Some plants can get damaged at temperatures above freezing. Learn how and which ones. Chilling damage occurs mostly to tropical fruit if temperatures are like a refrigerator. Freezing Temperatures This is the time of year, the second week of December, we normally experience the start of freezing temperatures. The likelihood of freezing temperatures increase through the winter. Sometimes freezing temperatures occur in November, as it did a few years ago, but that’s rare and not “normal”. What I mean by “freezing temperatures” is plant damage that occurs anytime the air temperature drops below 32 degrees Fahrenheit (F). Those familiar with Celsius or centigrade, may realize this temperature is the same as 0 degrees on the Celsius scale. What is Chilling Injury? As a reference point, the temperature inside most refrigerators is set to around 40 degrees F, or 8 to 10 degrees above freezing; too cold for most tropical fruit and plants but not too cold for temperate fruit like apples and peaches. Chilling injury (plant damage that occurs because air temperatures are too cold for the plant but not yet freezing) is one reason many ripe tropical fruits, like tomatoes and (more obviously) bananas, should not be exposed to the 40-degree F temperatures of a refrigerator. All parts of tropical plants such as tomatoes and bougainvillea, experience “chilling injury” when temperatures drop a few degrees above freezing and may extend to 50+ degrees F. Chilling injury (as opposed to freeze or frost damage) occurs at different temperatures and depends on the plant. Chilling Damage Chilling injury damage to tropical and subtropical plants include small stem and leaf discoloration, leaf roll, poor growth, and susceptibility to some diseases like root or collar rot. Symptoms of chilling injury include a change in color such as yellowing or bronzing of leaves that ultimately result in leaf scorch or drop, the slowing or halting of growth, leaf drop, water-soaked patches in soft and semi-hard tissues, susceptibility to diseases, and wilting. Chilling injury is due to cooler or cold weather (above freezing) temperatures to tropical plants growing outside of, or close to, the fringes of their normal range. As a side note, I noticed leaf and stem discoloration (closer in color to leaf “bronzing”) in mesquite, palms, citrus and a wide range of plants growing at different temperature ranges. Warm Season Vegetables Usually Have More Chilling Damage Vegetables can exhibit chilling injury and freezing damage as well. So-called “warm season vegetables” such as tomatoes, peppers and eggplant can show chilling injury anytime the air temperature drops into the damaging temperature range I mentioned earlier. Cool season vegetables, on the other hand, may sail through the same temperatures, or lower, or require a crop cover when temperatures are below freezing. Vegetable varieties may differ in their chilling injury by a few degrees. The ‘Dragon’s Tongue’ variety of bush bean is more susceptible to collar rot (chilling injury) when grown in garden soil a few degrees cooler than other bush bean varieties. Refrigerator Temperatures Result in Chilling Damage Temperate fruit like apples and pomegranates, unlike tropical fruit like tomatoes and bananas, are not damaged at refrigerator temperatures (around 40 degrees) because fruit from these trees can handle these lower temperatures. The ideal storage temperature for these types of fruit is somewhere close to freezing (0 degrees F) and combined with high humidity. A high humidity slows water loss and helps delay some fruit from shriveling. Freezer Temperatures Result in Freezing Damage The freezer part of our refrigerator is set to around 32 degrees F, or about 10 degrees below the “refrigerator temperature.” Our nighttime winter temperature frequently drops to a “refrigerator temperature” range at night during the late fall, winter and early spring and occasionally into the “freezer temperature” range during the early morning hours of December, January and occasionally early February. When nighttime temperatures reach the “freezer range” is when we often times see plant damage or experience fruit loss, but we may not know it yet. Open Flowers are All Subject to Some Type of Damage Open flowers of any fruit (citrus, peach, apple and others) can’t handle temperatures below freezing (32 degrees F) even though most plants or trees might show no damage at all! When flowers are simply buds and not yet open, there is a small amount of freeze protection provided to the developing flower. This freeze protection starts disappearing as the flower buds mature into open flowers. As the flower begins opening, and the frost-sensitive ovary is surrounded by the freezing night air, is when we experience damage or fruit loss. Fruit loss due to a frozen flower ovary can happen in a few seconds. This is why sprinklers, ultimately resulting in applied water turning to ice on the flowers, are used in orchards to prevent freeze damage to flowers (ovary). The act of water freezing releases a small amount of heat that protect flower ovaries from death. If you are curious if the ovary of a flower from your fruit tree was damaged during a freeze, pull the flower apart a few days after a suspected freeze and inspect the ovary for death. Ovaries that eventually turn into fruit will be robust and green. Dead flowers drop from the tree early or have a dull, water-soaked appearance if they are still attached. Just because the flower you inspected was “dead” doesn’t mean there will be no fruit produced at all. It takes about two or three weeks for all the flowers to open in spring flowering plants. Several consecutive light freezes in a row (or only one hard freeze) are needed to totally wipe out a crop of fruit from a mature tree.  There is a temperature difference between the freezing death of open flowers and the freezing damage or death of the plant or tree. For citrus this difference can range from the same temperature as flower death (32 degrees F; limes and true lemons) to lower temperatures (mid 20 degrees F; Myer lemon, grapefruit, and kumquat).