Here’s How Building a DIY Cold Frame Can Extend Gardening Season
Set up a cold frame to protect your less established plantings in early spring and fall.
Gardening doesn’t have to be restricted to summer, even in colder climates. While your plants may not normally welcome crisp, cool temperatures of fall or the winter days that dip below freezing, you can help them weather the changing seasons. Spending just a weekend building a cold frame will allow you to start your garden earlier and grow later into the fall. Heed these guidelines for your DIY cold frame for the best chance at success.
What are cold frames?
By design, a cold frame is just a see-through five-sided box that covers the garden to protect it from the elements. Its transparent roof invites sunshine, then traps the sun’s heat and the earth’s moisture, keeping the environment five to 10 degrees Fahrenheit warmer than the outdoor temperature. A flip-top door grants access to the gardener for tending to his or her plants.
Simplest versions of these structures have been used for centuries to start cold-tolerant plants in the early spring, harden off seedlings before transplanting, shelter tender perennials, and even overwinter plants and cuttings.
When can you begin growing season using a cold frame?
The growing season will depend on the types of plants your cold frame contains and their preferences. Cold frames will generally keep the garden five to 10 degrees Fahrenheit warmer than the outdoor temperature. So, if your plants are hardy enough to grow in 40-degree weather, you may be able to start while there’s still frost out.
Hotbeds do one better to extend the growing season. Electric cables and/or a bed of manure (which will release enough heat to warm the soil a few degrees as it decays) located just below grade will heat things up well enough to keep nearly any plant alive year-round.
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How big should you make your DIY cold frame?
The ideal size for a cold frame is 3 by at least 6 feet, about 12 inches deep along the front sloping to 18 inches at the back.
You can build a cold frame out of 1×12 pressure-treated lumber, assembled with nails and screws for permanency or prefabricated corners for easy dismantling and storage. For the cover, old windows work great, but a panel of Plexiglas or fiberglass—even a double layer of plastic sheeting on a frame—will do the job. If creating your own pane, check out our handy guides for cutting Plexiglas and glass.
Even easier, check out your local home improvement store or favorite gardening catalog for a kit with all the pieces included, such as the Easy-Fix Polycarbonate Cold Frame from The Home Depot!
Where should you build a cold frame?
Orient your cold frame toward the south or southwest, near the house so you can water it easily and monitor the temperature inside.
What temperatures should a cold frame reach?
Spring and fall plants do best at about 60 degrees Fahrenheit, while summer plants thrive at around 75 degrees Fahrenheit. Invest in a pair of simple outdoor thermometers, placing one inside and one outside of the DIY cold frame to help you keep tabs on the growing conditions.
Plants will wither if they get too hot, though, so you’ll want to start making adjustments in early spring.
- Once the outdoor temperature goes above 40 degrees Fahrenheit, prop the lid open about six inches.
- When the temperature reaches a consistent 50 degrees Fahrenheit, you can remove the lid altogether during the sunny hours and replace it on those chilly spring nights.
- When temperatures consistently surpass 60 degrees Fahrenheit, you can disassemble entirely and store until fall.