Cold Frames – Moving Seedlings Outside Weeks Earlier
A cold frame is the cheapest way to stretch a short season: a box with a clear lid, facing south, doing the job of a greenhouse for one tray at a time.
TL;DR
- Sited against a south wall, lid sloping toward the sun, on ground that drains.
- Vent every sunny day — overheating is the usual killer, not cold.
- Buys roughly two to four weeks at each end of the season; not a frost-proof greenhouse.
Siting and building the frame
Position matters more than construction. Put the frame against a south-facing wall, tilted a few degrees toward the sun, so the low winter light hits the lid at something close to a right angle. That wall also radiates heat back into the frame at night, which is half of what the box is for. Ground underneath should drain — a frame sitting in a puddle loses more plants to rot than to cold.
Construction can be four boards and an old window. Depth of about 12 inches at the back and 8 to 10 at the front, a lid on hinges, and a way to prop it open. Salvaged storm windows, shower doors and double-wall polycarbonate all work; the lid needs to shed water and let light through, and nothing else.
Venting, and the mistake everyone makes once
A closed frame on a sunny February afternoon can pass 100F inside while the garden is at 40F. Seedlings that were fine at breakfast are limp at lunch, and it is not recoverable. The rule is mechanical: if the sun is out and you can be home, open it. If you cannot be home, fit an automatic vent opener, which uses a wax cylinder to lift the lid as it warms and close it as it cools.
Cold is the easier problem. Insulate on frost nights — an old blanket over the lid, straw bales along the sides — and accept that a hard freeze will still get through. Hardy crops (lettuce, kale, spinach, radish) tolerate far more than tomato and pepper, which should not be in a frame until the frost date has passed for that crop regardless of what the box does.
Cold frame or nothing
For a small garden, the frame is the difference between setting out seedlings on the last frost date and setting them out ten days earlier — and between pulling the last lettuce in September and in October. It is also the cheapest hardening-off tool there is: put trays in it with the lid propped for a week and you have done half the work of moving them in and out of the house every day.
Pair it with the indoor seed starting guide for the front half of the season and the soil blocker guide if you want potless blocks to set out from.
Tools for this job
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Polycarbonate cold frame
A snap-together frame with a vented lid, for a first season without tools.
Automatic vent opener
Opens the lid on heat and closes it as it cools, so a workday does not cook the tray.
Soil thermometer
Tells you when the soil is actually warm enough to plant into, not just the air.
Related guides
Frequently asked questions
What does a cold frame actually do?
It traps daytime solar heat and keeps the worst of the wind off, so the air around the plants stays several degrees warmer than the garden. In practice that buys you two to four weeks at the start and end of the season, without electricity.
When do I open and close it?
Open the lid mid-morning once the outside air is above about 40F and close it mid-afternoon before the temperature drops. On a bright day even in February the inside can pass 100F quickly, and that is what kills seedlings in a cold frame, not cold.
Do I still need to harden plants off?
Yes, but you are partway there. A week of days in the frame does most of the hardening work; add a few days with the lid fully open, then a night or two before planting out. Plants moved straight from indoor heat to a closed frame still scorch.
Can I build one cheaply?
Yes. Four boards, a sheet of clear polycarbonate or an old window, and two hinges. Old storm windows and salvaged shower doors are common free lids. The lid does not need to be beautiful, it needs to shed water and let light through.
Will a cold frame keep tomatoes alive through frost?
No. It buffers a light frost for hardy crops like lettuce, kale and spinach, but a hard freeze with temperatures in the twenties will still kill tender plants. Add a blanket over the lid on those nights or bring the plants in.