Coolness, even without frost, can damage some plants. Plants accustomed to warm temperatures can suffer chilling injury at cold temperatures. Houseplants left out on a cool night, tomato seedlings put outside too soon, or bananas left in the refrigerator can all suffer chilling injury.
Different plants can tolerate different levels of cold. Many tender plants suffer chilling injury at temperatures below 55 degrees. Others can tolerate a little more cold, but suffer chilling injury at 40 degrees or higher.
The cold damages plants in several ways. If severe, it damages some of the cellular internal membranes, causing phenolic compounds to leak out of the cell. The phenolic compounds oxidize when exposed to air, causing the damaged tissue to turn black. Slightly damaged plants might cup their leaves downward or wilt. Fruit quality is hurt by chilling injury, damaging the texture and flavor.
Tropical Plants
Plants adapted to tropical and subtropical regions have a low tolerance for cold. Most houseplants are native to tropical forests, and suffer chilling injury on cold nights. Houseplants can be left outside during the summer, but must be brought indoors in the fall before the nighttime temperature drops to 55 degrees.
Starting Seedlings Indoors
Seedlings started indoors are susceptible to chilling injury, too. Temperate seedlings like broccoli and lettuce tolerate quite a bit of cold if hardened to it, but can suffer chilling injury if exposed to cold temperatures without a period of hardening off. Tropical seedlings like tomatoes and squash are more tender yet; they should be hardened off, also, but shouldn’t be put outside until the nights are warmer than 55 degrees.
Tropical Fruits and Vegetables
Harvested or purchased tropical fruit should be stored as cool as possible, but at temperatures higher than 55 degrees. Bananas, papayas, mangos, and other tropical fruit will last longer and taste better if stored outside the refrigerator. Tomatoes—although they do not darken—also suffer from refrigerator temperatures; some of their more delicate chemicals are destroyed, damaging their quality and flavor.
After cutting, it is necessary to store any fruit in the refrigerator to avoid spoilage. Uncut fruit, though, will remain of better quality if stored at cool, but not cold, temperatures.
Apples, pears, and other temperate fruit tolerates cold temperature well, and store best in the crisper section of your refrigerator.