Cold Frame Gardening For a Longer Growing Season

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.

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 a 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 grown 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.

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.





How to Build Cold Frame in your Home Garden?

Cold Frame Plans

With our cold frame plans, you can start and harden off seedlings and can add weeks to your growing season. Our easy DIY plans help you improve your gardening experience.

With these cold frame plans, you can garden on sunny apartment balconies, in urban yards, or for large rural gardens. The basic structure is simple and can help protect plants for early planting.

Ours is a box about 4 feet (1.2 meters) square. The back panel is 15 inches (38 cm) high and the front is 9 inches (23 cm) with sides sloping down from back to front.

The top is covered with a sheet of clear plastic with squared-off corrugation and is reinforced by a strip of molding designed to go with the clear panel. There are many choices of clear and translucent materials that are satisfactory for the top. Just remember the function is to protect the little plants while allowing the sun to shine through the top.

The clear plastic top is mounted on a frame of dimension lumber. We used 1” x 4” boards (actually 3/4 inches by 3 1/2 inches or 2 cm by 9 cm). The frame corners are simply butted together and fastened using flat steel plating with pre-drilled holes that are fastened across the joint. We used 1/2 inch (about 1.3 cm) #8 lath screws to fasten the plates. Be sure to reinforce both the top and bottom of each corner. Serious woodworkers might use a glued and screwed lap joint or a mortise and tenon joint for this.

Remember to orient the top material so that the corrugations are sloped to allow rainwater to run off. We attached the top and bottom of the plastic to the frame by driving 1 inch (2.5 cm) hex head gasketed screws through the plastic and brace into the frame. We used the shorter lath screws to secure the sides to the frame. Remember to screw ONLY in the valleys of the corrugated material.

The sides for our cold frame were cut from a piece of 1/2 inch (1.3 cm) construction grade plywood. Using a chalk line, we marked two adjacent trapezoids 4 feet by 15 inches by 9 inches. The two trapezoids form a rectangle 4 feet by 2 feet or 1/2 of the plywood piece. We then marked two rectangles, one measuring 4 feet by 9 inches and the other measuring 4 feet by 15 inches. The two trapezoids are the sloping sides of the box and the rectangles are the front and back.

From a 4-foot-wide roll of metal window screen, we cut a piece 54 inches (137 cm) long and stapled this to the bottom of the box. The excess length was overlapped and stapled to the sides for additional strength. We chose the screening because we had it on hand. Hardware cloth or any strong mesh that will keep mice and gophers out and let water drain will work. We suggest a material not prone to rusting such as aluminum or galvanized metal.

Following our cold frame plans to strengthen the box, we cut four pieces of scrap wood from a piece of 2 inches by 2-inch fir. Two pieces were cut 8 inches long and two were 14 inches. The corner braces were clamped to the front and back pieces and screwed in place with 1 1/2 inch (about 4 cm) sheetrock screws that we always have in our shop. Any wood screw of appropriate length will work for this. We then clamped the sides to the corner braces and screwed them into place. This completed our basic box.

After the bottom mesh was secured, we attached the top by affixing the hinges at the back or higher end. We placed the hinges approximately 1 foot (30.5 cm) in from each side. We used a hook and eye to secure the front of the top in the closed position. We placed our cold frame in front of a south-facing porch post for securing the top in the fully open position.

For venting, a variety of options are available. (read more on venting) Commercial automatic venting systems are available. An adjustable system may be installed for manual temperature control. There are many ways of propping open a top, including just using a piece of scrap wood under the front of the top with a rock on the corner for weight. Use one that is easy, convenient, and secure enough to avoid damage from spring winds.

Tools and Material for Cold Frame Plans

The building process for our cold frame plans required about five hours with simple materials and some basic hand tools.

Tools we used included

Electric hand drill screw driving head

Hammer Heavy stapler with 1/2” staples

Chalk line

Power hand saw

Clamps Tape measure

Phillips screwdriver

1/4″ nut driver for gasketed screws

The hardware we used for the project

Hinges – 2 @ 2” x 3” hinges (galvanized or brass avoids rust)

1 1/2 inch sheetrock screws – about 40 of these

Flat steel corner reinforcement – two per corner – 8 pieces

Appropriate screws for corner reinforcement and top construction

Hook and eye – 2 sets

The material we used for the project to Build Cold Frame

1 piece of 1/2 inch CDX (construction-grade) plywood – 4 feet by 4 feet

1 sheet of corrugated plastic – 26 inches x 8 feet (66 cm x 2.4 m)

Corrugation gasket for top and bottom – enough for 8 feet

Aluminum screening (1/4” hardware cloth optional)

In our desert region of southern Arizona USA, where the sun can be strong, we built a second top frame for our box and covered it with shade cloth. This enables us to start seedlings for summer and fall rotational plantings without frying the tender little plants.





With our basic cold frame plans, you can build an inexpensive and durable shelter for your young plants. Modifications can easily be made to suit your local requirements.

Happy Gardening!!

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