How To Build Garden Cold Frames

A cold frame is like a mini-greenhouse that you can open during warm days.

A cold frame is a small, portable, unheated greenhouse that insulates tender plants from cold weather, allowing you to start your spring garden a few weeks early. It has a clear top that allows sunlight in to trap the heat.

In moderate climates, a cold frame can allow you to grow some herbs and vegetables year round! Cold frames are easy to build and use, here's how to go about it.

Why Use a Cold Frame?

Cold frames are generally used for:

Early Start: Cold frames usually keep the air about 5-10 degrees warmer than the outside air, and they protect plants from light frost. This usually gives about a month's head start on spring and summer vegetables.

Start seedlings in a cold frame.

Germinate Seeds: Most vegetable seeds germinate in soil temperatures of around 70° F. A cold frame warms the soil to allow you to start seeds early in the season.
Harden Seedlings: If you're even more ambitious and have started seeds indoors during the winter, a cold frame is a great way to acclimate young seedlings gradually to the outdoors to prevent shock.
Root Cuttings: The warmer temperatures of a cold frame can be great for rooting cuttings of shrubs, herbs, and perennials during the winter.
Tender Plants: By covering tender plants with a cold frame, northern gardeners can enjoy plants that normally wouldn't survive their winters. They can also be used as winter storage for semi-tender potted plants.

Vegetables to Start in a Cold Frame

You can grow or start most anything in a cold frame, but they're especially useful for early spring vegetables like:

Salad greens in a cold frame.

Broccoli
Cabbage
Eggplant
Fingerling carrots
Green onions
Lettuce
Peppers
Radishes
Spinach
Tomatoes
Other root vegetables

How to Build a Cold Frame

Cold frames are easy to build from recycled materials. They can be any size and shape, and they can be as fancy or as simple as you like. A cold frame consists of:

An old window unit works great.

Clear Lid: Old window sashes, glass shower doors, storm doors, polyethylene sheeting, and clear fiberglass can all be used to construct the lid for your cold frame. This is a great way to recycle!
Frame Box: Once you have your lid, you need to build a box for the lid to sit on. Pressure treated 2" thick lumber is a common choice, but you can also use recycled lumber and even stacks of bricks or cinder blocks.

The dimensions of your cold frame will depend on the size of the lid, and it really can be as simple as sitting an old window over a wooden box. However, if you're building one from scratch, useful features include:

Slope Lid: By making the back higher than the front, you can maximize sun exposure.
Hinge Lid: While you could make the lid so it lifts off, hinging will provide easier access and allow you to prop it open with a stick during warm days.
Easy Access: Limit the width of your cold frame to 3' or less, so you can easily reach all the way to the back without having to step inside.

Drawing of sample cold frame with hinged lid against south facing wall, 36
A simple cold frame design, vary length as needed.

Where to Locate a Cold Frame

Some people nestle a permanent cold frame against the south wall of their house or shed, while others build portable frames directly in the garden. Wherever you put your cold frame, the location should have:

Full sun: Southern exposure is best, with western exposure as a second choice.
Drainage: Make sure the soil in and around your cold frame drains well, or the spot is on a slight slope, to keep plants from sitting in soggy soil.
Rich soil: Dig the soil under the cold frame to a depth of 12" or more. Work in plenty of organic matter, or add a few bags of high-quality potting mix to the soil.
Insulation: You can rest the cold frame directly on the ground, or increase the insulating properties by burying several inches of the frame in the ground.

Cold Frames Throughout the Season

These savvy gardeners started salad greens (left) in their cold frame in early spring. Once the weather warmed into the 60s F and the greens could handle the weather, they moved the lid over onto a new base and planted summer vegetable seedlings.

Cold Frame Tips

Ventilate: Open the lid on your cold frame on warm days to provide air circulation.
Temperature: Place a thermometer inside your cold frame (or a soil thermometer if you're starting seeds). Open or close the lid to keep the temperature right for the plants you're growing.
Freezing Weather: Remember that cold frames give only 5-10 degrees of warmth. If temperatures are going to plunge into the 20s F, your plants will be in trouble. If this happens, simply pile some old blankets on your cold frame, then remove the blankets when temperatures warm up again.
Excess Heat: Those 5-10 degrees can also mean trouble when temperatures start heating up. Don't forget to open the lid of your cold frame on warm days to keep the plants from baking.
Water: Plants in a cold frame will need extra attention to make sure they aren't getting too much or too little water. Because you'll have to water them by hand, you'll soon get a feel for the needs of your plants.
Harden Plants: To prepare your seedlings for life outside, open the lid of your cold frame longer and longer each day, until you've finally removed it completely. Keep the lid handy in case of a cold snap!

Further Information

How to Get an Early Start on Your Spring Garden
How to Protect Your Garden from Frost and Freeze
Cold Frame Gardening (Vegetable Gardener)
Vegetable Garden: Planning and Layout
Vegetable Garden: Growing Warm-Season Vegetables
Vegetable Garden: Growing Cool-Season Vegetables