Simple to use and easy to make, cold frames are an asset in any garden.
If you’ve ever wanted to extend the growing season, the simplest way is to add cold frames to your garden. Cold frames are simple wooden boxes built with an open bottom and a solid, clear top. They capture and concentrate the sun’s rays, creating a microclimate inside that’s more hospitable to plants when the mercury plummets.
If this sounds like a greenhouse, you’re not far off. Cold frames are smaller than greenhouses, but they have some of the same benefits. Gardeners commonly use cold frames as a temporary measure in the spring, fall and winter.
What are the benefits of cold frames?
In The Winter Harvest Handbook, Eliot Coleman notes that adding a cold frame to your garden is like moving your beds 500 miles south: it creates an environment that’s one and a half zones warmer than where you live. This means you can plant seeds earlier in the spring. You can also hold and harvest crops later when peak gardening season has passed.
But more than just warming soil and air, cold frames protect tender plants from frost and garden pests. They’re inexpensive and often portable, meaning almost anyone can buy or build them with little impact on the wallet.
Related: Winter Gardening – Best Crops to Extend Your Harvest
How do cold frames work?
Like a greenhouse, cold frames capture and hold the sun’s rays, warming the soil, air, and plants inside using solar energy. Unlike a greenhouse, you can’t walk around inside them. In fact, having as little headspace as possible inside your cold frames is best, because there will be less air to keep warm.
In most cases cold frames are built higher at the back and placed facing south to maximize aspect. The average cold frame is between one and two feet high, with a gently sloping lid mounted on hinges that can open and close. Some models are partially buried in the ground for added insulation.
That leads us to a few important cold frame rules.
- Always put your cold frames in a location with full sun.
- Be sure to include ventilation. This can be as simple as a prop inserted beneath your lid on warm days.
- Keep the tops of your cold frames free of leaves and snow to maximize heat and light.
- Don’t overwater. Cold frame environments tend to be moist, since evaporation can’t easily escape.
What can you grow in a cold frame?
Cold frames are typically used for cold-hardy plants that need protection from frost and severe temperatures near the beginning or end of season. You can sow seeds directly into a cold frame weeks earlier than the surrounding garden beds. In the same way, you can keep your plants alive 2 to 4 weeks longer in the fall.
Use your cold frame to extend your gardening season by growing a range of cold hardy vegetables.
How to use a cold frame in the spring
When growing early season crops in a cold frame, sow directly into the soil rather than using seeds started indoors or purchased from a garden supplier. This prevents the shock of moving tender seedlings into a fluctuating environment. Sow the following plants in your cold frame in springtime:
- Lettuces
- Spinach
- Mesclun blends
- Cole crops
- Mache
- Radishes
- Peas
- Carrots
- Beets
- Leeks
Later in the spring, you can use empty cold frames for hardening off seedlings grown indoors, providing you’re close to your region’s last frost date.
Using a cold frame in late summer
In August and early September, when spring crops are a distant memory, it’s time to fill your cold frame again, this time with crops you plan to hold through the winter. Plant them early enough so they can reach maturity before the days get too short, spacing them close together for added insulation. Be sure to leave the lid open so the seedlings inside don’t overheat. This is the perfect time of year to plant:
- Spinach
- Head lettuce
- Kale
- Arugula
- Overwintering cabbages
- Sprouting broccoli
Cold frames in the fall and winter
Now that the chill has set in, keep your cold frame closed during the day unless the temperature climbs markedly. Once the weather in your region dips below 40 degrees, the lid should remain shut.
For added protection, mulch around the base of your plants with dry leaves, straw or your favorite organic mulch material. Clear the lid of your cold frame when snow or leaves threaten to block out daylight. Harvest your crops as needed.
Best cold frame materials
Some of the earliest cold frames were made of stacked stone or bricks with glass on top. These days you’re likely to find a mixture of materials available, from wooden frames with polycarbonate lids to metal and fiberglass. The materials you choose will depend on your preferences for height, weight, and insulation value.
| Lower | Plastic, recycled wood, used concrete blocks, straw bales | Plastic sheeting, old shower doors, recycled windows, storm windows |
| Higher | New wood, aluminum, fiberglass | New glass, polycarbonate |
How to build a DIY cold frame
For many people, building a DIY cold frame is the easiest solution. If you have the materials and are handy with tools, you can construct a single cold frame in an afternoon.
Wooden cold frames
The classic cold frame is built from long-lasting wood (like cedar) and looks something like a raised bed with a hinged lid. The difference is that the back is higher, causing the characteristic sloping that helps trap the sun’s energy and warm the space inside. For basic instructions check out this DIY cold frame plan from the University of Minnesota. For something smaller and lighter, see these plans for a portable garden cloche that’s light and easy to make.
Straw bale cold frames
Straw bale cold frames are relatively inexpensive and can be set up in a few minutes. Simply arrange the bales in a rectangle around your garden bed. Cover the top with glass or polycarbonate panels that span the distance between the sides. While these cold frames lack the sloping top and hinged lid of commercial cold frames (making them harder to prop open), they have excellent insulation value.
Stacked block cold frame
Like the straw bale model above, stacked concrete blocks make another good cold frame with added insulation value. If you’re short on time and looking to make use of leftover building materials, this might be the option for you. As for straw bales above, arrange your blocks in a rectangle and top with clear panels. Sunlight will warm the blocks during the day. This heat will release slowly overnight, giving your plants an added boost.
Best cold frame on the market
If you don’t have the time or materials to build a cold frame, several styles are available today. Our favorites include those below, chosen because they are easy to use and long lasting.
Year-round cold frame
The Year-Round Cold Frame features a dual function lid with two covers for crop protection during cold and warm weather.
Year-round cold frames combine sturdiness with portability to offer gardeners solid weather protection that keeps crops through the winter months. This year-round cold frame made in Austria is one example of a versatile material combination. Polycarbonate panels with an aluminum frame offer a sturdy but portable construction. The metal frame supports light snow in winter. In the summer, the clear lid is easy to remove, exposing an insect screen that continues to shelter and protect plants.
To support snow loads up to six feet, consider a double-walled cold frame that works on the ground or attached to raised garden beds.
Double-wall panels provide insulation to resist frost and are strong so they won’t collapse under heavy snow.
To combine raised bed and cold frame gardening into one easy unit, check out this integrated cold frame design made in Canada from Outdoor Living Today. Made from solid cedar, the bed offers enough depth for all garden vegetables. The easy-fold sides mean the bed can be used all year round and is easily converted as the weather changes. The lid makes a nice trellis when not in use.
This raised bed offers a removable cold frame option to extend the gardening season.
Adding hot beds to your cold frames
Many people choose to give their plants an extra boost by warming the soil inside their cold frames using something known as a hot bed. The heat in a hot bed is generated by materials layered below your garden soil. Hot beds can be passive (using heat from decomposing organic matter) or active (using heat from electrical cables).
Before adding your soil to your cold frame, dig a hole the length and width of your bed about 12 to 18 inches deep. Fill with manure and bedding and tamp down. Top with another 12 inches of soil and allow to rest for 7-10 days. Cover with your cold frame and plant. Be sure to test the soil temperature before planting to ensure you don’t overheat.
For more information, check out this resource from the University of Missouri Extension on building and using hot beds in cold frames.
Frequently asked questions
What is the best time of year to use a cold frame?
While you can use a cold frame all year round, you’ll need to keep the lid open in summer so your plants don’t overheat. Most people use cold frames in spring, fall and winter to extend the gardening season.
Can you start seeds in a cold frame?
Yes, absolutely. It’s a good idea to sow your seeds directly into the cold frame for the earliest and best results.
Can you put a cold frame on raised garden beds?
Yes. Raised beds tend to warm earlier in the spring than in-ground beds, so cold frames used at this time of year can hasten the season even more. Some cold frames are designed to fit on top of raised beds. Others look more like a mini greenhouse but perform the same function.
How do you keep winter cold frames warm at night?
Insulating your frame with soil or straw on the outside and mulch on the inside, around plants, is one good way to keep plants warm at night. Using framing materials with good insulating values (such as masonry blocks or straw) and spacing your plants close together are other good ways.
Is a cold frame better than a greenhouse?
A cold frame is different than a greenhouse, but not necessarily better. In general, cold frames are small, which makes them easy to move around. They’re ideal for urban yards that may lack the space for a greenhouse, but they offer less planting area for gardeners looking to grow a large variety of warm season vegetables.
Are you ready to make the leap into cold frames? Visit our product page to learn more.
