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GardenSMART :: Using a Cold Frame

Using a Cold Frame

By Therese Ciesinski, GardenSMART

Are you eager to get your spring garden started, but it’s still too cold to put plants outside? A cold frame can give you a jump on the season.

A cold frame is simply a box with a clear lid that holds plants that need protection from bad weather such as freezing temperatures, snow, and wind, while exposing them to sunlight, warmth and circulating air. The sun provides the heat – there is no supplemental heating. A cold frame creates a microclimate that allows plants to get a strong head start on their growth.

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Cold frames are usually used in spring to start seeds and harden off seedlings, in fall to extend the growing season, and in winter to overwinter tender plants. If you start seeds indoors in flats, having a cold frame means you can start them as much as six weeks earlier than you could without one.

With a cold frame, you can adjust the temperature by opening or closing the lid. This also helps you monitor the amount of light, humidity, moisture, and moving air the plants receive. It can be partially or completely open during the day, depending on the weather, and closed for warmth at night. 

A cold frame is good for:

  • Hardening off seedlings before planting them outside.
  • Starting seeds of cool-weather loving vegetables, herbs, and flowers.
  • Growing very early spring vegetables such as lettuces, arugula, and spinach.

Site a cold frame somewhere that gets full sun, but ideally is out of the way of the wind. Keep in mind that temperatures above 90 degrees F can damage seedlings, so venting is essential. When the sun is too strong, you can deflect it by covering the lid with a white sheet.

When using it to seed directly into the soil or to move seedlings started indoors outside to harden off, place the cold frame over the site two weeks or so beforehand to allow the soil to warm up.

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Don’t forget to keep the seedbed or seed flats evenly moist. Seedlings have thin leaves and fragile root systems that can’t tolerate drought. What’s deceptive about a cold frame is that while the outside air temperature may be cold, inside the warmth generated by the sun can dry out the soil.

Not all plants are suited for a cold frame. Plants that already tolerate some cold do best. Warm season crops such as tomatoes and peppers don’t do well because in most climates the level of heat these plants require to flower and fruit cannot build up to sustainable levels. Also, as a practical matter, tall plants or extremely large plants won’t fit, since a standard cold frame usually isn’t all that tall or wide.

You can buy ready-to-assemble cold frames from a garden center, big box store, or online, or build your own from a window sash and wood or polycarbonate. The lid can be made out of a translucent plastic instead of clear glass.

Low-tech cold frames can be made from stacked bricks and plastic sheeting, cinder blocks with a window laid on top, hay bales, plastic bottles, or old glass doors. The amount of heat you can trap, and cold you can keep out, depends on what the cold frame is made of. You can find many articles and videos on how to build one on the Internet.

Whether you make or buy it, a cold frame can give you a head start on enjoying the fruits of your gardening labor.

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