Focus On: Using a Cold Frame

What is a Cold Frame?

Cold Frames have been used for hundreds of years by both vegetable and ornamental gardeners for a host of gardening needs. Once gardeners start using a cold frame, they don't know how they ever did without it. It is an essential component of successful gardening, or gardening made easy.

A cold frame is, in essence, a protected bed that will provide you with a shelter during cold temperatures that protects young plants in both spring and fall. Cold frames have no moving parts or electrical components: They rely on heat generated by solar energy, in which the sun's warmth is absorbed by the soil within the cold frame and then released slowly as the temperature drops. Cold frames provide a few degrees of warmth while keeping frost off tender plants prior to their transplant to the garden, which is critical to gardening success.

What types of plants are best grown in a Cold Frame?

The cold frame is an exceptional transitional structure from indoor to outdoor gardening. Its number one use is for acclimating indoor-sown seedlings to the outdoors, by providing protection against frost and allowing the seedling to adjust to cooler outdoor conditions and natural sunlight. Seedlings grown in a cold frame will not be shocked when permanently planted; they will grow uninterrupted, which equates to earlier harvests and higher yields.

Gardeners have had much success using a cold frame for tomatoes, peppers, and all vine crops, such as squash, cucumbers and melons. Gardeners also use them for all the popular flower varieties such as zinnias, sunflowers, petunias and impatiens, just to name a few.

Many gardeners use cold frames as a method of production for cool season vegetables. Nearly all salad greens such as lettuce, spinach, arugula and mesclun can be direct sown up to a month earlier in a cold frame, bringing your favorite healthy leafy greens to a much earlier harvest. In the fall, a cold frame can protect these same leafy greens for up to a month from cold weather, again providing extended harvest. Between spring and fall, you can easily extend your garden harvest by 60 days.

Cold frames can be used to overwinter your harvested root crops. Root crops can be healed into the soil for easy digging on warm, sunny winter days. Potatoes, onions and carrots store exceptionally well in the cold frame. Tender bulbs dug from the garden and non-hardy perennials can be overwintered as well, so you will not have to buy those bulbs or plants every spring.

What is involved in operating a cold frame?

When you situate the cold frame, it needs to face south so the sun's rays enter the cold frame as long as possible during the day. The cold frame needs to be located in a very well drained location with no standing water at any time. Providing a 2-4'' layer of sand or gravel will enhance drainage. Inside the cold frame, use high-quality, well composted compost or purchased potting soils, free from weed seeds.

Ideally, butt the cold frame up against a structure that will block the wind, or use bales of straw to create a northern windscreen. Do not allow the windbreak to shade the cold frame. Not all cold frames are above the soil: Many of the historic cold frames were below the soil line, which allowed for even more protection and took advantage of the heat given off by the earth.

Ventilation is very important. During a bright sunny day, the temperature inside the cold frame will build. A slight opening of 2-4'' will provide ample ventilation to allow the excess heat to escape, but will still protect your cold frame contents. Ventilation will also prevent condensation from forming and dripping water onto the contents. In a very short amount of time, you'll understand the simple needs of the cold frame, which is basically adjusting the doors based on the anticipated weather.

Protecting Plants with a TunL Cover

Want to know more?

Most gardeners start with 1 or 2 Cold Frames, learn the operational requirements, and install additional ones as their gardening needs increase. Some gardeners use cold frames as a large seed-starting bed. Many vegetables and flowers can be direct sown into a cold frame and transplanted very successfully to the garden. During extreme cold in spring or late fall, a blanket can be placed over the cold frame in the evening for additional insulation. When training in the use of your cold frames, a high-low thermometer is very useful to see the temperature fluctuations. The thermometer will help in managing the ventilation timing. When watering inside a cold frame, morning watering is best to allow for the leaves to dry during the day. In Zone 6, spinach has been grown all winter in a cold frame. Cold frames are also a great place to store your recently delivered plants: Gardeners often use them when they receive their orders from catalog shopping.