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Extend the growing seasons with a cold frame

A peony ready to bloom is seen through the glass of a cold frame and reflections of mature trees April 29, 2016, at Brucemore in Cedar Rapids.
A peony ready to bloom is seen through the glass of a cold frame and reflections of mature trees April 29, 2016, at Brucemore in Cedar Rapids.
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Extend your Iowa growing season by weeks with a cold frame.

A cold frame is basically just a miniature greenhouse. And like a greenhouse, a cold frame can extend your growing season. A greenhouse, if heated, allows you to grow plants all year long. A cold frame in Iowa will expand the growing season by two to four weeks in the spring and again in the fall.

Construction is simple: A piece of glass or plastic that is laid on top of a frame. Old windows are often repurposed for this use. The frames are usually made of wood, though they can also be made of concrete block or even brick. The frames should be at least a few inches tall, but sometimes they are a foot or two tall to allow for ample plant growth.

Cold frames can sit on top of the soil. But the most efficient types are buried by a few to several inches in the soil to take advantage of its insulating properties. (If you add heat to the cold frame, through heated cables or another method, you have a hot bed, and extend the growing season by months rather than weeks.)

The frame is constructed, usually, so that the top slopes to shed rain and snow. With most cold frames, the transparent top is tilted and facing south so that it can get the most sunlight possible, using passive solar energy. This passive solar is key. Some people, in fact, even put plastic jugs of water in their cold frames so that the water heats up during the day and slowly releases a little bit of warmth at night. If your cold frame has transparent sides, siting it to face the south is not critical. No matter what type of cold frame you have, you need to place it in full sun.

There is no bottom in a cold frame. It rests on or is partially filled with soil. This allows you plant seeds or transplants directly into the soil. You can also grow plants in the cold frame in pots or flats, started elsewhere, and set into the cold frame.

How to use a cold frame in Iowa

In a very cold climate like Iowa, the most practical and popular use for cold frames is for getting a head start on growing greens like lettuce and spinach.

Conditions can vary from year to year, but as a rule, a cold frame can allow you to plant two to four weeks earlier than normal in the spring and protect plants for two to four weeks later in fall.

With radishes, spinach and lettuce, for example, you can plant them as soon as the soil can easily be worked. Without a cold frame, in the Cedar Rapids area, that’s usually the end of March. With a cold frame, planting time would be early March.

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Cold frames also can be used to “harden off” tender seedlings of warm-climate annuals, such as marigolds, tomatoes, peppers or petunias. When you grow or buy seedlings that can’t tolerate frost (the last frost in Cedar Rapids is usually mid-May), you need to keep them indoors, where sunlight is limited. You can, of course, carry the plants outside every single day and then bring them every night, but that can be a time-consuming hassle. With a cold frame, the frost-sensitive plants spend their days inside the warm sunny cold frame and at night, the glass protects them from frost. No carrying.

In the fall, plant cool-season vegetables in October, leaving the lid off or fully open. Then, when hard frosts threaten, close the cold frame at night and open again at sunrise. You’ll be able to harvest into November.

What kind of cold frame for you?

There are many types of ready-to-assemble cold frames out there, but they can cost hundreds of dollars. Cold frames are easy even for beginners to make. Base your design on any old windows you can find, using one or two or even three. You can also use a piece of plexiglass, new or repurposed. Look online for building directions.

You don’t even need to build a cold frame, depending on what you want to use it for. Some years, I’ve relied on a makeshift cold frame. I simply propped glass windows up at an angle along the south side of my garage. On either end, I put pieces of scrap plywood, held in place with stacked bricks. I then used it to harden off several flats of seedlings I’ve purchased, removing the plywood each day for ventilation.

Depending on how you are going to use it, a cold frame should be generously sized. Unfortunately, too many ready-made cold frames are undersized. The most useful cold frames stretch at least 5 or 6 feet horizontally, are about 3 feet across, and measure at least 12 inches from the interior soil to the lowest part of the glass top. This allows you plenty of room to plant rows of greens or several flats.

Ventilation is a must

Like a car on a sunny day, cold frames can get so hot that they damage or even kill plants. To combat this, cold frames must be properly vented.

Many gardeners do this the old-fashioned way. On warmer days, they go out and prop up open the cold frame top and close it again at night. Fancier cold frames have automatic venting systems that lift the lid when the interior temperature reaches a certain point.

Cold frames aren’t for everyone. If you are a beginning or small-scale gardener, a cold frame probably isn’t for you. But if you want to experiment with extending our Iowa growing season, and enjoy the bounty of your garden longer, consider a cold frame.

l Veronica Lorson Fowler is co-publisher of The Iowa Gardener website at theiowagardener.com.

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