Damage to plants from cold temperatures above freezing (32°F) is called chilling injury.
Identification
Plants of tropical and subtropical origin are most susceptible to chilling injury. African violet, avocado, banana, mango, okra, princess flower (Tibouchina urvilleana), tomato, and certain others can be damaged by temperatures above freezing. To help diagnose chilling as the cause of injury, learn the low-temperature tolerance of the plants grown and whether temperatures colder than plants tolerate recently occurred in the area.
Many plant species can be injured by a rapid, substantial drop in temperatures. The duration of cold, minimum temperature, rate at which temperatures drop, time of year, and plant characteristics (e.g., age, hydration, part affected) influence the occurrence and severity of cold damage.
Damage
Chilling commonly kills flowers, injures fruit, and causes leaves and shoots to wilt or discolor purplish to red. Other symptoms of chilling damage can resemble those observed in more cold-hardy plants after exposure to freezing or frost (≤ 32°F).
Solutions
Choose species well adapted to the climate and seasonal temperatures for the location. For information on plants’ tolerance to cold, consult the California Master Gardener Handbook and the Sunset Western Garden Book. Compare the cold tolerance of the species you wish to grow to the USDA Plant Hardiness Zone Map, which presents the historical, minimum temperatures at your location.
To increase a soil's ability to absorb heat and warm plants, control weeds and during winter rake away mulch and keep soil bare around cold-sensitive plants. Irrigate dry topsoil at least 3 days before cold weather is expected; this increases the soil's ability to retain heat. Cover sensitive plants overnight with cloth or similar material other than plastic to reduce heat loss to the atmosphere, but leave covers open at their bottom so heat from soil can help warm plants. Consult Abiotic Disorders of Landscape Plants for more information.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
