simple! The advantages are obvious and the prices can be very reasonable if you understand the basics. There is a Fact Sheet, called “Cold Frame, Hot Bed Construction and Use” (HYG-1013–88) that can be downloaded for free at http://ohioline.osu.edu/hyg-fact/1000/1013.html if you would like information on some simple cold frame projects to build yourself. If you are not interested in a building project, they can be purchased from many different suppliers.

Spring catalogs will be coming soon, but there’s a real sense of loss as the days shorten and the nights grow cool. Before your soil gets too cold, you might want to consider a cold frame. Cold frames are useful for growing cool season vegetables, like broccoli, lettuce, and peas. In an Ohio winter, there is no greater joy than trudging through snow out to the garden to clip some fresh greens for your salad.

Wendy Wolpert is an OSU Extension Master Gardener volunteer.