Cold Frame Gardening
For a Longer Growing Season

Cold frame gardening offers an affordable alternative to a greenhouse while offering many of the same advantages. Protect your bedding plants and keep them warm to extend your growing season.

A cold frame, or mini-greenhouse, is basically a large bottomless box with a transparent cover that sits on the ground and is used to provide a protected place for plants where they can get light and warmth, particularly during the winter season. (See our building a cold frame page.)

It resembles a greenhouse in that the transparent cover allows sunlight through the lid and traps heat inside. These qualities make both cold frames and greenhouses valuable for growing plants during the cold season when they would otherwise languish (or perish) outdoors.

We use the cold frame to shelter young seedlings that we're growing to be transplanted later into the vegetable garden. This allows us to get a head start on the growing season while it's still too cold to plant outdoors.

Our climate in southern Arizona is mild enough that we didn't install a source of additional heat inside the cold frame.

Our lowest expected winter temperature is 2°F (-17°C) and we have lots of sunshine year-round.

If you live in a colder and/or cloudier climate, you may want to install an electric heating cable or hot pad under the floor of your mini-greenhouse.

We do have occasional pocket gophers and other rodents that relish young seedlings, so we have covered the bottom of the box with metal screen to deter them. This means our cold frame gardening doesn't include planting directly in the box. All plants we used for our cold frame gardening are in containers of some kind.

Situate the Cold Frame to Catch Sunlight

Your mini-greenhouse should be set on level ground (ours is on a raked gravel pad), oriented with the lid facing south to capture the warmth and light of the low-angle winter sun. Make sure you place a thermometer inside so you can track the temperature. You want to protect your plants without overheating them.

While providing sunlight without overheating your seedlings, you also have to protect the little things from birds.

To protect our cold frame gardening efforts from birds who find new seedlings irresistible, we have built a simple screened frame which is placed over the cold frame to protect vented plants.  

If you will be growing directly in the box be sure that you have cultivated and amended the soil to be welcoming to the new plants. Lettuces and other salad veggies are perfect candidates for such an arrangement. Seed them directly in the soil.

Growing onions: great for cold frame gardening.

In late January we plant onion seeds in flat trays of potting soil indoors, and place them over a heating pad set on low.

After they germinate, we place them in the cold frame during the day and bring them in every night. When they are about 4 inches (10 cm) tall they can be planted in the garden.

For about a week before this, they are left in the cold frame overnight to get used to outdoor conditions, or harden off.

We wait until later in the spring to start our tomato, pepper, and bean seedlings in individual pots.

They won't be put out in the garden until the soil temperature is at least 70°F (21°C), so we wait to put them in the cold frame until about three weeks before that.

Don't Cook Your Seedlings!

Cold frame gardening offers an affordable alternative to a greenhouse while offering many of the same advantages. Protect your bedding plants and keep them warm to extend your growing season.

Your mini-greenhouse will ordinarily trap heat very efficiently on sunny days, and will easily become overheated if you don't provide some way of venting it. A closed cold frame will heat to well over 100˚ very quickly. Vent your cold frame for temperature control.

It would be a shame to cook the seedlings you've tended so faithfully! Watch the temp inside, and when it gets over 80°F (27°C), prop open the lid so some cool air can come in. See our building a cold frame page for detailed instructions for building both the cold frame and a manual venting system.

For starting an early garden, we recommend cold frame gardening. You can enjoy nature’s bounty without any time wasted waiting for cold garden soil to warm up enough for planting seeds.

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