How do I use a cold frame properly?

A cold frame is the cheapest, most useful piece of protected growing equipment there is, and it is more useful to most gardeners than a greenhouse because it addresses the actual bottleneck: the handover between a warm windowsill and a cold garden. It gives very little frost protection on its own; what it gives is shelter, and shelter is what young plants need.

Safety

What you'll need

• A cold frame, bought or built
• A block or notched stick for propping the lid
• Horticultural fleece or an old blanket
• Max-min thermometer (optional)
• Slug control appropriate to the site (optional)

Step by step

1. Site it against a south-facing wall if you can.The wall gives shelter and re-radiates heat at night. Sloping the lid towards the south catches low spring sun. Level, firm ground, and somewhere you will actually walk past every day.
2. Understand what it does and does not do.It gives you perhaps two to four degrees over the open garden and takes the wind and rain off. That is enough for hardening off and for overwintering many cuttings; it is not enough to keep tender plants alive through a hard frost without extra covering.
3. Ventilate earlier than feels necessary.This is the whole skill. On a bright April morning a closed frame reaches temperatures that cook seedlings within an hour, and it will feel cold outside while it happens. Prop the lid in the morning as a matter of routine.
4. Close it in the late afternoon.Shutting it while the air is still warm traps the day's heat for the night. Waiting until dusk means you shut cold air in.
5. Use it to harden off, over about a week.Days one and two: lid propped a crack through the day, closed at night. Days three and four: lid half open all day, closed at night. Days five and six: lid fully open all day, propped at night if no frost is due. Day seven: lid off day and night. Then plant out.
6. Time the planting out by your region's last frost, not the calendar.Roughly mid to late May in southern England, late May in the Midlands and the north, and into early June in Scotland and in any frost pocket or valley bottom. Local frost dates vary by a fortnight over a few miles.
7. Cover it when frost is forecast.Fleece over the plants inside and an old blanket or bubble wrap over the lid. Check the forecast every evening during the hardening-off period and be prepared to go back a step.
8. Use it through the whole year.Spring for hardening off and early sowings. Summer for rooting cuttings under shade netting. Autumn for sowing hardy annuals and sweet peas. Winter for overwintering cuttings, alpines that hate wet, and half-hardy perennials kept just dry.
9. Keep winter contents dry rather than warm.Most overwintering losses are rot, not cold. Ventilate on mild days even in January, water very sparingly, and space plants so air moves between them.
10. Shade it in high summer.Shade netting or a coat of shade paint. A frame in July sun cooks anything in it, and cuttings in particular need bright shade rather than direct sun.
11. Watch for slugs, mice and vine weevil.A frame is warm, sheltered and full of soft young plants, so it concentrates pests. Check under pots and around the frame regularly. Mice take pea and bean seed sown in a frame with great enthusiasm.
12. Build one cheaply if you do not want to buy one.Four scaffold boards or a stack of bricks with an old window on top works perfectly. Make the back higher than the front so the lid sheds rain and faces the sun, and use polycarbonate rather than glass if children are around.

Tips

• An automatic vent opener made for a cold frame costs little, needs no power, and prevents the single most common disaster.
• A max-min thermometer left in a closed frame for one sunny day tells you more than any amount of advice.
• Bricks or paving slabs inside the frame store heat during the day and release it at night, which is a genuine if modest improvement.

Common mistakes

• Leaving the lid shut on a sunny spring day — The temperature inside can pass 40°C within an hour while the air outside is still cold. This kills more plants in cold frames than frost does.
• Hardening off in two days because the weather turned nice — Leaves need time to build the surface structures that resist wind and ultraviolet. Rushed plants scorch, shred and check badly.
• Keeping overwintering plants damp and shut up — Still, humid, cold air is what rots them. Ventilate on mild days and keep them barely moist.
• Using horticultural glass where children play — It breaks into sharp shards. Polycarbonate is safer, lighter and only marginally less clear.

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