Frost damage
Injury caused when ice forms inside plant tissue, turning leaves, flowers and shoots black, limp or papery after a cold night.
Also known as: Frost injury, Frost burn
Symptoms and identification
Leaves and shoots look water-soaked at first, then turn dark green, brown or black and wilt or collapse as they thaw. Flower buds and open blossoms turn brown, and fruit may fail to set. Damage often shows only on the exposed top and outer parts of the plant.
Causes
Frost occurs when temperatures fall to or below freezing, and it is worst on clear, still nights, in low spots where cold air collects, and on tender plants or soft new growth after a warm spell.
Organic treatment
Do not prune damaged growth straight away. Wait until new growth shows in spring so that you can see how far back the damage goes, then cut back to healthy buds.
Prevention
Check forecasts in spring and autumn, cover tender plants overnight with fleece, sacking or cloth, and bring pots under shelter. Avoid planting tender crops out before the last frost date and avoid frost pockets.
Chemical treatment (use with care)
No chemical treatment repairs frost damage. Avoid feeding with nitrogen until recovery starts, as it pushes soft growth that is easily hurt.