• TABLE OF CONTENTS
HIDE
 Front Cover
 Table of Contents
 Cold injury symptoms
 Plants response to freezing...
 Principles of heat transfer
 Enviromental conditions leading...
 Measuring environmental condit...
 Methods of cold protection
 Treatment of plants after cold...
 Reference
 Back Cover






Group Title: Florida Cooperative Extension Service bulletin 201
Title: Cold protection for nursery crops
CITATION PAGE IMAGE ZOOMABLE PAGE TEXT
Full Citation
STANDARD VIEW MARC VIEW
Permanent Link: http://ufdc.ufl.edu/UF00024817/00001
 Material Information
Title: Cold protection for nursery crops
Series Title: Bulletin Florida Cooperative Extension Service
Physical Description: 17 p. : ill. ; 23 cm.
Language: English
Creator: Ingram, Dewayne L ( Dewayne Lebron ), 1952-
Yeager, Thomas H ( Thomas Henry ), 1952-
Hummel, Rita Lynn
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida
Place of Publication: Gainesville
Publication Date: [1985?]
 Subjects
Subject: Crops and climate   ( lcsh )
Plants, Effect of cold on   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
bibliography   ( marcgt )
non-fiction   ( marcgt )
 Notes
Bibliography: Bibliography: p. 17.
Statement of Responsibility: Dewayne L. Ingram, Thomas H. Yeager, Rita L. Hummel.
General Note: Cover title.
Funding: This collection includes items related to Florida’s environments, ecosystems, and species. It includes the subcollections of Florida Cooperative Fish and Wildlife Research Unit project documents, the Sea Grant technical series, the Florida Geological Survey series, the Coastal Engineering Department series, the Howard T. Odum Center for Wetland technical reports, and other entities devoted to the study and preservation of Florida's natural resources.
 Record Information
Bibliographic ID: UF00024817
Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
Rights Management: All rights reserved, Board of Trustees of the University of Florida
Resource Identifier: aleph - 000872580
oclc - 14720329
notis - AEG9841

Table of Contents
    Front Cover
        Front Cover 1
        Front Cover 2
    Table of Contents
        Page 1
        Page 2
    Cold injury symptoms
        Page 3
    Plants response to freezing temperature
        Page 4
        Page 5
    Principles of heat transfer
        Page 6
    Enviromental conditions leading to cold injury
        Page 7
    Measuring environmental conditions
        Page 8
    Methods of cold protection
        Page 9
        Water used for cold protection
            Page 9
            Page 10
        Air movement for cold protection
            Page 11
        Cold protection structures
            Page 11
            Page 12
            Page 13
            Page 14
        Supplementary heat
            Page 15
    Treatment of plants after cold stress
        Page 16
    Reference
        Page 17
    Back Cover
        Page 18
Full Text

Bulletin 201


COLD PROTECTION

FOR NURSERY CROPS


Dewayne L. Ingram
Thomas H. Yeager
Rita L. Hummel


For Commercial Use Only


Florida Cooperative Extension Service
Institute of Food and Agricultural Sciences
University of Florida, Gainesville
John T. Woeste, Dean for Extension

















































Dewayne L. Ingram, Associate Professor, Extension Ornamental
Horticulturist; Thomas H. Yeager, Assistant Professor, Extension
Ornamental Horticulturist, and Rita L. Hummel, Assistant Pro-
fessor, Ornamental Horticulture; Ornamental Horticulture Depart-
ment, IFAS, University of Florida, Gainesville, 32611.








Table of Contents




Cold Injury Symptoms .............................. 3

Plant Response to Freezing Temperatures ................ 4

Principles of Heat Transfer ............................ 6

Environmental Conditions Leading to Cold Injury .......... 7

Measuring Environmental Conditions .................... 8

Methods of Cold Protection ............................ 9

Water Use for Cold Protection .......................... 9
Air Movement for Cold Protection ....................... 11
Cold Protection Structures ............................ 11
Supplementary Heat ................................. 15

Treatment of Plants After Cold Stress .................... 16








Winter temperatures are frequently low enough to cause cold injury to
tropical, subtropical, and temperate plants that are produced in Florida.
This publication provides information for ornamental plant producers
regarding symptoms of cold injury, plant adaptation to cold,
environmental conditions leading to cold injury, and methods to alter the
environment to avoid or minimize cold injury.
Cold injury includes damage from temperatures above and below
freezing. Many tropical and herbaceous plants do not adapt to withstand
freezing temperatures and may be injured by temperatures below 50F
(100C). Injury caused by low temperatures above freezing is chill injury,
and damage caused by freezing temperatures is freeze injury.
Cold Injury Symptoms
Cold injury symptoms usually occur after exposure to critically low
temperatures, not during the cold exposure. Direct injury is inflicted at a
cellular level, and the response of plant tissues to this injury is revealed
through visual or measurable symptoms. The rate at which these
symptoms develop depends upon the severity of the exposure and the
environment after the exposure. Continued cool temperatures and high
humidity after an exposure to cold may slow the symptom development,
while high light intensity and warm temperatures may accelerate
symptom development.
Chilling. Many chilling injury symptoms are common to other stresses
such as drought stress, root rot diseases, phytotoxicity to chemicals, heat
stress, and light stress. General symptoms of chill injury to plant leaves,
stems, and fruits are listed below.
Surface lesions, pitting, large sunken areas, and discoloration. These
symptoms have been reported on several orchids.
Water-soaking in tissues results from disruption of cell structure and
release of cell solutes into spaces between cells, and is commonly
followed by wilting and browning.
Internal discoloration (browning) of pulp, pith, and seed.
Accelerated rate of senescence (natural death), but with otherwise
normal appearance.
Increased susceptibility to attack by fungi and bacteria not commonly
found on the plant.
Slowed growth, or limited growth flush. This symptom may be
difficult to detect without non-chilled plants for comparison or a
thorough knowledge of normal growth rate.
Freezing. Symptoms of freeze injury could include desiccation or
burning of foliage; water-soaked areas that progress to necrotic spots on
leaves, stems, or fruit; and death of sections of the plant or the entire
plant. Close examination of woody plants several days or weeks after
freezing may reveal a dead or weakened root system or split bark on
stems or branches. Obvious symptoms on plant foliage may not be








present until after the plant has been stressed by warm temperatures. A
hot, bright day could increase transpirational water loss beyond the
ability of injured roots or stem conductive tissue to replace. Subsequent
symptoms might include wilting and/or desiccation, as caused by direct
drought stress.

Plant Response to Freezing Temperatures
When considering new plant material for use in the landscape or as a
possible nursery crop, cold hardiness should be determined. Hardiness
indicates a plant's resistance or ability to adjust to cold stress in order to
tolerate freezing temperatures.
The timing and degree of cold hardiness is determined by
environmental conditions and the genetic makeup of a particular plant.
Inherent, genetic potential is the first limiting factor in development of
hardiness, with plant species and genotypes within species differing in
their tolerance to cold. Most tropical plants fail to develop hardiness
regardless of preconditioning environmental conditions. Some species
are always killed by freezing, while others tolerate temperatures as low as
-3200F (-1960C) in midwinter.
The geographic source of a plant plays an important role in the timing
of hardiness. Northern plants sensitive to daylength start to cold harden
sooner in autumn than southern plants grown on the same site.
Therefore, selection of seed or cutting stock of desired plants from cold
hardy genotypes is an important consideration.
Plant reaction to environmental conditions leading to increased cold
tolerance is called cold acclimation, and plant reaction to environmental
conditions resulting in less tolerance to cold is called cold deacclimation.
Although in more northern climates acclimation occurs primarily in the
fall and deacclimation in the spring, rapid changes in plant cold tolerance
occur throughout the fall, winter, and spring months in Florida as
environmental conditions change rapidly.
Environmental factors, such as daylength, temperature, nutrition, water
availability, light intensity, and physiological maturity of a plant or plant
part are known to play a role in cold acclimation. Once the effect of these
environmental factors on cold acclimation is understood, sound cultural
practices directed toward increasing cold tolerance can be developed.
Generally, plant growth slows or ceases before cold acclimation begins.
Decreasing daylength provides the primary stimulus or trigger for cold
acclimation in many plants. Phytochrome, a light receptive sensory
pigment in leaves and bark, detects decreasing daylengths of autumn and
initiates biochemical and physiological changes that slow vegetative
growth and increases cold acclimation. Plants that react primarily to
daylength slow growth at approximately the same time each year,
regardless of temperatures. Temperature is the primary stimulus for cold








acclimation in some subtropical plants such as citrus. However, in most
plants there is an interaction between photoperiod and temperatures on
plant growth and development, and the most rapid cold acclimation is
produced by short photoperiods and low temperatures.
Plants should withstand cold best if fertilized with a balanced ratio of
plant nutrients that produce optimum growth (Pellet and Carter), but the
rate of fertilization should be reduced slightly in fall and winter to reflect
the lower nutrient requirements during the cold months. Plants under
severe nutrient deficiencies, or plants receiving nutrients at near toxic
levels do not withstand cold or recover from cold injury as well as plants
receiving properly balanced nutrition.
Moderate drought stress can result in increased cold tolerance in some
plants by slowing growth and initiating dormancy. Many plants native to
the dry plains respond to this treatment because in nature the plants are
commonly subjected to dry conditions before the onset of winter.
Although water stress may increase cold tolerance in some plants, such
stress may result in an unacceptable decrease in quality of plants such as
azaleas.
The maximum cold tolerance level is seldom reached in Florida, even
with temperate plants, because of the loss of hardiness during extended
warm periods in winter months. Plant metabolic activity slows during
extended cold periods, but a period of warm temperatures can stimulate
rapid deacclimation. Temperature seems to be the primary environmental
factor controlling deacclimation, and plants can reacclimate if a
subsequent slow temperature drop occurs. A rapid temperature drop
following a warm period, a common event in Florida, may produce injury
and death to plants.
Evaluation of cold hardiness is further complicated because different
tissues and organs of the same plant may display varying degrees of
hardiness. Flower buds are often damaged by freezes while vegetative
buds or stem tissues are uninjured. Plants grown for their floral display
may be hardy, but if their flower buds are killed every year they will be
unsatisfactory for landscape use. Leaves of broad-leaved evergreens may
be injured by cold, yet the stems remain unharmed. Root tissues are less
resistant to freezing injury than stem tissues, with young roots being more
sensitive to cold than mature roots. Stems and leaves of Pyracantha
coccinea 'Lalandii' became acclimated to -150F (-260C), while young
roots failed to survive below 230F (-50C) (Wiest and Steponkus). Lack of
cold resistance in roots is not usually a problem in field production or in
the Florida landscape; however, it may be the limiting factor to winter
survival of containerized plants when prolonged freezing temperatures
occur.
Plants that survive freezing temperatures must either avoid or tolerate
the formation of ice in their tissues. The primary mechanism by which
plants avoid freezing is supercooling. Supercooling occurs when the








plant's temperature drops below its freezing point without ice formation.
Even pure water will supercool. The lowest subfreezing temperature
recorded before ice formation is the supercooling point. Unfortunately
this point is not a constant value but varies for repeated tests on the same
solution. Under field conditions supercooling generally allows
nonacclimated plants to avoid freezing when temperatures in the 31 F to
26F (-1C to -30C) range occur.
Cold resistant plants tolerate water freezing in their tissues as long as the
ice crystals form between cells extracellularr freezing) and not inside them
intracellularr freezing). Extracellular ice formation (Figure 1A) is the type
of freezing encountered in nature and is tolerated by hardy plants in their
cold acclimated state. Intracellular freezing (Figure 1B) disrupts the cell
and is always fatal. Generally, freezing rates in nature are too slow to
allow intracellular freezing. Cooling rates of 3.6F (20C) per minute or
faster are required for intracellular freezing, and slower cooling allows
sufficient time for water to move through the surrounding membrane and
form ice crystals in extracellular spaces. Thus at the cellular level, the
requirements for freezing resistance in any hardy plant are the avoidance
of intracellular freezing and the tolerance of extracellular freezing.

---- ~Cell Wall
: Cell Membrane
Protoplasm
Vacuole
Normal Cell
,** .,* Cell Wall
8-* Cell Membrane
r ;Protoplasm
H20
II3 Ice Crystals
Figure 1A- Extracellular Ice Formation

Cell Wall
.:)- Cell Membrane
j Protoplasm
SIce Crystals

Figure 1B- Intracellular Ice Formation


Principles of Heat Transfer
Heat loss by plants involves heat transfer. This principle should be
understood before formulating and evaluating cold protection methods.
Heat may be transferred by conduction, convection, or radiation.








Conduction is the process of heat moving from one molecule in a
substance to an adjacent molecule. The denser the material, the closer
the molecules and the greater its potential to conduct heat. Heat
movement from a warm area within a soil to colder adjacent areas is an
example of conduction.
Convection is the process of heat transfer within a fluid or air that
results in mass motion of molecules in that fluid or air. Heat is transferred
from air at the soil surface to air above the earth by convection. Air
becomes lighter when heated, rises, and is replaced by heavier, cooler
air. This mass motion of air is called convective mixing, and explains why
air just above the earth's surface does not become extremely hot on a
sunny, summer day.
Radiation is the process of heat transfer from one object to another
without the aid of a transfer medium. The sun's energy heats the earth's
surface and can burn human skin by radiant heat transfer. The surfaces in
a greenhouse are warmed by absorption of short wave solar radiation.
These surfaces reradiate heat to the air above them as long wave or
infrared radiation.

Environmental Conditions Leading to Cold Injury
Cold conditions in Florida are a result of cold air masses moving down
through more northern states and pushing into Florida. Temperatures
associated with these fronts depend upon where the pressure systems
originate and the rate at which they move into Florida. Environmental
conditions created by these cold fronts can be categorized into one of
two general types. These fronts can be characterized by moist air with
considerable cloud cover or by dry air and clear skies. Windy conditions
can accompany either of these conditions, and wind is an important
consideration in developing plant protection strategies.
Daily temperature fluctuations are greatest when clear skies exist. Solar
radiation warms the earth's surface during the day, and air temperatures
of 600F to 700F (160C to 21 C) are common even though the minimum
temperatures at night may be less than 300F (-1 OC). This large fluctuation
is primarily due to radiational cooling.
Radiational cooling occurs primarily at night when the heat absorbed
by the earth's surface during the day is reradiated into the atmosphere.
Air at or near the soil surface is warmest during the day and coldest during
the night under such conditions. Heat lost from surfaces by radiational
cooling moves away from the earth's surface due to convective mixing.
The soil continues to lose heat until it is colder than the air just above it,
then the soil absorbs heat from the air. The existing condition is a cold air
layer near the earth's surface with a rapidly cooling soil surface, and the
warmest air may be from a few feet to 100 feet above the soil surface.
Plant leaves close to the ground may sustain freeze injury even though








the temperature a few feet above the plant is above their freezing point.
This condition is called a "temperature inversion" because it is an
inversion of normal daytime conditions where the warmer air is near the
ground.
Moist air and cloud cover reduce the fluctuation of daily air
temperatures. Cloud cover reduces the amount of solar radiation
reaching the earth's surface during the day, and heat radiating from
surfaces on a calm night is absorbed by clouds and reradiated back to the
earth's surface. The primary cooling process of plants and other objects
during cloudy, calm cold weather is conduction of heat from the leaf to
the colder air surrounding it. Leaf temperature generally is not lower than
the air temperature in these conditions.
Wind increases the rate of temperature drop. A 5 to 10 mph wind on a
cold, cloudy night constantly replaces the warmer air on leaf surfaces
with cold air, and this accelerates the rate of heat loss from the leaf. A
temperature gradient will develop between the leaf and the air if the air is
calm, and the rate of heat loss from the leaf would be reduced slightly.
Wind on a clear night prevents or reduces the formation of an inversion
layer by mixing the warmer air above the crop with colder air at the crop
surface, thus slowing the rate of leaf temperature drop.
The lowest temperatures occur in Florida when cold, dry (clear skies)
air masses move rapidly across the United States and into Florida.
Generally, as such a cold front moves from north Florida to south Florida
the temperature of the air mass increases, but the amount of temperature
increase depends upon how fast the front is moving. Clear skies at night
greatly increase the chance of crop injury by allowing considerable
radiational heat loss from the crop environment.
The terms frost injury and freeze injury are often confused. The injury
mechanism in both is the freezing of cellular water, but freeze injury can
take place even if frost is not present. Frost occurs when the dew point of
the air (the temperature at which air is saturated with water) is reached at
freezing temperatures. Air holds less water vapor as it gets colder. When
the dew point occurs at freezing temperatures, the water vapor in the air
changes to ice crystals on exposed surfaces. When the air contains a lot of
moisture, the dew point may be reached before freezing temperatures
occur, and water vapor will condense as a liquid on exposed surfaces.
Dew may freeze after it has condensed on leaf surfaces if the air
temperature drops below freezing, but this type of ice formation is less
damaging to plants. When the humidity is very low, freeze injury can
occur without frost. Freezes without frost are often called "black frosts."
Measuring Environmental Conditions
Proper measurement of environmental conditions is essential for
predicting or assessing plant response and for optimum management of
plant protection systems. Temperature is the most important








environmental parameter to be measured by the nursery manager.
However, temperature measurements related to relative humidity, and
wind speed and direction can provide the manager with more insight into
current and expected conditions.
Relative humidity is the quantity of water vapor present in the
atmosphere, expressed as a percentage of the quantity which would
saturate the air at the same temperature. Relative humidity affects plant
response to cold temperatures, how rapidly temperatures drop, and how
well specific protection methods work. Air with a high relative humidity
will resist temperature change more than dry air, and plant water lost to
desiccating winds will be lessened by high relative humidity. Relative
humidity sensors and recorders are available in a wide range of prices
with varying accuracies. One practical way of measuring relative
humidity at a given temperature is with a wet-bulb thermometer. A wet-
bulb thermometer has a bulb covered with a moist muslin bag, thus
lowering the measured temperature by loss of latent heat through
evaporation. The lower the wet bulb temperature compared to the dry
bulb temperature, the lower the relative humidity.
Care should be taken to purchase high quality thermometers that
should be routinely calibrated in an ice water bath. Electronic sensors
such as thermocouples and thermisters can be purchased or made to
sense a temperature at a particular point. Microprocessors are available
that can be programmed to scan a large number of temperature sensors
at predetermined time intervals and record the temperatures.

Methods of Cold Protection
Water Used for Cold Protection. The unique physical properties of
water as a vapor, liquid, or solid make it a primary factor in plant
protection from freezing or chilling temperatures. As water cools at
temperatures above freezing, "sensible" heat is released. Actually, 1000
calories of heat energy are released as 1 liter of water is cooled from
37.40F to 35.60F (30C to 20C), which is 8.3 BTU's/gallon/F. A BTU,
British Thermal Unit, is defined as the heat required to raise the
temperature of 1 pound of water 1 F, or to raise 816 grams of water 1 C.
As water cools from 600F (160C) to 320F (0C), 16 kcal per liter (232
BTU's per gallon) of water are released into the surrounding
environment. Fogging, flooding, and sprinkling (at temperatures above
freezing) utilize the sensible heat in water to moderate temperature drop
in the nursery.
When water changes from a liquid to solid state (ice), a tremendous
amount of energy is released. This energy is called the "heat of fusion"
and is equal to 80 kcal per liter or 1200 BTU's per gallon. Sprinkling when
air temperatures are below or approaching 320F (0C) is sometimes
called icing, and utilizes the heat of fusion to provide cold protection for
plants.








environmental parameter to be measured by the nursery manager.
However, temperature measurements related to relative humidity, and
wind speed and direction can provide the manager with more insight into
current and expected conditions.
Relative humidity is the quantity of water vapor present in the
atmosphere, expressed as a percentage of the quantity which would
saturate the air at the same temperature. Relative humidity affects plant
response to cold temperatures, how rapidly temperatures drop, and how
well specific protection methods work. Air with a high relative humidity
will resist temperature change more than dry air, and plant water lost to
desiccating winds will be lessened by high relative humidity. Relative
humidity sensors and recorders are available in a wide range of prices
with varying accuracies. One practical way of measuring relative
humidity at a given temperature is with a wet-bulb thermometer. A wet-
bulb thermometer has a bulb covered with a moist muslin bag, thus
lowering the measured temperature by loss of latent heat through
evaporation. The lower the wet bulb temperature compared to the dry
bulb temperature, the lower the relative humidity.
Care should be taken to purchase high quality thermometers that
should be routinely calibrated in an ice water bath. Electronic sensors
such as thermocouples and thermisters can be purchased or made to
sense a temperature at a particular point. Microprocessors are available
that can be programmed to scan a large number of temperature sensors
at predetermined time intervals and record the temperatures.

Methods of Cold Protection
Water Used for Cold Protection. The unique physical properties of
water as a vapor, liquid, or solid make it a primary factor in plant
protection from freezing or chilling temperatures. As water cools at
temperatures above freezing, "sensible" heat is released. Actually, 1000
calories of heat energy are released as 1 liter of water is cooled from
37.40F to 35.60F (30C to 20C), which is 8.3 BTU's/gallon/F. A BTU,
British Thermal Unit, is defined as the heat required to raise the
temperature of 1 pound of water 1 F, or to raise 816 grams of water 1 C.
As water cools from 600F (160C) to 320F (0C), 16 kcal per liter (232
BTU's per gallon) of water are released into the surrounding
environment. Fogging, flooding, and sprinkling (at temperatures above
freezing) utilize the sensible heat in water to moderate temperature drop
in the nursery.
When water changes from a liquid to solid state (ice), a tremendous
amount of energy is released. This energy is called the "heat of fusion"
and is equal to 80 kcal per liter or 1200 BTU's per gallon. Sprinkling when
air temperatures are below or approaching 320F (0C) is sometimes
called icing, and utilizes the heat of fusion to provide cold protection for
plants.








Sprinkling for cold protection is becoming increasingly popular in
Florida nurseries. It can be used to moderate temperatures above freezing
because of the sensible heat in water, and can maintain plant
temperatures just above freezing due to the heat of fusion released as
water freezes.
Heat is released as long as water is freezing, and this prevents plant
temperatures from dropping below 32F (0C). It is important to
emphasize this fact because plants freeze below 320F (0C) due to
dissolved solutes in plant sap. The key to successful use of sprinkling is for
ample water to cover the plant surface so ice forms and heat is released
continuously. Therefore, a nursery operator must have an adequate
water supply, pumping capacity, and irrigation delivery system so that
ample water for continuous freezing is delivered to plants. Formation of
milky-white ice rather than clear ice is an indication that the application
rate is too low to provide protection. Many nurseries do not have the
irrigation capacity to water the entire nursery at one time, which is
needed for sprinkler protection, and water management boards may limit
the amount of water used for freeze protection. Therefore, a nursery
might group less hardy species together in an area to receive sprinkler
freeze protection. Detailed information on delivery rates is available in
Circular 348, Sprinkler Irrigation for Cold Protection. The delivery rate
must increase as the air temperature becomes lower, because the water
loses heat faster. Inadequate water for freezing could result in more plant
damage than if sprinkling had not been used, because the ice on leaves
without a constant supply of water for freezing would pull water from the
leaves resulting in freeze injury and desiccation injury as well.
When windy, dry conditions are prevalent, sprinkling should begin
before the leaf temperature drops to the critically low temperature for
that crop plant, because evaporation of the applied water may lower the
leaf temperature 20F to 40F (10C to 20C) or more. Sprinkling should
continue until after thawing or until the wet bulb temperature rises above
freezing, especially if windy, dry conditions prevail. Evaporative cooling
occurs because heat energy is lost to the atmosphere as water changes
from a liquid to a vapor. Do not rely on a household window
thermometer to monitor leaf and air temperatures.
The water must be delivered uniformly, with allowances for changes in
wind velocities and direction. Wind adversely affects the sprinkler
distribution pattern and causes the heat from the heat of fusion to be lost
by evaporation. This means that up to 7 times the amount of water used
for a freeze on a calm night must be applied to compensate for heat loss
due to evaporation and conduction when a 5 to 10 mph wind exists.
The greatest disadvantage of sprinkling is breakage of plant limbs due to
ice weight. Easily broken container plants may be placed on their side
and iced to prevent breakage. These plants should be placed upright as
soon as possible after the freeze.








Sprinkling for freeze protection can be used effectively in Florida. Plants
do not have to be repositioned, and there are no structures to erect.
Therefore, the reduced labor requirement for sprinkling is an advantage.
However, frequent sprinkling may leach nutrients and/or cause water
logged soils or container media which may result in plant stunting or
death. The large amount of water required for this practice could be a
limiting factor in some areas of Florida.
Water applied to aisles of shade structures or greenhouses increases the
moisture content of the air and soil surrounding the plants (increases wet-
bulb temperatures), thus slowing the rate of temperature drop. The water
absorbs heat during the day which is released slowly during the night.
The water should be applied in the late afternoon of a warm day. Sides of
adequately constructed shade houses can be covered with ice by
sprinkling on freezing nights to reduce the effect of wind.
Fog also retards the loss of heat from soil and plant surfaces to the
atmosphere. Natural fogs create a barrier to radiant heat loss much like
clouds, although their effectiveness varies with the size of the suspended
water particles. Fog can provide up to 80F (40C) of protection outdoors
during radiational cooling. Applying ground water with an average
temperature of 700F (21 C) to a shade house or greenhouse can create a
ground fog if the ground surface is several degrees cooler than the water.
This applied water adds heat to the plant environment and/or buffers
temperature change by increased humidity. Fogging is most effective in
an enclosed structure such as a greenhouse or a partially enclosed
structure such as a saran house, but the fog must be uniformly distributed.
Temperatures can be elevated as much as 9F (50C) in these unheated
structures. High pressure, low volume systems are the best means
available to create a uniform fog. A low volume system dramatically
reduces water requirements compared to sprinkling.
Air Movement for Cold Protection. Wind machines have been used for
many years in citrus and vegetable industries, and recently in the
ornamental industry, as a means of cold protection. Wind machines are
only effective in the advent of radiational freezes characterized by winds
less than 5 miles per hour.
Denser cold air settles in low areas resulting in temperature strata with
warm air above the cold. Wind movement can disturb this inversion
existing on calm nights. This forced air movement will mix the cold and
warm air, resulting in warmer air surrounding the plant. Air movement
also helps distribute and circulate heat added by orchard heaters or other
sources.
Cold Protection Structures. Structures for cold protection are used to
prevent plant desiccation caused by winds associated with severe freezes,
to trap heat present, and to contain supplied heat energy. They should be
constructed to withstand high winds and minimize heat loss. These
structures are expensive because of construction materials and required








Sprinkling for freeze protection can be used effectively in Florida. Plants
do not have to be repositioned, and there are no structures to erect.
Therefore, the reduced labor requirement for sprinkling is an advantage.
However, frequent sprinkling may leach nutrients and/or cause water
logged soils or container media which may result in plant stunting or
death. The large amount of water required for this practice could be a
limiting factor in some areas of Florida.
Water applied to aisles of shade structures or greenhouses increases the
moisture content of the air and soil surrounding the plants (increases wet-
bulb temperatures), thus slowing the rate of temperature drop. The water
absorbs heat during the day which is released slowly during the night.
The water should be applied in the late afternoon of a warm day. Sides of
adequately constructed shade houses can be covered with ice by
sprinkling on freezing nights to reduce the effect of wind.
Fog also retards the loss of heat from soil and plant surfaces to the
atmosphere. Natural fogs create a barrier to radiant heat loss much like
clouds, although their effectiveness varies with the size of the suspended
water particles. Fog can provide up to 80F (40C) of protection outdoors
during radiational cooling. Applying ground water with an average
temperature of 700F (21 C) to a shade house or greenhouse can create a
ground fog if the ground surface is several degrees cooler than the water.
This applied water adds heat to the plant environment and/or buffers
temperature change by increased humidity. Fogging is most effective in
an enclosed structure such as a greenhouse or a partially enclosed
structure such as a saran house, but the fog must be uniformly distributed.
Temperatures can be elevated as much as 9F (50C) in these unheated
structures. High pressure, low volume systems are the best means
available to create a uniform fog. A low volume system dramatically
reduces water requirements compared to sprinkling.
Air Movement for Cold Protection. Wind machines have been used for
many years in citrus and vegetable industries, and recently in the
ornamental industry, as a means of cold protection. Wind machines are
only effective in the advent of radiational freezes characterized by winds
less than 5 miles per hour.
Denser cold air settles in low areas resulting in temperature strata with
warm air above the cold. Wind movement can disturb this inversion
existing on calm nights. This forced air movement will mix the cold and
warm air, resulting in warmer air surrounding the plant. Air movement
also helps distribute and circulate heat added by orchard heaters or other
sources.
Cold Protection Structures. Structures for cold protection are used to
prevent plant desiccation caused by winds associated with severe freezes,
to trap heat present, and to contain supplied heat energy. They should be
constructed to withstand high winds and minimize heat loss. These
structures are expensive because of construction materials and required








labor for movement of plants in and out as the conditions or seasons
change.
Florida nursery operators should analyze nursery production systems
for each plant species, the risk of cold damage, and the projected dollar
return before investing in structures for cold protection. Certain high
value crops warrant structures specifically for cold protection, but in
other cases dual purpose structures should be considered. A structure
used for shading in summer often can be used for cold protection during
winter.
Structures can be constructed of wood, galvanized pipe, conduit, PVC
pipe, or concrete reinforcing rods, and cost will be the overriding factor
in determining which to use. Size of structures depends on the size of
plants to be protected, growing bed width and length, and the
production system used. Detailed plans for various greenhouses and cold
frame structures can be obtained from the Extension Agricultural
Engineer through your local Cooperative Extension Office.
A common winter protection structure is the quonset type constructed
of bent galvanized pipe. For this structure '/2-inch pipe joints are used for
bows which are placed inside larger pipe studs that protrude 6 inches (15
cm) from the ground. This results in a house 14 feet (4.3 meters) wide and
6 feet (1.8 meters) tall. The bows are usually 2 feet (61 cm) apart and the
house is usually long enough to accommodate common size
polyethylene. One purlin down the center is adequate for support.


Figure 2. Quonset-style greenhouse constructed with PVC pipe bows.








Similar construction with PVC pipe bows also has become popular due to
reduced cost of PVC pipe (Figure 2). The house should be oriented in a
north-south direction to distribute the light uniformly within the structure.
A clear polyethylene covering (4 to 6 mil) is usually pulled over the ends
and secured along the sides. A door at one or both ends is used for entry.
Venting may be done by raising the plastic on the side opposite prevailing
winds, and closing it during cold weather.
Frames for such permanent structures are usually built on a portion of
the container production area, and plants from areas adjacent to the
quonsets are crowded into these structures to reduce the number of
houses needed. The irrigation system for the container production area is
usually not flexible enough to use to irrigate plants enclosed in these
quonsets, and expensive hand watering may be required.
One may elect to construct small, lightweight, portable structures
which can be placed over beds of cold sensitive plants during cold
weather and removed during warm weather (Figure 3). Such portable
structures may be small quonsets constructed of conduit, PVC pipe, or
concrete reinforcing rods covered with concrete reinforcing wire for
support. They should be wide enough to span a bed and 6 to 10 feet (1.8
to 3.0 meters) long. Quonsets made to stack on top of each other will
make storage easier (Figure 4). Polyethylene coverings can be attached to
wooden strips at the bottom of each side. A small piece of plastic may be
secured over ends of the structures and opened during the day for
ventilation. Portable quonsets have definite advantages to the
nonportable, galvanized pipe structures, since plants are not repositioned
and the structure may be removed for watering.


Figure 3. Lightweight, portable structure for cold protection.























Figure 4. Lightweight, portable cold protection structure, designed to stack for con-
venient storage.



Winter temperatures in Florida are not consistently low enough to
warrant placing plants on their side in structures and covering with
Styrofoam or polyethylene material for the entire season. Nursery
operators might consider placing high value container plants on their
sides in the event of a severe freeze. The plants may then be covered with
1 or 2 mil polyethylene, Styrofoam or other insulating material supported
just above the plants so the cover is not in direct contact with the foliage.
The insulating material should be removed and the plants placed upright
after the freeze.
Shade structures are most effective in providing protection during cold
weather with little air movement. Saran structures may raise the ambient
temperature under them 20F to 40F (1C to 20C) by reradiated heat
radiating from the ground and objects within the structure. Lath houses
are less efficient than saran structures at reradiating heat, but both
provide some cold protection. Sides of shade structures may be covered
with water during freezing conditions since the ice forms a windbreak.
Care should be taken that ice loads do not crush the structure. Some
shade structures are designed so they can be covered with polyethylene
film during winter months or when cold weather is expected.
Plants may be placed in cold frames for protection from rapid
temperature fluctuations (Figure 5). Small plants, such as liners, can be set
upright in the frame while larger plants (1 gal. etc.) may be placed on their
side. Placing larger plants in the frame is an expensive operation and one
should contemplate placing the plants on their side and covering in the
field rather than transporting to a cold frame. In either case, the plants
should be placed upright as soon as cold weather passes. Frames may be




























Figure 5. Cold frame for protecting nursery crops.

economical for protection of liners that were propagated and/or held in
the frame through the winter. Plants in cold frames may be covered with
glass, polyethylene, Styrofoam or other insulating materials to trap radiant
heat. Polyethylene or other light transmitting covers with a southern
exposure permit solar radiation to warm the structure. The frame
covering should be removed on warm days to prevent excessive heat
build up.
Supplementary Heat. Air temperatures inside unheated quonsets made
of a single layer of plastic are usually about 5F (30C) warmer than outside
air on a cold night. This small temperature differential can be critical to
the survival of many plants. However, supplemental heat ensures that
plant environment temperatures are above critical levels. Adding heat to
an enclosed structure is more feasible than heating an outside growing
area with orchard heaters, but energy costs may prohibit the heating of
any woody ornamental growing area. If outside heating is used, it should
be used in combination with other protection techniques such as wind
movement, fog, or some barrier created to reduce radiant heat loss. The
decision to add supplemental heat must be based on crop value and rate
of return on investment.
Heat sources for structures include solar radiation, well water, and
boilers fired by oil, gas, or wood. Solar heating systems, warm water units,
and unheated well water circulation offer the greatest potential for Florida
woody ornamental growers. Nursery operators should consult their
Water Management District personnel before installing unheated well








water circulators. Several passive solar heating designs are available that
collect the sun's rays and store this heat energy in some medium such as
stone or water. The heat collected during the day is then circulated in the
structure at night.
Circulation of warm water (1100F to 130F [430C to 540C]), not hot
water, in enclosed growing and/or storage areas has gained popularity in
recent years. Warm water is more economical than forced air heat to
keep plant environments above a critical minimum temperature. This
water often is circulated through PVC pipe installed in some material like
sand or concrete under plant containers.
Well water temperatures in Florida during winter months range from
68F to 750F (200C to 240C). A system has been designed and tested at
the Agricultural Research Center in Monticello, Florida, that circulates
unheated well water through PVC pipe in a propagation cold frame. This
system kept the minimum cold frame soil temperature above 54F (12C)
and the minimum air temperature above 46F (80C) while outside
temperatures were below 20F (-70C) for several hours. Minimum soil
and air temperatures in cold frames without this water circulation system
were 450F (70C) and 40F (4'C), respectively, for the same time period.
The decision to add heat to the plant growing environment must be
based on economics. Costs and returns for a given heating system,
structure and crop plant must be known or estimated before a system is
constructed or an existing system is used.
Treatment of Plants After Cold Stress. The environment to which
plants are subjected after cold stress affects the degree of injury and rate
of symptom development. Importance of post exposure environment
varies with the severity of cold stress. Plants exposed to temperatures
below their cold tolerance level will not recover; however, damage to
plants exposed to near critical temperatures may be influenced by post
stress handling.
Intense light, low humidity, and high temperatures following chilling of
some tropical plants result in increased water loss through transpiration.
Extreme water stress can develop if the chill exposure has disrupted water
absorption, temporarily or permanently.
Root systems of plants in field production are seldom frozen in Florida,
but roots of container-grown plants can be frozen for several consecutive
hours. Clear skies are common when extremely low temperatures occur
in Florida. Sunny conditions on mornings after night freezes can result in
rapid transpiration (water vapor loss) as leaves are warmed, but the
soil/root mass may be frozen and unable to provide ample water to
leaves, resulting in excessive water stress and leaf desiccation. Symptoms
may not occur for several days and may be manifest as marginal leaf
scorch or overall browning. Watering container-grown plants can thaw
the growing medium/root mass and allow water absorption and transport
to the leaves. Excessive water, however, can leach nutrients and cause








root injury by waterlogging the growing medium.
Cold injury to roots may not be evident until spring when plants are
stressed by high temperatures. Failure to initiate a spring growth flush
may be the only visual symptom of winter injury, and little can be done to
minimize the effect of winter injury at this time. Weakened or injured
plants are more susceptible to disease attack, so growers should increase
frequency of inspection and implement a preventative fungicide program
if justified. Increased shade may also reduce heat or water stress during
recovery periods. Justification of such efforts should be determined on an
economic basis.
Pruning apparently damaged branches should be delayed until new
growth appears or the degree of wood damage can be determined. This
ensures that an excessive amount of live wood is not removed. Cold
injured wood can be identified by examining the cambium layer (tissue
just under the bark) for black or brown coloration. Pruning these
branches 2 to 3 cm behind the point of discoloration reduces the
potential for disease development and maintains aesthetically pleasing
plants.








Trade names are mentioned with the understanding that no discrimina-
tion is intended nor endorsement implied by the Cooperative Extension
Service.

References

Harrison, D., J. F. Gerber, and R. E. Choate. 1974. Sprinkler irrigation for
cold protection. University of Florida, IFAS, Technical Circular 348,
p. 19.
Levitt, J. 1980. Response of plants to environmental stress. Volume I.
Chilling, freezing and high temperature stresses. Academic Press,
NY, NY. p. 23-290.
Pellett, H. M. and J. V. Carter. 1981. Effect of nutritional factors on cold
hardiness of plants. Horticultural Reviews 3: 144-171.
Wiest, S. C. and P. L. Stephonkus. 1976. Acclimation of Pyracantha tissues
and differential thermal analysis of the freezing process. J. Amer.
Soc. Hort. Sci. 101: 273-277.
Wright, R. D. 1977. Physiology of plant tops during winter. Proc. Int. Plant
Prop. Society. Vol. 27: 287-290.



























































This publication was promulgated at a cost of $1,763.00, or 22 cents per
copy, to inform the Florida nursery industry of principles of cold injury to
plants, heat transfer in the environment and methods of protecting crop
plants from cold. 5-8M-85

COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORI-
DA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, K. R.
Tefertlller, director, in cooperation with the United States Department IFAS
of Agriculture, publishes this Information to further the purpose of the
May 8 and June 30, 1914 Acts of Congress; and Is authorized to pro-
vide research, educational Information and other services only to Indl-
viduals and Institutions that function without regard to race, color, sex or national ori-
gin. Single copies of Extension publications (excluding 4-H and Youth publications) are
available free to Florida residents from County Extension Offices. Information on bulk
rates or copies for out-of-state purchasers Is available from C. M. Hinton, Publications
Distribution Center, IFAS Building 664, University of Florida, Gainesvllle, Florida
32611. Before publicizing this publication, editors should contact this address to deter-
mine availability.




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