AMES, Iowa -- In the fall, cooler temperatures signify an end to the harvest of many vegetables in the garden. However, if you use cold frames you can extend the growing season. These simple structures allow you to continue to garden and harvest fresh produce even when temperatures turn cooler. In this article, horticulturists with Iowa State University Extension and Outreach answer questions about building and using a cold frame in your garden.
What is a cold frame?
A cold frame is a raised bottomless box with clear covers that acts much like a small greenhouse. These structures protect plants from wind, rain, snow, ice and cool temperatures. The ground and frame of the cold frame are heated by the sun during the day and radiate that heat during the night, keeping the inside warmer. They are used to extend the growing season by two to four weeks in both the fall and the spring, allowing you to start plants outdoors sooner and continue to grow plants later in the growing season.
Hotbeds are similar structures used to extend the growing season, but they rely on external or active heat sources, like electric heating cables, rather than solar heating.
How do you build a cold frame?
A number of materials can be used to create a cold frame. Kits and pre-assembled cold frames are available for purchase. Most often these simple structures can be created using repurposed materials. The basic cold frame has two components: a frame and a transparent covering.
Most cold frames have frames built of wood. Although treated lumber will last longer, wood treated with creosote or similar chemicals should be avoided. Untreated wood can be painted to reduce decay from contact with soil and improve appearance, especially if leftover scraps are recycled to construct the frame. White latex paint is commonly used because it also reflects light. However, the inside walls could be painted black, which absorbs and radiates heat back into the structure. The frame can also be constructed from cinderblocks, bricks, poured concrete, plastic lumber, or straw bales.
The transparent covering can be any number of materials purchased or repurposed. An old glass storm window, window sash, or shower door are great recycled coverings. Clear plastic or other durable material such as rigid plastic sheets or fiberglass panels can also be used. Mount the covering using hinges to make opening and closing the cold frame easier.
Locate a cold frame over good, well-drained soils that don’t flood or collect water. Ideally, the cold frame is oriented to face south. A southern exposure provides maximum sunlight and heating capacity. To reduce heat loss, position the north end of the structure near a home, garage or fence. For convenience, a nearby water source is helpful.
Often the dimensions of the cold frame are dictated by the materials being repurposed. Be sure the frame is not more than 3 feet deep to allow you to easily reach the back without stepping inside. Ideally, the frame is about 12 inches high in front and 18 inches high in the back. This slope allows more sunlight to enter the protected area. Taller frames can be built to accommodate taller plants but if boxes are too large it will be difficult to reach inside. For seasonal use, consider constructing the cold frame utilizing pins or eye hooks so it can be easily assembled or dismantled for storage.
What can I grow in a cold frame?
Cold frames are great for growing cool-season vegetable crops like lettuce, spinach or radishes. Typically in Iowa, you can plant these crops in early April. When using a cold frame, you can start these vegetables two to four weeks earlier, depending on the weather conditions. This allows for an earlier harvest and a much longer harvest period. In the fall, uncovered cold frames can be used to start cool-season vegetables in late August or early September and as temperatures cool or when the threat of a hard frost comes, the cold frame can be closed overnight to protect the plants. This can allow you to continue to grow and harvest well into November most growing seasons.
Cold frames are also great for hardening off tender seedlings or transplants grown indoors or recently purchased from a garden center. Warm-season vegetables like tomato or pepper, as well as summer annuals like marigolds or petunias, will not tolerate a frost but need to be placed outside to slowly acclimate them to outdoor conditions (harden them off) before planting in mid-May. This process often requires bringing plants back indoors overnight when temperatures dip below freezing and then shuttling them back outside once the freezing temperatures have passed. When plants are hardened off in a cold frame, all you have to do is close it up at night to hold in heat and keep plants from getting cold damage when frost threatens.
How do I garden in a cold frame?
Temperature and ventilation must be controlled to avoid overheating plants. This is an active process and at times may be time intensive since the temperature inside can change rapidly. Check cold frames frequently, even on cloudy days, to prevent temperatures from getting too warm and killing the plants inside.
Use a thermometer to determine when to open the cover for ventilation. The sash should be raised when the air temperature inside the cold frame rises above 85 degrees Fahrenheit. It should be closed before sunset to retain heat. A simple board or dowel can be used to prop the cover open. Automatic openers are also available. The simplest types consist of wax-filled cylinders that expand and push open the cover as temperatures rise and then contract and close the cover when it cools off. Insulating the frame with straw bales or mounds of soil can help moderate temperatures inside the cold frame, reducing drastic temperature swings.
Photo Credit: Cold frame, by Martina Berg/stock.adobe.com