Cold frames and low tunnels are low transparent covers that trap sun-warmed air and block wind over a bed, so hardy crops survive winter and seedlings start earlier in spring. How much warmer the air gets depends on the glazing, the thermal mass inside, the ventilation and how clear the night sky is, and on a clear, calm night the gain can shrink to nothing or turn negative. Measure it on your own structure with one thermometer inside and one outside, and expect the cover to overheat on a sunny day before it ever freezes.
A cold frame is a bottomless box with a transparent lid, and a low tunnel is a row of hoops covered with film or fabric over a bed; both use the sun to warm a small volume of air around plants.
Cover Gains Heat By Day And Can Lose It By Night
Sunlight passes through the cover, warms the soil and plants, and the covered air cannot mix freely with the outside, so the space warms. At night the soil and plants radiate heat back toward the sky. Glass blocks most of that long-wave radiation, but thin plastic film lets much of it pass, so on a clear, calm night the air under a film tunnel can end up no warmer than the outside air, and even a little colder. Cloud, wind, wet soil and heavy mulch all change the result. This is why no single figure for the temperature gain of a cover is safe to quote.
The carrier guide, Four-Season Growing & Season Extension, gives temperature gains for each cover type in degrees Fahrenheit. It cites no source; treat them as unverified and replace them with your own readings.
What Each Structure Is Good For
| Structure | What it does well | Limit |
|---|---|---|
| Cold frame | Protects a small bed of hardy crops through winter; hardens off seedlings | Small, needs lifting or propping to vent |
| Low tunnel | Covers a whole row cheaply; easy to move | Light cover, collapses under snow, weak against wind |
| Greenhouse or high tunnel | Walk-in space, bigger and more stable | Costlier; see Greenhouse & High Tunnel |
Measure Your Own Gain
Step 1: Put one thermometer inside at plant height, shaded from direct sun, and one outside on a post in open air.
Step 2: Read both at dawn (the coldest time) and mid-afternoon (the hottest), and write down the sky: clear, cloudy, windy, calm.
Step 3: After a few weeks compare. Note when the gain is largest and when it is zero or negative. That tells you when to add a second layer, a blanket, or ventilation.
What You Need
- Transparent cover: glass, clear rigid sheet or plastic film.
- A frame: boxes for frames, hoops for tunnels.
- Two thermometers.
- A way to ventilate — see Ventilation and Temperature Management.
Common Mistakes
- Forgetting to vent on a sunny day — the interior can climb far above the outside temperature within an hour and cook the plants.
- Trusting a quoted degree gain — measure.
- Covering wet, bare soil with a thin film and assuming it will hold heat on a clear night — it may not.
What’s Next
- Learn next: Building Cold Frames from Salvage — making one from old windows
- Combine with: Cold Frame Design — a second design guide