Why it matters
A cold frame captures solar warmth and reduces wind exposure. It has no automatic promise of frost protection and can overheat quickly in sun. The skill is daily management rather than merely putting a transparent lid over plants.
What you’ll need
Tools
- Thermometer
- Tape measure
Materials
- Purpose-made cold frame
- Seedling trays
Before you begin
Safety & limits
- Use safe glazing and reliable lid supports. Keep children clear of pinch points.
Work carefully, in order
The method
- 01
Choose a sound unit
Use a stable frame and safe glazing intended for the purpose. Avoid damaged old windows with unknown coatings or fragile glass. Check that the lid cannot slam onto hands or blow away in a gust.
- 02
Place for access
Choose a drained site with useful light and easy daily access. Leave room to open and secure the lid. Do not place it where roof runoff floods trays or where opening it obstructs a path.
- 03
Measure inside conditions
Put a thermometer where it reflects the plants’ conditions rather than touching sun-heated glazing. Record morning, midday, and evening readings. Outside air temperature alone does not reveal the temperature beneath the lid.
- 04
Vent and water
Open ventilation as conditions require and secure it against wind. Check compost moisture in trays directly. Warm air can dry small containers quickly, while a closed damp frame can encourage disease. Match ventilation and watering to the crop.
- 05
Transition plants gradually
When using the frame for hardening seedlings, increase exposure gradually according to the crop and weather. Move tender plants to appropriate protection before damaging cold. Review the forecast daily and arrange care when you are away.
Mistakes to avoid
- Leaving the lid closed all sunny day
- Assuming the frame prevents every frost
- Using unsecured glass
If it isn’t working
Seedlings wilt in sun
Check heat and moisture promptly.
Condensation persists
Review ventilation and crowding.
Then & now
Historical context
Cold frames extended seasonal growing before controlled greenhouses were common.
The modern equivalent
Automatic vent openers help but do not replace weather checks and inspection.
Sources & review
Editorial draft — specialist review pending. Written 2026-09-20. Sources support the stated scope; this page has not been professionally certified.
This entry describes general practice. A topic-specific source review is still required before it can be marked reviewed.
How this manual handles evidence →
