Cold Frames for Beginners: Your First Step Toward Year-Round Gardening

What a Cold Frame Actually Is

A cold frame is a bottomless box with a transparent lid, set directly on the ground. Sunlight passes through the clear cover and warms the soil inside. That warmth stays trapped, keeping the plants inside a few degrees to twenty degrees warmer than the air outside, depending on the season.

It is not a greenhouse. A greenhouse is heated, insulated, and often climate-controlled. A cold frame is just a box with a clear lid. It costs less than fifty dollars to build, and you can do it in a weekend. That is what makes it useful for beginners who want to push their season longer without a big investment.

Why You Would Want One

There are three main reasons gardeners use cold frames:

Start seeds earlier in spring. The soil inside a cold frame warms up weeks before the ground outside does. You can sow seeds that need warm soil as soon as the ground thaws, instead of waiting until late spring.

Extend the harvest later into fall and winter. Hardy greens survive inside a cold frame long after the open garden is done. You can pull fresh salad greens in January and February, well into the coldest part of the season in Zone 7a.

Harden off seedlings. Plants started indoors need a few weeks to adjust to wind and direct sun before going outside. A cold frame gives them a sheltered step between the windowsill and the garden.

Where to Put It and Which Way to Face It

Position matters more than you might expect. In the Northern Hemisphere, the best spot faces south or south-southeast, so the glass or plastic catches the lowest winter sun. Even a slight south slope helps.

Put it where it gets morning sun. A frame on the east side of a building gets the full morning sun but loses some afternoon heat in winter, which keeps it from overheating. A south-facing location gets more total sun but needs more attention to ventilation on warm days.

Make sure the spot drains well. Standing water under a cold frame rots the wood faster and creates a muddy mess around the plants. If your ground does not drain well, build the frame on a bed of gravel.

How to Build a Simple Cold Frame

A basic four-by-eight foot cold frame is a good starting size. It fits standard lumber cuts and gives you enough room to grow a real salad.

Materials

• Eight 2x4s cut to four feet (for the short sides)
• Six 2x4s cut to eight feet (for the long sides and one support beam in the middle)
• Exterior-grade plywood or pressure-treated boards for the floor
• A clear cover. An old window works. So does a sheet of polycarbonate or fiberglass greenhouse panel, which you can cut to size at the hardware store
• Wood screws, exterior wood glue, hinges for the cover, and a latch
• Optional: two bricks or wooden blocks to prop the lid open

Assembly

Build a rectangular frame with a lower side that is about eighteen inches tall and an upper side that slopes from about twelve inches at the back up to eighteen inches at the front. The slope lets water and snow slide off the cover.

You can also make a simpler flat-topped version with sides all the same height, but the sloped design handles rain and snow much better.

Place the frame on the ground or over a shallow bed of topsoil and compost. Fill the inside with good garden soil before planting. The soil itself holds the daytime warmth and releases it slowly at night.

What to Grow Inside a Cold Frame

The crops you put in a cold frame depend on the season.

Early Spring

Sow directly when the soil inside is workable, usually late February to March in Zone 7a:

• Radishes
• Spinach
• Lettuce (especially winter varieties)
• Arugula
• Snap peas
• Scallions
• Cabbage (early varieties like Golden Acre)

Summer

Remove the cover entirely during warm weather and use the frame as a shade structure. Many gardeners grow lettuce or other cool-season crops under the partial shade it provides. The frame keeps rabbits and deer out too, if you forget to fence them.

Fall and Winter

Plant in late summer so the greens are established before the coldest weather:

• Tatsoi
• Mizuna
• Kale
• Spinach
• Chard (especially 'Fordhook' or 'Bright Lights')
• Winter lettuce varieties like Winter Density or Slobbovia

A cold frame in Zone 7a with no extra heat will reliably hold plants down to about 20 to 25 degrees Fahrenheit. Below that, you may need an extra layer of insulation on the cover, like a quilt or burlap, at night. You do not need it every night, but having it ready for hard freezes saves your plants.

Managing the Inside Temperature

Overheating is the number one reason beginners kill plants in cold frames. On a sunny spring day, the temperature inside a closed cold frame can jump from forty degrees outside to over one hundred inside in less than an hour. Your plants will cook.

Prop the lid open on any sunny day when the temperature reaches fifty outside. Even a two inch gap lets enough air circulate to keep things from baking. Use bricks, a stick, or a purpose-made prop to hold it open. As spring turns into summer, leave the lid fully off and use the frame only as a raised bed with a removable cover.

In winter, keep the lid closed during the day unless it is extremely cold. Open it briefly on mild afternoons to let fresh air in and reduce condensation. Close it tight before sunset. The soil and the structure itself hold enough heat to keep things safe through the night, down to about 20 degrees. Below that, add insulation on top of the cover.

Common Mistakes to Avoid

Planting tomatoes or peppers inside a cold frame in winter. Warm-season crops need warmth and long days. They will not survive a cold frame through a Zone 7a winter, no matter what you do. Save them for spring planting inside or outside once the danger of frost has passed.

Ignoring ventilation. This is the most common cause of dead plants. A cold frame is a solar oven, and ovens need to be vented. Check it every day you can. If the lid is fogged with heavy condensation, it is too hot or too humid. Prop it open.

Building it too small. It sounds backwards, but a too-small frame heats up and cools down too fast to be stable. A four-by-eight frame has enough soil mass and volume to moderate temperature swings. Smaller frames work, but they require more frequent attention.

Placing it in the shade. A cold frame needs full sun. Under a tree or behind a fence, it will not trap enough heat to make a difference. It just becomes a damp box that grows moss.

Getting Started

A cold frame costs less than most gardening books and takes an afternoon to build. It gives you more usable growing days per year than almost any other single tool. You can start with secondhand windows and scrap lumber if you want. The first one does not have to be pretty. It just has to be on the ground, facing the sun, with the lid closed when it is cold and open when it is warm.

Once you have one, you will find ways to put it to use that you did not expect. Maybe you start seedlings under it before moving them outside. Maybe you grow salad greens in February. Maybe you use it to harden off tomatoes in May. All of those things are what it is there for.

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— C. Steward 🌱