Vent a cold frame or tunnel on sunny days by reading a thermometer, not by the calendar: the carrier guide says to vent whenever outside air is above 40 degrees Fahrenheit and the sun is out, propping the lid open 4-6 inches, and to close it before sunset to hold the day’s heat. A closed space with a small volume of air heats in minutes once the sun comes out, and the plants suffer from heat and damp before they ever suffer from frost. On a clear, calm night add a second layer or a blanket, since a thin cover may give little or no gain.
Ventilation and temperature management in a season-extension structure means opening and closing the cover through the day and night to keep plants between their heat and cold limits and the air dry enough to avoid disease.
A Small Volume Of Air Heats Faster Than Plants Can Cope
Air holds little heat. Take a bed 1 m² under a cover 0.5 m high: 0.5 m³ of air at a density of 1.2 kg/m³ is 0.6 kg, and with a specific heat of 1.005 kJ/kg·K that is 0.60 kJ per kelvin. If the sun delivers a net 100 W into that air, it heats at 100 W ÷ 600 J/K = 0.17 K per second, or 10 K per minute, until the soil, the cover and venting take the heat away. The input of 100 W is an illustrative figure, not a measurement, but the order of magnitude shows why a closed frame can spike once the sun comes out, and why soil and water, which hold far more heat per volume, are what steady it.
The carrier guide (Four-Season Growing & Season Extension) states: on sunny days above 40°F you must vent; interior temperatures can spike to 100°F or more; prop the lid open 4-6 inches; close before sunset. Those are the carrier’s rules of thumb; adjust them to the crop and the thermometer.
What You Need
- A thermometer inside at plant height, shaded from direct sun.
- A prop stick with notches, or a wax-cylinder opener, which uses the expansion of wax to lift the lid automatically (the carrier mentions this).
- Extra cover: blankets, straw mats or a second film layer, for cold nights.
Method
Step 1: Know your crop’s limits. Lettuce and spinach tolerate cool conditions and suffer in heat; check the crop. Write the upper limit on the frame.
Step 2: Vent early. Open the lid before the interior reaches the limit, not after. Open a little on cool sunny days and more on warm ones.
Step 3: Close before sunset so the soil’s stored heat stays in, as the carrier advises.
Step 4: Water in the morning, so leaves dry before night, and avoid watering late in the day.
Step 5: Watch humidity. Damp, still air favours grey mould and mildew. Open the lid even on cold, dull days for an hour when it is not freezing.
Step 6: Add cover on clear, calm nights. A blanket or straw mat over the glass reduces radiative loss. It must come off in the morning.
Step 7: Read the dawn minimum. The gain from the cover is smallest at dawn; if the dawn reading is near or below the outside reading on clear nights, add a layer.
Diagnosing Problems
- Soft, pale, leggy growth — too warm or too dark; vent more.
- Scorch or wilt on a sunny day — the frame was closed too long.
- Mould and rot — too damp; vent and water earlier.
- Frost damage despite the cover — a clear, calm night with a thin cover; add a layer.
Common Mistakes
- Opening at noon only — by then the damage is done on a sunny morning.
- Never opening on cold days — humidity builds and disease follows.
- Trusting the 40°F rule everywhere — it is the carrier’s rule of thumb; your crop and structure may differ.
What’s Next
- Learn next: Season Extension Crops — what to put under the cover
- Combine with: Thermal Mass Heating — steadying the temperature with water and stone