Frost forms when the temperature at ground level drops to freezing and water vapour in the air turns to ice on surfaces. For a farmer, the sight of white on young leaves in spring is rarely welcome.

Ice crystals pierce cell walls inside plant tissue. When they melt, the cells leak and collapse. Tender seedlings blacken overnight. Open blossoms on fruit trees can be wiped out in a single cold morning, taking the whole year's crop with them.

Protection methods

• Wind machines mix warmer air from above with cold air at ground level
• Sprinklers coat plants in ice, which releases heat as it freezes
• Row covers trap warmth close to the soil
• Site selection: cold air drains downhill, so slopes are safer than hollows

Growers watch forecasts closely in spring and autumn. A few degrees can mean the difference between a full harvest and none. Breeding programmes have also produced varieties that flower later or tolerate brief freezes.