August 5, 2026 · Updated September 12, 2026
How a cold frame for plants holds a few extra degrees
A cold frame traps the sun's heat during the day and gives it back slowly after dark, so the air inside holds a few degrees above the open garden through the night. That small buffer extends your season at both ends. What it saves on a given night depends on how cold your yard runs.
How a cold frame works
A cold frame is a low box with a clear, sloped lid. During daylight, sunlight passes through the lid and warms the soil and the air inside. The soil is the store here. It absorbs heat all day and radiates it back through the night, and the walls and lid slow how fast that warmth escapes to the sky.
After sunset, plants and soil in the open garden give their heat back to the sky and cool faster than the surrounding air. Inside a frame, the lid blocks that direct view of the cold sky and cuts the wind that would otherwise strip warmth away. So the frame runs warmer than the bed beside it, most on still, clear nights when open ground loses heat fastest.
The buffer is small, so your real low decides
A frame buys you a few degrees, and a hard freeze still passes through it. This is why the buffer means nothing until you know how cold your yard gets. A published frost date averages a wide region, and your own ground can sit several degrees colder or warmer than that regional figure on any given night.
In our own garden the station reads about 7\u00b0F colder than the forecast. Our first fall frost landed on October 3 last year, well before the published date of October 22. Our coldest night of 2026 hit 3\u00b0F. A cold frame does nothing against a number like that for a tender crop, and it changes everything for hardy greens sitting a few degrees above the open bed.
Read the buffer against your own low, not the calendar. If your yard drops to 25\u00b0F, a frame that holds four degrees leaves the inside near 29\u00b0F, cold enough to damage anything but the hardiest crops. The frame cannot tell you what tonight's low will be, so pair it with a real read of your site before you trust it with plants you would rather not lose.
On a sunny winter day a closed frame can climb far above the outside air and cook the plants inside. Prop the lid open in the morning and close it before the afternoon warmth drains away.
What to grow, and when to open it
Match the frame to plants that already tolerate cold and keep going in low light. Greens and roots fit best.
- Hardy greens. Spinach and lettuce hold through frost and regrow when the sun returns. We direct-seeded both in our ground bed in late July.
- Quick roots. Radish sizes up fast and shrugs off cold. We seeded ours across three raised beds in early July.
- Overwintering starts. A frame shelters seedlings you want to harden before spring planting.
Venting is the daily job. Sunny days overheat a sealed box, so open the lid in the morning and shut it well before dusk to keep the day's warmth. On cold, overcast days you can leave it closed. For a wider list of crops that carry a cold season, see plants that grow in winter and your microclimate.
What this changes for a gardener
A cold frame turns a few degrees into a longer harvest, but only if you aim it at the right nights and the right plants. That means knowing your real low before the season narrows. The public microclimate checker pulls recent overnight readings from home stations near you, sets them against the forecast low, and shows the gap for your area.
For a per-yard number, a connected station learns the gap between your site and the forecast after about fourteen overlapping nights, and Siltwell then adjusts your frost alerts to your own ground. Start there, then set your frame where you can vent it easily. See how to connect a station to begin building that record.
Common questions
- How does a cold frame work?
- Sunlight passes through the clear lid and warms the soil and air inside the box. The soil stores that heat and releases it slowly after dark, while the sealed walls and lid slow the loss to the night sky. The result is air a few degrees warmer than the open garden through the coldest hours.
- What plants can I grow in a cold frame in winter?
- Cold-hardy greens and roots do best, because they tolerate cold and keep growing at low light. Spinach, lettuce, and radish all handle the conditions a frame creates. In our own garden we direct-seeded lettuce and spinach in late July and radish in early July, and those crops carry the cold better than any summer fruit.
- How cold is too cold for a cold frame?
- A cold frame holds only a few degrees, so a hard freeze still reaches the plants inside. Match the buffer to your real overnight low rather than the calendar. On the coldest nights, add a layer of fleece over the crops inside the frame or move tender pots elsewhere.
- Do I need to open a cold frame during the day?
- Yes, on any sunny day the sealed box can climb far above the outside air and cook the plants. Prop the lid open in the morning and close it before the afternoon heat leaves. On overcast, cold days you can leave it shut.
Related guides
The plants that grow in winter depend on how cold your yard gets
Learn which winter crops survive outside by matching each plant's rated low to your yard's real coldest night, not the zone map.
Frost cloth for plants and how long to leave it on
How frost cloth protects plants, which material to pick, how long to leave it on, and how to tell the nights your own yard actually freezes.
How frost covers for plants work
How frost covers trap ground heat, what fleece, blankets, and plastic each buy you in degrees, and how to match a cover to your yard's real low.
Free planting tools
- When to plant — the last date to sow each crop before your fall frost
- Microclimate checker — how much colder your yard runs than the forecast
- GDD estimator — whether a crop will ripen where you are
