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Cold Frames Extend Summer

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Oh, my aching muscles...

Did your summer vegetable garden just get going when the hot weather left and fall arrived, wiping out hopes for more fresh vegetables this year?
Do not give up—all is not lost: You can build a cold frame. Wait! It’s easier than it sounds.

In the old days, cold frames often consisted of nothing more than old storm windows, placed over young plants to protect them from wind and frost, and to keep warmth and moisture close by.

Today, cold frames are still easy to make, and they’re an obvious way to extend the Northwest growing season throughout fall and winter. They can be as simple as plastic-covered huts for your tomatoes, or as complicated as the creative gardener’s imagination permits.

Experienced gardeners say frames should be built in the warmest area of the yard, with southern exposure to provide maximum light, and with a sloped lid to provide runoff for rain or snow. All cold frames really are is a little added protection. The premise is basic: cover your crops, and the added warmth will provide just the right boost to convince your garden that summer is not over yet.

Cold frames are like boxes to cover your plants, comprised of four walls and a lid. Walls can be constructed of just about anything: different materials will provide different levels of heat retention. The top should be made of a transparent material—plastic is the easiest to work with—to allow sunlight in. Look around the house for materials: 8-mil plastic works well to top the cold frame. Because the plastic is flexible, you will need to brace it. Lighter plastics  also work, but don’t expect them to last beyond a single season. And the thinner the plastic, the more structural support is needed to keep it from blowing away in a strong wind.

Cedar, cypress and redwood are naturally rot-resistant, but don’t feel you have to use any one type of wood for the frame—or wood at all, for that matter. Some gardeners favor concrete, cinder blocks (on their sides to prevent air from leaking through) or other materials that may hold the heat longer and better than wood. Think heat, and insulation. Ventilation can be built in, or it can be as simple as propping open the lid on sunny winter days.

Your structure doesn’t need to be gigantic: make it as large or small as you need. Most cold frames are no more than a few feet wide and deep—enough to cover a small vegetable bed.

If you want additional longevity and support in your cold frame, try using corrugated plastic (often used as deck covering). Clear corrugated plastic is available at your local home-improvement store.

If you enjoy a challenge, cold frames can be quite elaborate.  Some gardeners build cold frames that look more like mini-greenhouses, building them adjacent to walls, backyard sheds or garages.

Some gardeners build their cold frames in pits. In our rainy climate, be sure to build in adequate drainage. Putting a cold frame into the ground also requires a lot of digging (recommended depth is 6-8 inches), and it may be a more permanent alternative than you’re looking for.

If you’re really not the go-out-and-build-it type, a single-season cold frame can be made from bales of straw. Place them in a U-shape around your garden bed, and cover the top with plastic or glass. Then, when spring comes, use the straw as a mulch on your soil.

Just because your plants have a little added protection to help them grow and stay warm in cooler months doesn’t mean they  don’t need all the care and tending plants regularly demand. Check your frame periodically to ensure that your garden is getting all the fresh air, water and sun it needs throughout the year.

What to Grow?
Some gardeners say just about any crop will work. Others recommend non-hybrid species, or short-season crops, such as lettuce. Here are a few suggestions for crops that work well:
•  Greens. Tunnels can help revive some of last year’s crops, such as Swiss chard. Spinach, too, works well.
•  Lettuce. Try head lettuce, as well as specialty varieties such as arugula and radicchio.
•  Bulbs. Cold frames can be used to force bulbs into early bloom. Place dormant bulbs in decorative pots, and force into bloom for holiday gifts.

For more information:
For a copy of the King County Cooperative Extension’s free publication on Plastic Plant Protection, (206) 296-DIAL (3425) or, outside the Seattle area, call (800) 325-6165.

Cold frame supplies and frames are available from garden-supply companies, including The Yard Works in Lynnwood and Charley’s Greenhouse Supply in Mount Vernon.
NWGN archive published October 1996.

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