xt7bg737258c https://exploreuk.uky.edu/dips/xt7bg737258c/data/mets.xml   Kentucky Agricultural Experiment Station. 1973 journals 214 English Lexington : Agricultural Experiment Station, University of Kentucky Contact the Special Collections Research Center for information regarding rights and use of this collection. Kentucky Agricultural Experiment Station Progress report (Kentucky Agricultural Experiment Station) n.214 text Progress report (Kentucky Agricultural Experiment Station) n.214 1973 2014 true xt7bg737258c section xt7bg737258c Progress Report 214
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I Introduction ........................................... 3
Freezing Temperatures and Plant Injury ............................. 3
Freeze Classification ....................................... 6 _
Favorable Conditions for Late Spring and Early Fall Freeze .................. 6
Source ol` Freeze Data ...................................... 8
Explanation of Charts and Graphs ................................ 8 ,
Proper Use of These Data ................................... 24
Literature Cited ........................................ 24
The author acknowledges the assistance of personnel ol the National Climatic Center, NOAA,
lor their statistical analysis of Kentucky’s temperature records, and Mrs. Catherine DeBoer for
her help in assembling the tables.

‘R1sk of "Frost and ‘Freez1ng Temperatures 1Il Kentucky
During the course of producing and marketing a profitable crop, the Kentucky farmer has
, many hazards to deal with. One of the more important of these is, in many instances, the risk of J
a damaging late frost or freeze in the spring and the risk of a similar occurrence early in the fall.
The truck farmer and fruit grower are possibly the most concerned with late spring and early fall
freezes, but those who raise some of the other more basic field crops such as cotton, corn, and
tobacco sometimes experience losses from unusually late freezing temperatures in spring. Like-
' wise, some late-maturing crops are also subject to damage by early fall freezing.
Since it is not always economically feasible to attempt to prevent the occurrence of freezing
temperatures in Kentucky fields and orchards, it would be desirable for the farmer to be able to
· plan and carry out his operations in such a way that his risk of damage will be minimized. To do
this he must have information regarding the chance or risk of freezing temperatures occurring in _
his part of the state after certain dates in the spring and before certain dates in the fall. In some
instances, the length of the growing season (the number of days between spring and fall freezes)
determines the economic feasibility of growing certain crops. The purpose of this study is to
provide the needed information concerning growing season length and spring and fall freezing
While the effect of sub-freezing temperatures on agriculture is of great importance, other M
significant interests such as construction, sale of seasonal items, maintenance, heating, etc., are
affected to varying degrees.
The data in this pamphlet will inform the farmer, home gardener, or orchardist of the risks
involved in early and late planting dates. These data should also be of considerable value to
certain industries in planning their respective operations.
It is generally agreed that injury to plants by freezing usually occurs when ice crystals form
within the plant tissues as a result of the freezing of the liquid in these tissues. The freezing point
of the liquid within the plant tissues varies greatly from one plant to another and at different .
stages of development in the same plant. For this reason there is a wide range in the temperature
necessary to cause major damage to growing plants. A temperature of freezing or slightly below
will damage and possibly kill very tender young plants but may have little adverse effect on more
hardy, older plants. Peach blossoms usually sustain little or no damage from a temperature of 300
F, but serious damage may be expected when temperatures drop to the 24-270 F range.
The duration of sub-freezing conditions is also an important factor in determining the
extent of damage. If the temperature remains below freezing for only a short time, the damage
may be negligible, while the same temperature over a period of several hours could cause major
· damage.
1This publication is a revision of Kentucky Agricultural Experiment Station Progress Report 139, "Late Spring
and Early Fall Freezes in Kentucky," by Doyle Cook published in 1964.
2Advis0ry Agricultural Meteorologist, National Weather Service, Lexington, Ky.

 Other factors that seemingly have a definite bearing on the amount of damage are the r
temperatures present prior to the sub-freezing conditions, the suddenness of the temperature »
drop, and the wind speed. Several days of unseasonably warm weather in early spring will often ;
cause perennials to come out of dormancy prematurely and become especially vulnerable to later
freezing temperatures. Some types of plants seem to be more susceptible to sudden freezes than
to slowly dropping temperatures, even though the same temperature may be reached in both ‘
cases. Drying winds, that often accompany unusually severe outbreaks of cold weather, can add
considerably to the damage. ‘
The appearance of frost has commonly been associated with plant injury. Frost is defined as
the deposit of atmospheric moisture in the form of feathery ice crystals on the ground or other
surfaces, whose temperatures have fallen to 32° F or lower. Frost and dew are both caused by `
condensation of atmospheric moisture (water vapor) on the cool ground or plant surface. In the _
case of frost, the water vapor is deposited as ice crystals instead of water droplets when the
temperature of the air at the surface is at the freezing point (32° F) or below. Dew or frost can
be expected to occur only on clear still nights. Since sub-freezing temperatures can and often do ¤
occur without the appearance of frost, plant injury may occur in the absence of frost. For this
reason, air temperature is usually considered a better criterion for measuring the extent of plant _
injury than the occurrence or non-occurrence of frost.
Recent research (2) has determined the critical temperatures for various fruits during their
early development when freeze damage is most likely to occur. Some of these are given in Tables
Stage of Development
Buds Small
Fruit Closed But Full Green `
Showing Color Bloom Fruit
°r °r °r
Apricot 25 28 29
Blueberry 25 26 27
Cherries 28 28 30 A
Grapes 30 31 31
Plums 25 28 30
Raspberries (black) 28 30 28
Raspberries (red) 27 29 27
Strawberries 28 31 28
Walnuts, English 30 30 30
More detailed information (3), (4) has been gathered for apples, peaches, and pears during
their blossom stages (Tables lb-d).

Average Temperature Average Temperature
Bud Development Stages* for 10% Kill for 90% Kill
°r °F
Silver tip 15 2
Green tip 18 10
Half-inch green 23 1 5
Tight cluster 27 21
First pink 28 24 ’
Full pink 28 25
First bloom 28 25
Full bloom 28 25
Post bloom 28 25
V *For Red Delicious. Golden Delicious and Winesap are approximately 1
degree hardier and Rome Beauty 2 degrees hardier, except after petal
i fall when all varieties are equally tender.
Average Temperature Average Temperature
Bud Development Stages* for 10% Kill for 90% Kill
°r °r
First swelling 18 1
Calyx green 2 1 5
Calyx red 23 9
First pink 25 1 5
First bloom 26 21
· Full bloom 27 24
A Post bloom 28 25
* For Elberta.
Average Temperature Average Temperature
Bud Development Stages* for 10% Kill for 90% Kill
°r °r
Scales separating 15 0
Blossom buds exposed 20 6
Tight cluster 24 l 5
- First white 25 19
Full white 26 22
First bloom 27 23
Full bloom 28 24
Post bloom 28 24
*For Bartlett. Anjou is similar in hardiness but may bloom earlier and,
therefore, may be more tender than Bartlett at the same date.

 Very little specific information is available regarding the critical temperatures for truck-
garden crops. However, these crops are usually classified as tender, semi-hardy, or hardy
according to their ability to withstand low temperatures. Tender plants will be damaged or killed
by any temperature of freezing or below. Included in this group are watermelons, tomatoes,
beans, and peppers. Temperatures slightly below freezing are necessary before injury to semi-
hardy plants occurs. This group includes carrots, lettuce, and celery. Hardy plants, which include
cabbage, turnips, and garden peas, can withstand a fairly hard freeze without being killed.
Freezes have been classified (1) as light, moderate, or severe, as follows:
Light freeze—temperatures 29° through 320 {
Moderate freeze—temperatures 25° through 280
Severe freeze—temperatures 24° or lower
It may be generally assumed that a light freeze will kill only the tenderest plants; a moderate _
freeze will damage most plants to some extent, with heavy damage to fruit blossoms and tender
and semi—hardy plants; and a severe freeze will cause heavy damage to most plants.
On clear nights, under favorable conditions, the heat lost from the surface of leaves and
plant buds will cause the temperature of those parts to drop below freezing while the air
temperature remains about 32° F. Thus there is still a risk of cold weather injury in the spring
when there is little chance of a freeze. Similarly, frost risk in the fall increases even before there is
a significant chance of having the first 32 degree reading.
Under conditions of clear skies, low humidity, and light wind, frost becomes a slight
possibility when air temperatures drop near 40 degrees. In the 35- to 36-degree range, frost
becomes even more of a possibility. To indicate when frost risk may become critical even before `
freezing temperatures are likely, temperature values of 360 F and 400 F have been included in
this study.
In late spring and early fall, temperatures in Kentucky usually remain above freezing during
the daylight hours and drop below freezing only at night. The last spring freeze and the first fall
freeze are more likely to occur on clear nights. Fair skies permit the invisible heat waves (long-
wave radiation), which are constantly leaving the earth’s surface, to pass more readily through the
atmosphere. Each night a large amount of heat is lost from the soil and plant surfaces by these
heat waves. Since the soil is a good insulator, the rate of replenishment of heat to the surface is
much lower than the rate of heat loss by longwave radiation, resulting in a net cooling effect.
Because of the loss of heat, the earth’s surface and the air near the surface gradually become
cooler ou clear nights. 'l`he cooling process continues throughout the night, with the lowest
temperature occurring near sunrise. If the air is sufficiently cool to start with, below—freezing
temperatures will result. The temperature profiles which develop near the ground on clear nights
are shown in Fig. l.
When skies are overcast, the heat leaving the earth is absorbed then radiated to the earth in
some degree by the cloud cover and not so much heat is lost from the surface. By this means,

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