Lesson 2
Understanding frost
Frost is not one thing. A light ground frost (-1°C for thirty minutes at dawn) will blacken dahlias but leave cabbages untouched. A hard frost (-4°C for three hours) will kill the crowns of perennials you thought were safe. An air frost (temperature below zero a metre above ground) is rarer and more dangerous — it finds hanging baskets, wall fruit, anything elevated.
Frost forms when the air is still, the sky is clear and heat radiates up and away from the ground overnight. This is why valleys frost before hilltops, why plants under trees survive when ones in the open do not, and why a single bedsheet over your courgettes can save them. The sheet does not warm them — it traps the earth's residual heat beneath.
In the UK, the last spring frost typically arrives in early May in the south, late May in the north, and mid-June in cold pockets of Scotland and the Pennines. But these are averages. I have seen lethal frost in London on May 19th. The only safe approach is to have a plan the night before — fleece, cloches, or the willingness to carry pots indoors at 10pm.
Common mistakes
- Hardening off for three days, then leaving tender plants out on a frost night
- Assuming 'frost-free date' means no more cold nights (it doesn't — it means no more frost)
- Protecting only the plant, not the pot (roots in small containers freeze solid)
- Using plastic directly against leaves (it conducts cold; use fleece)
- Trusting a forecast that says 2°C (your garden's cold spot will be colder)
What to do when frost is forecast tonight
- 1.At 4pm, check the night's forecast low — if 3°C or below, act.
- 2.Move all tender container plants (tomatoes, peppers, basil, dahlias) into a shed, porch or greenhouse.
- 3.Drape horticultural fleece or old bedsheets over fixed plants — weight edges with bricks.
- 4.Water the soil beneath vulnerable plants lightly — moist soil holds heat better than dry.
- 5.Set an alarm for dawn; remove covers by 9am so plants do not cook in morning sun.
Frost prediction improves if you know your garden's cold spot — usually a dip where cold air pools overnight.