Plants that are not adapted to cold climates are killed or damaged by freezing temperatures. This group includes most tropical and subtropical plants. Gardeners refer to these as “tender” plants, in contrast to “hardy” plants that can tolerate freezing temperatures.
Tender plants turn black and wilt dramatically as they thaw the morning after a frost. Once they have reached this state, they are dead and cannot be saved.
Plants can also be damaged by exposure to temperatures above freezing. This damage is called Chilling Injury.
Damage from Freezing
Freezing damage is caused by the water inside the plant freezing. The water in the space between cells freezes first, drawing water through the cell walls and causing the cells to shrink. The shrinkage may kill the cells. The cold may also freeze the fluid within the cells, rupturing the cell walls.
Damage is often done when the ice crystals in the intercellular spaces thaw suddenly. Water rushes back into the cells, causing them to rupture from the pressure. When cell walls rupture, the cells deflate like pricked balloons, spilling their contents into the space between the cells. Phenolic compounds in the cells turn black when they are exposed to oxygen, causing the blackening usually seen in freeze-damaged plants.
Adapted plants can tolerate a light frost by increasing the dissolved solids—sugars, salts, protein, and other substances—in their cellular fluid. The dissolved material acts like antifreeze, lowering the point at which the cell contents freeze. Cells also reduce the amount of water they contain, concentrating the fluid within and lowering its freezing point.
After a Freeze
The worst freezing damage often occurs when the morning sun warms frozen plants rapidly after a frosty night. You might be able to save the plants by thawing them slowly. Place blankets or other insulating material over the plants to allow them to thaw slowly.
Do not wash the frost off with water. Water thaws the cells rapidly, causing more damage. The frost itself is not doing the damage; the cold that caused the frost is what damages the plants.