Some plants can get damaged at temperatures above freezing. Learn how and which ones.
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| 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).
Much of this depends on how many minutes or hours a killing freeze lasts and at
what temperature.
Temperature, and duration of that
temperature, are what is critical. With open flowers, freezing temperatures are
needed for just a minute. With plant or tree death, the same temperature is
needed for a longer time period. With some fruit trees (such as citrus), the
temperature difference between flower and tree death can be small (0 to 7
degrees). In other fruit trees (such as apples, pear, peach and apricot) it can
be much greater (20 degrees or more).
What to do about freezing temperatures and
fruit or plant loss? Track and use weather station apps during times when
freezing temperatures might occur. Buy a recording thermometer for your
backyard and hang it in your important trees. If you are into growing
vegetables, use a soil thermometer to judge when to precisely plant.
Move fruit trees to a new location. If
your fruit trees are less than three to four years old, now is the time of year
to move them. South and West-facing sides of walls and buildings are tough to
grow plants because of the heat they reflect. Some citrus are in landscape
“wind tunnels” that remove flowers during strong winds. North and east sides of
buildings and walls as well as backyards have a gentler environment. In cooler,
gentler areas of the landscape expect flowering to be two to three weeks later.
Group plants together so they can share water, fertilizer, and protect each other.
If they can’t be grown together, consider constructing wind barriers.
Try kumquats which are a cold hardy, subtropical
citrus trees. The small fruit can be freshly harvested and eaten from the tree
but still gives you that citrus “kick”. Kumquat flowers several times during
the summer instead of just once and yet is small enough to grow in a container.
Flowering more than once during a single growing season increases your chances
of having a successful crop if you have late or multiple freezes.
Grow fruit trees that flower continuously.
Pomegranate, fig and jujube are temperate cold and heat tolerant fruit trees
that flower continuously through the growing season.
Always group plants and fruit trees
together. Improve the soil at the time of planting and every couple of years. The
water, soil improvement and shade that they provide and share is a big benefit
to them when grown in the desert.