Cold frame gardening is one of the simplest ways to stretch your growing season on both ends. A cold frame is a low box with a clear lid that catches the sun’s warmth.
That lets you grow salad greens long after frost and start spring crops weeks early.
In this guide, I’ll explain what a cold frame does and how it compares to other protection. I’ll also show you how to build a simple one, what to plant in it, and how to vent and water it.
By the end, you’ll know how to keep it productive in fall, winter and spring.
What a Cold Frame Is and What It Does
A cold frame is a bottomless box with a clear, sloped lid that traps solar heat. It shelters plants from wind and frost and creates a warmer pocket of air and soil for cold-hardy crops.
How a Cold Frame Works
Sunlight passes through the clear lid and warms the soil and air inside. The lid and walls hold that heat in and block cold wind. On a sunny day, a cold frame typically runs about 10 to 20°F warmer than the outside air.
At night, the gain is much smaller. The soil releases stored heat slowly, but a cold frame has no heat source of its own. That’s why it’s called a “cold” frame and not a hotbed.
What It Can and Can’t Do
A cold frame protects hardy plants from frost, snow, ice and drying winter winds. It keeps greens tender and growing when unprotected plants would stall or get damaged.
It won’t turn winter into summer, though. You can’t ripen tomatoes in January in a cold climate. Think of it as a tool for cool-season crops, early starts and hardening off seedlings.
Helpful Tools to Get Started
If you don’t want to build your own, a mini cold frame kit is an easy way to test the idea. These usually have a lightweight frame with clear panels and fold flat for storage.
The second tool I recommend is a min/max thermometer. It records the highest and lowest temperatures inside your frame. That tells you when you’re overheating plants during the day and how cold it gets at night.
Cold Frame vs. Greenhouse vs. Row Cover
A cold frame sits between a row cover and a greenhouse. It gives more protection than fabric, costs far less than a greenhouse and suits small spaces.
How They Compare
Each option has its place. Row covers are cheap and quick to throw over a bed. Greenhouses let you walk inside and grow year-round with added heat. Cold frames are compact, sturdy and easy to build in a weekend.
| Feature | Row Cover | Cold Frame | Greenhouse |
|---|---|---|---|
| Typical cost | Low | Low to moderate | High |
| Frost protection | A few degrees | Moderate | High, especially with heat |
| Wind and snow protection | Limited | Good | Excellent |
| Space needed | Fits any bed | About 3 by 6 feet | Large footprint |
| Setup effort | Minutes | A weekend | Major project |
| Best for | Light frost, pest exclusion | Winter greens, hardening off | Year-round growing |
When to Choose Each One
Choose row cover if you just need a few degrees of protection on a frosty night. It also works as a pest barrier in summer.
Choose a cold frame if you want to harvest greens through fall and winter or start crops early. It’s a great fit for small yards and budgets.
Choose a greenhouse if you want to grow lots of plants year-round and have the space and money. Many gardeners start with a cold frame and see how much they use it first.
Using Them Together
You don’t have to pick just one. Draping row cover over plants inside a cold frame adds another layer of warmth on bitter nights. For more ways to shield crops, see my guide on protecting plants from frost.
Choosing a Site and Design
Put your cold frame in a sunny, sheltered spot facing south, with the back higher than the front. A standard size is about 3 by 6 feet.
Pick a Sunny, Sheltered Spot
Sun is everything for a cold frame. In winter the sun sits low in the sky, so look for a spot with clear southern exposure and no buildings or evergreens blocking it.
A spot against a south-facing wall, fence or foundation is ideal. The structure blocks north winds and gives off a little stored heat. Just make sure it doesn’t cast shade on the frame.
Good drainage matters too. Avoid low spots where water pools or cold air settles. If your yard has soggy areas, place the frame on slightly higher ground.
Get the Shape and Size Right
Build the frame higher at the back so the lid slopes toward the sun. A typical design is 18 to 24 inches tall at the back and 12 to 18 inches tall at the front.
That angle lets sunlight in and helps rain and snow slide off.
About 3 by 6 feet is a handy size. At 3 feet deep, you can reach the back without climbing in. Longer frames are fine, but split the lid into sections so each one is easy to lift.
Choose Your Materials
Use untreated wood or straw bales for the walls. Never use treated lumber near food plants. Cedar and other naturally rot-resistant woods last longer without chemicals.
For the lid, an old window or a sheet of polycarbonate works well. Polycarbonate is lightweight and shatter-resistant. Old windows are free or cheap, but they’re heavy and can break.
Straw bales make a quick, well-insulated frame. Arrange them in a rectangle and lay a window across the top. They’ll break down over a season or two, and you can use the old straw as mulch.
How to Build a Simple Cold Frame
A basic wooden cold frame takes just a few boards, some screws and a clear lid. You can build one in an afternoon with simple tools.
Gather Materials and Tools
Here’s what you’ll need for a basic 3 by 6 foot frame:
- Untreated boards, about 1 inch thick, for the walls
- Corner posts, such as 2×2 lumber, for strength
- Exterior screws
- A clear lid: an old window or a polycarbonate panel on a wood frame
- Two hinges for the lid
- A notched stick or block to prop the lid open
- A drill, saw, tape measure and pencil
Measure your lid first if you’re using an old window. Then build the box to match the window size. It’s much easier than trying to find a window to fit a box.
Build the Box
Cut the back wall to about 18 to 24 inches tall and the front wall to about 12 to 18 inches. Cut the two side walls so they slope from the back height down to the front height.
Mark a straight line from corner to corner and saw along it.
Screw the walls to the corner posts to form a sturdy rectangle. Check that the box sits square, then set it in place with the tall side facing north. The low side should face south.
Settle the frame into the soil a little so cold air can’t sneak in under the edges. You can also bank soil or mulch along the outside base.
Add the Lid
Attach the lid to the back wall with two hinges so it opens from the front. Make sure it closes snugly against the top edges. Gaps let heat escape, but a few small ones aren’t a disaster.
Keep a notched prop stick handy for venting. Notches at different heights let you open the lid a little or a lot.
If you use an old window, check it for peeling paint. Old windows may contain lead paint, so the EPA advises taking care around older painted surfaces. I’d avoid scraping or sanding them, and skip any with chipping paint.
What to Grow in a Cold Frame
Grow cold-hardy greens like spinach, kale, lettuce, arugula, mache and claytonia, plus radishes. A cold frame is also perfect for hardening off seedlings and starting crops early.
Best Cold-Hardy Crops
Leafy greens are the heart of cold frame gardening. They handle cold well, stay compact and produce a lot in a small space. Many of the same crops on my list of easy vegetables for raised beds also thrive in a frame.
| Crop | Cold Tolerance | Notes |
|---|---|---|
| Spinach | Very high | One of the hardiest winter greens |
| Kale | Very high | Pick outer leaves as needed |
| Mache (corn salad) | Very high | Small rosettes with a mild, nutty flavor |
| Claytonia (miner’s lettuce) | Very high | Tender leaves, grows well in cold |
| Arugula | High | Fast and peppery |
| Lettuce | Moderate to high | Choose cold-hardy types |
| Radishes | Moderate to high | Quick crop for fall and early spring |
Many cool-season greens can handle lower light, too. If you’re curious which ones manage with less sun, see my list of shade-tolerant vegetables. Still, give your frame as much sun as you can in winter.
Hardening Off Seedlings
Seedlings started indoors need time to adjust to outdoor conditions before planting. A cold frame makes this easy. Set your trays inside and open the lid a bit more each day.
Over about a week, the seedlings get used to sun, wind and cooler nights. Then they’re ready to move into the garden. This works well for tomatoes, peppers, broccoli and cabbage.
Starting Crops Early
In late winter, the soil in a cold frame warms sooner than open ground. That means you can sow cool-season crops several weeks early. Lettuce, spinach, radishes and peas are good choices.
Timing still matters. Base your schedule on your local frost dates. My guide on when to plant vegetables can help you plan.
Venting, Watering and Temperature Control
Vent your cold frame when the inside climbs above about 70°F, and close it in the afternoon to trap heat for the night. Water in the morning and only when needed.
Why Venting Matters
A cold frame can overheat fast, even on a cold, sunny day. A closed frame in bright sun can jump well past 70°F inside while it’s freezing outside. That heat can wilt or scorch tender greens in a few hours.
Prop the lid open when daytime temperatures inside rise above about 70