How to Build a DIY Cold Frame to Keep Your Garden Going Into Winter
Published September 15, 2026 · Updated September 23, 2026
The first frost doesn’t have to end the harvest. A cold frame is a bottomless box with a clear, sloped lid that sits on a garden bed. It traps the sun’s warmth in the soil and air during the day and slows heat loss at night, so cold-hardy crops keep growing weeks or even months past the point where an open bed shuts down. It’s one of the simplest garden builds there is, and salvaged windows or a single polycarbonate sheet make it inexpensive.
Quick answer: Build a rot-resistant wooden box about 3 × 4 or 3 × 6 feet, taller at the back (about 16–18 in) than the front (about 10–12 in), and face the low side toward the winter sun. Hinge a clear lid of twin-wall polycarbonate or a salvaged window to the back. Seal the gaps, and vent the lid on sunny days, since a closed frame overheats quickly. Grow spinach, lettuce, kale, mâche, scallions and root crops, and plant them early enough to reach good size before the coldest weeks.
How a Cold Frame Works
A cold frame has no heater. The clear lid lets sunlight warm the soil and air inside, the walls block wind, and the soil stores heat and releases it overnight. The result is often several degrees of protection compared with an open bed, plus shelter from wind and snow. That’s enough to keep hardy greens alive and growing slowly through much of winter in many climates. Adding a layer of row cover inside the frame on the coldest nights adds more protection.
Project Overview
| Detail | Estimate |
|---|---|
| Size (example) | About 3 × 4 ft, 18 in tall at the back and 12 in at the front |
| Difficulty | Beginner |
| Build time | 3–4 hours |
| Main costs | Lid material and rot-resistant lumber; salvaged windows can make it very cheap |
Choose the Lid Material
| Material | Pros | Cons |
|---|---|---|
| Twin-wall polycarbonate | Light, shatter-resistant, insulates better than single glazing, diffuses light | Costs more than film; can scratch |
| Salvaged window sash | Often free; rigid, clear | Heavy, can break; old painted windows may contain lead paint, so avoid sanding them and don’t use them near food crops without checking |
| Clear corrugated PVC or polycarbonate | Cheap, light | Less insulating; needs a frame |
| Greenhouse film on a wood frame | Cheapest, easy to replace | Tears in wind; lasts a season or a few |
What You’ll Need
Materials
- Rot-resistant boards such as cedar, about 1x10 or 2x10, for the walls
- 2x2 lumber for corner cleats (and lid frame if building one)
- Lid material (see above), sized to overhang the box slightly
- 2–3 exterior hinges and a prop stick or adjustable lid support
- Exterior screws; washered screws if attaching polycarbonate
- Foam weatherstripping
- Optional: a min/max thermometer and an automatic vent opener
Tools
- Circular saw or handsaw
- Drill/driver, tape measure, square, level
Cut Plan (3 × 4 ft frame)
| Part | Qty | Size | Notes |
|---|---|---|---|
| Back | 1 | 48 in long × 18 in tall | Two boards stacked, or one wide board plus a strip |
| Front | 1 | 48 in long × 12 in tall | |
| Sides | 2 | 36 in deep, tapering from 18 in to 12 in | Cut the slope along the top edge |
| Corner cleats | 4 | Match the height at each corner | 2x2, trimmed to the slope |
| Lid | 1 | About 50 × 38 in | Slight overhang all around |
Adjust sizes to fit your raised bed or your lid. Building the box to fit a salvaged window is often easiest.
Step 1: Pick the Spot
Place the frame in the sunniest location you have, with the low side facing the winter sun (south in the Northern Hemisphere). A wall or fence behind it adds wind protection and reflected warmth. Make sure water drains away rather than pooling around it.
Step 2: Cut the Sloped Sides
The slope sheds rain and snow and tilts the lid toward the low winter sun. Mark each side panel at 18 inches at the back and 12 inches at the front, connect the marks with a straightedge and cut along the line.
Step 3: Assemble the Box
Screw the front and back to the sides through 2x2 cleats at each inside corner rather than into end grain. Check for square by measuring diagonals. Set the box on the bed, level it, and press it slightly into the soil or bank soil against the outside so wind can’t blow under it.
Step 4: Build and Hinge the Lid
- Salvaged window: hinge it directly to the back wall.
- Polycarbonate: build a simple 2x2 frame to match the box, attach the panel with washered screws every 8–12 inches, and hinge the frame to the back wall.
Let the lid overhang slightly so rain drips outside the box. Add a prop stick with notches at different heights, or an automatic vent opener, which uses a wax cylinder to open the lid as it warms.
Step 5: Seal and Finish
Run foam weatherstripping along the top edges where the lid closes. It makes a large difference in overnight warmth. Leave cedar unfinished, or use a plant-safe exterior finish on the outside only.
Venting: The Most Important Habit
A closed cold frame on a sunny day can overheat quickly, even when it’s near freezing outside.
| Conditions | What to do |
|---|---|
| Sunny, above about 40°F (4°C) outside | Prop the lid open a few inches |
| Sunny and mild | Open wider or remove the lid for the day |
| Cloudy and cold | Keep closed |
| Hard freeze forecast | Close early in the afternoon to trap warmth; add row cover inside or a blanket over the lid at night |
| Heavy snow | Brush snow off the lid so light gets in and the lid doesn’t crack |
Most plants inside prefer daytime temperatures in the 50s to 70s °F (10–25°C). A min/max thermometer inside helps you learn how your frame behaves.
What to Grow
| Crop | Notes |
|---|---|
| Spinach | Very cold-hardy; one of the best winter crops |
| Mâche (corn salad), claytonia | Thrive in cold frames |
| Lettuce | Choose cold-tolerant varieties; harvest outer leaves |
| Kale, collards | Hardy; smaller varieties fit best |
| Arugula, mustard, Asian greens | Quick and tough |
| Scallions | Slow but steady |
| Carrots, beets, radishes | Plant in late summer; harvest through winter |
| Spring seedlings | Harden off transplants started indoors (see our seed starting guide) |
Plant crops in late summer or early fall so they’re close to harvest size before days get very short; growth slows dramatically in midwinter. A cold frame extends the season but doesn’t replace good timing.
Watering and Care
Soil in a cold frame dries more slowly in winter, so water sparingly, in the morning on mild days, and only when the top inch is dry. Watch for slugs and aphids, which enjoy the shelter too. Keep the lid clean for maximum light.
Troubleshooting
| Problem | Likely cause | Fix |
|---|---|---|
| Plants wilted or scorched on a sunny day | Overheating | Vent earlier and wider; add an automatic opener |
| Plants frozen overnight | Gaps, very cold night, lid open | Seal gaps; close earlier; add row cover inside |
| Mold or rot on leaves | Too humid, too wet | Vent more; water less; remove damaged leaves |
| Little growth in midwinter | Short days | Normal; plant earlier next year so crops are larger |
| Condensation dripping | Warm, damp air inside | Vent daily when possible |
FAQ
How much frost protection does a cold frame give?
Often several degrees, depending on how well it’s sealed, the lid material and how much sun it gets. Adding row cover inside or insulating the lid on very cold nights adds more.
Does a cold frame need a floor?
No. It sits on the ground so plants can root into the soil and the soil can store heat.
Which way should a cold frame face?
The low side should face the winter sun, south in the Northern Hemisphere.
Can I start seeds in a cold frame?
Yes. In late winter and early spring, cold frames are great for cool-season crops and for hardening off seedlings before transplanting.
