The first time I saw a cold frame, just for a moment, I thought I was looking at a magical portal to the center of the earth. Then my father lifted the frame and its heavy pane of steamy glass, revealing a treasure trove of early spring lettuces and herbs inside.
Cold frames aren’t portals to anywhere, but they are a little bit magical. Akin to tiny unheated greenhouses, cold frames use solar energy to create a warm microclimate in your garden. Like most magical contraptions, they look simple but produce remarkable results.
Whether you want a place to start seedlings, a bed for winter greens, or a refuge for questionably hardy ornamental plants, a cold frame is a neat trick to transform your garden.
They are easy to build yourself or create from a kit, without the expense and hassle of a greenhouse. Plus, they allow you to grow plants in zones where they would otherwise not be hardy.
Above all, cold frames extend your growing season.
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What Is a Cold Frame?
A cold frame is essentially a five-sided box. The bottom is a garden bed, and the top is made of a light-permeable material, such as glass, plexiglass, or greenhouse fabric. The sides may be made of wood, glass, brick, garden soil (if built into a hillside), or any other material that will provide a solid structure to elevate the glass off the ground and plants.
Cold frames are typically small, often only a couple of feet high. One common technique for a DIY cold frame involves building a wooden frame and topping it with an upcycled window (still its frame) that has been torn out of a house during a remodel.
If you want to make a DIY cold frame, watch the video:
They can be larger, but you’d want the frame to be structurally sound so that the glass won’t break.
Although the plants in a cold frame are usually planted directly into the ground, there is another option. You can use a cold frame with a bottom, essentially a planter with a glass lid. This would work, for example, if you wanted to create a cold frame in a rooftop garden. However, the box must have drainage holes in the bottom.
In essence, a cold frame is a simple, unheated garden. Its size and lack of heat make it less useful in some ways than a greenhouse. If you’re ready to take the plunge and go for a full greenhouse, check out our guide on the best greenhouse kits available.
How Does a Cold Frame Work?
A cold frame, a greenhouse, and the earth’s atmosphere all work in fundamentally the same way. The process begins with sunlight (the energy source supporting all life on earth) entering through a transparent barrier and becoming trapped.
Inside a cold frame, the plants, ground, walls, and any other objects inside absorb the sunlight that passes through the glass panel (or through the air, if the cold frame is open). As the interior objects absorb sunlight, they convert light energy into heat energy.
When the sun sets and the temperature drops, the plants and soil radiate the heat, they absorbed back into the air inside the greenhouse. The heat rises, but, assuming you’ve closed the top window, the glass traps a lot of the heat inside the cold frame.
As the hot air rises, it pushes the cold air above it downward, where it is warmed up with more of the radiated heat. Then that warm air rises, displacing the cooling air above it, and so on. This heating cycle warms the greenhouse, creating a microclimate, keeping the interior warm long after sunset.
Where Should I Use a Cold Frame?
The best location for a cold frame is where you need it. In other words, focus on what plants you want to put in it and what environment they need. In general, you’re going to want to get as much sunlight as possible to heat the cold frame.
However, one of the most significant advantages of a good cold frame is its portability. Unlike a greenhouse, which is often built on a foundation, you can move a well-constructed cold frame more easily.
That means you might use your cold frame in the garden to protect late greens in the fall, and then when it gets cold enough that you give up on lettuce, move it to your succulent garden to protect your aloes overwinter.
How To Use a Cold Frame in Your Garden
A cold frame is useful in different ways over the course of a year.
Warm Up the Soil
Late in winter, when the snow is gone, but the vegetation is still dormant, is a great time to build a cold frame. If you’re eager to get planting, installing your cold frame at least a couple of weeks before you plan to sow will allow the solar energy in the cold frame to warm up the soil. This will allow you to start your seeds or seedlings a little earlier.
If you have a compost thermometer (I use this one), you can plant seeds at the right temperature to allow immediate germination.
Make Space for More Seedlings
Once you have warmed up the soil and are no more than a couple of weeks before your last frost date, you can use your cold frame as a temporary growing space for your potted seedlings, freeing up room under your grow light to start more.
This works best in a cold frame with well-insulated sidewalls. If your cold frame is not well insulated, and you get a late cold snap, you may need to cover your cold frame with blankets to protect the tender seedlings inside.
Harden Off Seedlings
The process of hardening off seedlings can be a real hassle. You want to move your seedlings outside to gradually get used to the heat and cold extremes, wind, precipitation, and other factors.
However, moving the plants from location to location can become tricky and time-consuming, especially if the temperature swings consistently or your spring weather is fickle.
Hardening off in a cold frame is a great solution. Depending on how much sun it gets, you may need to open the cold frame mid-day to prevent tender seedlings from overheating, then close it in time to capture the absorbed sunlight. However, occasionally opening and closing a cold frame is a lot less work than moving trays of seedlings multiple times every day.
Jump Start Heat-Loving Plants
If you live in a northern zone with a short summer, but you want to grow beefsteak tomatoes or other heat-loving veggies, a cold frame may be your best chance of producing ripe fruit by the end of the season (other than a greenhouse or hoop house).
Rather than simply moving your seedlings out into the cold frame in their pots before transplanting to their final homes, consider planting them directly into the space you’ve pre-warmed with the cold frame.
Keep the lid shut until the plants bump up against the glass. Then, either leave it open until the end of tomato season or, if you have a portable cold frame, disassemble it and move it to a new location.
Backup Your Veggies
We all know to backup computer files, but what about your veggies? One of the biggest mistakes beginning gardeners make is to plant everything at once. Best case scenario, all of your lettuce seedlings are ready the same week, and many of them go to waste.
Worst case scenario, a day of rogue weather or a brief pest infestation can wipe out your whole crop.
Cold frames are great for holding on to seedlings. When you end up with more seedlings of one crop than you need (because of a better germination rate than you expected), you can keep them in your cold frame for succession planting.
Or, if a late frost destroys your lettuce, you’ll be happy that you kept your extras in the cold frame instead of throwing them on the compost pile.
You can even make additional backups as the spring progresses. When you start breaking suckers off your favorite tomato bush, propagate them and keep them in the cold frame so that if your toddler tramples your main bush, you have a backup ready to plant.
Grow Fall and Winter Crops
One of the best uses of a cold frame is as a place to grow your fall and winter veggies. Greens, broccoli, carrots, kohlrabi, and scallion, for example, can be started in the late summer in an open cold frame, planted directly in the ground.
When the weather cools down, close the cold frame, and you have a mini-greenhouse to ensure you keep harvesting fresh produce long after the first frost date.
Cold frames are not only useful for veggies. They can also come to the rescue of marginally-hardy ornamental plants. For example, many succulents and cacti are resilient during brief cold spells but can’t stand the winter weather north of Zone 10. If you want to grow aloe in the ground in Zone 9, for example, try sheltering it with a cold frame. You can leave it open most of the time but close it in anticipation of any brief frosts.
Having a cold frame won’t turn you into a wizard since you don’t control the weather. Still, you’d feel like you’re working magic since you can protect your tender plants and sow tomatoes in April and harvest spinach in November.