HO-203

Purdue University

Cooperative Extension Service

West Lafayette, IN 47907



Effects of Cold Weather on Horticultural Plants in Indiana



Larry A. Caplan
Vanderburgh County
Horticulture Agent



Introduction

Every fall, winter, and spring, many fruit and vegetable crops, as well as ornamental landscape plants, run the risk of injury caused by cold weather. Depending upon the plant species, damage can be caused by anything from a light, overnight frost to a prolonged period of freezing temperatures.

While cold damage is hard to predict. you can count on the fact that it will occur sooner or later. It can range from the loss of a few early blossoms in a low lying field to the complete loss of hundreds of acres over several counties; from barely visible leaf burn on early spring vegetables and flowers, to the death of above and below ground plant tissues.

Depending on the crop and location, some economic loss from cold injury can occur every year. Losses can result directly from damaged or killed plants, and indirectly from reduced quality or delayed maturation. The severe freeze in December 1983 killed many fruit and landscape plants. Of the plants which survived, many were seriously weakened, making them more susceptible to borers, cankers, and other problems over the next several years.

The objective of this publication is to identify the types of injury caused by cold weather, the factors that influence the degree of injury, and ways to prevent or reduce injury. This information should help in planning a defense against cold injury, and explain past failures in freeze prevention.

Climatology and Site Selection

There are a number of factors that play a part in determining the risk of frost and freezing injury to horticultural crops. The regional and local climates are important, as is the area's topography and the conditions of the specific site. The actual hardiness of the plant also helps determine the risk of injury; this will be covered later.

The climate of a region, sometimes called the macroclimate, is affected by land and water masses (i.e., "lake effect"), prevailing wind patterns, and the latitude. The macroclimate of the Midwest is typified by cold winters, with large and rapid swings in air temperature caused by alternating warm and cool air masses. Compare this to the Southern states, which have cool winters with occasional cold snaps. Crops and landscape plants that thrive under one set of climatic conditions may not perform reliably in another. This is the reasoning behind the different plant hardiness zones found on the USDA Plant Hardiness Map (see Figure 1).

Figure 1. USDA hardiness zones of the United States.

Other regional factors, including terrain and elevation, can cause differences in the climate. The terrain in Indiana varies greatly, and includes hilly areas, broad flat plains, and both broad and narrow river valleys. Air temperatures, especially the daily minimums, can differ widely over these varying land forms. It is generally cooler at higher elevations than at neighboring lower locations; the growing season is also shorter.

On clear, windless nights, temperature inversions can cause cold air to pool in low areas, called "frost pockets." An inversion exists when the temperature is colder closer to the ground than it is higher up. As the sun sets, surface temperatures drop, and the air directly above the ground becomes cooler. Since cold air is heavier than warm air, it will form a layer above the ground. The cold air flows downhill and settles in valleys and low area, much like water. Often, the air in these frost pockets can be as much as 15°F cooler than that of the surrounding high ground. The Kankakee River Valley is an example of an area affected this way.

Another terrain effect that influences the climate is the presence of large bodies of water, such as the Great Lakes. Within several miles of the lake shore, the warming influence of Lake Michigan delays the average date of the first fall freeze in northwestern Indiana by about 2 or 3 weeks. Likewise, the tempering influence of the lake delays the advancement of spring, and reduces the risk of an unusually late spring frost.

On the other end of the scale is the microclimate of a particular site. Microclimates are the little weather variations from one side of a hill to another, from one street to the next, and even within different sites in the same yard. Frost pockets fit under this category. So would the presence or absence of shade, wind exposure, and even the direction of a slope. South facing slopes warm up earlier, reach higher temperatures, and have greater variations in temperature than north facing slopes, due to exposure to the sun's rays.

Soil type, drainage, and management can affect the maximum and minimum daily temperatures. A soil's ability to hold heat is determined by its water and organic matter content, texture, and color. For example, a wet soil holds more heat than a dry soil. A clay or clay loam soil, which has a considerable amount of permanently bound water, warms slowly in the spring and cools slowly in the fall. On the other hand, a sandy soil, which has a low waterholding capacity, tends to warm quickly in the spring and cool rapidly in the fall. Dark soils absorb more heat than light colored soils. Soils which contain large amounts of organic matter, such as mucks and peats, do not conduct heat well. These soils warm and cool only in a shallow surface layer, which leads to great temperature extremes near the surface.

Macroclimates are not easily changed by man. It takes an entire planet to change the global or regional weather, as is proposed under the theory of the "greenhouse effect." However, microclimates can be easily changed, or at least understood and manipulated, by individuals. Shade trees or structures can be added or removed; windbreaks can block or redirect wind currents; wet areas can have their drainage improved through tile, ditches, or raised beds; frost pockets can be avoided. A frost pocket caused by a wooded area surrounding the orchard can be at least partially corrected by cutting a swath 75 to 100 feet wide through the woods at the low points. These openings act as drains, allowing the cold air to flow out of the orchard.

Microclimate problems can also be avoided with proper plant selection, such as by planting shade or wet-site tolerant plants. Plants that are sensitive to cold, such as those native to the South, can be replaced with more hardy species or varieties; in the case of tender vegetables, their planting dates can be delayed.

A grower or landscaper must understand the plants' needs, recognize all the microclimate factors, understand what the limits are for changing the environment, and then plan accordingly, in order to be successful at avoiding cold weather problems. Researchers have kept records on weather patterns, and have developed maps which predict, on the average, when you may expect the last 32°F frost of the spring, the first frost of the fall, and the length of the growing season in your area. Maps showing this information for Indiana are found in Figures 2, 3, and 4.

Figure 2. Average dates of last 32° F temperature (normal frost) in spring.

Figure 3. Average dates of first 32° F temperature (normal frost) in fall.

Figure 4. Average length of the annual frost-free season (32° F, 0° C) in days.

One final note: weather station temperature readings, often taken at local airports, most often do not reflect the temperature in a grower's fields, especially in frost pockets. Growers should use their own thermometers, which they should check for accuracy before using. The thermometers should be placed in the crop canopy (within the strawberry foliage, or in the fruit tree) for valid temperature readings.

Plant Injury and Protection

Low temperatures can affect plants in several ways. First, temperatures near the minimum for plant growth will reduce the plant's rate of metabolism and growth. If the temperature, and therefore the metabolism, remain low for an extended period, plant quality will suffer, and death may occur. A period of cold weather may also alter plant growth, as when certain vegetables "bolt," or produce seed stalks, in response to several days at low temperatures.

Another type of injury occurs if the temperature falls below freezing (32deg; F, 0° C). Below 32° F, the water within and between the plant's cells freezes. The ice crystals which form puncture the cells' membranes; when the temperatures rise and the ice melts, the cell contents leak out, killing the cell. Plant tissues that freeze generally appear dark green and watersoaked at first, later becoming blackened and necrotic.

On perennial plants, cambial tissue and vegetative and flowering buds can also be injured by low temperatures, although this injury may not be obvious until the following spring, when the buds fail to open, or open and then immediately die. Some injured fruit buds may abscise before spring. If cold injury is suspected, cut a few buds open from several places on the tree and examine them. If the center of the bud is darkened or black, the bud has been killed by cold. Check several trees in the area to obtain a representative sample.

Following are more detailed descriptions of plant injury and protective measures for different horticultural crops.

Vegetables and Annual Flowers

Vegetable crops differ in their hardiness to cold temperatures, depending upon their genetics and origin. Warm season crops, such as tomatoes, snap beans, and the cucurbits, originated in tropical areas, and can be severely injured by even a light frost. On the other hand, cool season crops, such as broccoli, cabbage, peas, and onions, originated in northern areas, and can tolerate frost and light freezes of short durations with little damage. Table 1 lists the symptoms of frost injury on some common vegetables.

Some vegetables, such as the cole crops (cabbage, broccoli, and cauliflower) and onion sets, respond to cold weather by producing a seed stalk. This process, called "bolting," occurs when young plants are exposed to low temperatures for several days. This causes flower buds to form within the growing point. When warmer weather returns, the buds develop into flower and seeds stalks. This greatly reduces the quality and marketability of the affected crop.

Bolting occurs on plants that were set out too early in the spring. They put on enough growth to get out of the juvenile stage (pencil thick stems for cabbage, half-inch wide bulbs for onion); at this point, the plant is sensitive to a cold spell. Bolted plants should be discarded; cutting the flower stalks off will not prevent the deterioration in flavor and quality. Do not set out plants too early in the spring if you wish to avoid this problem.

Table 2 lists a number of vegetables commonly planted in the Midwest, based on their ability to withstand frost. Table 3 shows similar information for annual flowers. Knowing a plant's hardiness can help guide growers in deciding when to start producing transplants, or when to plant seeds or transplants in the field, garden, or landscape. More information can be found in Purdue University publications HO-186, Indiana Vegetable Planting Calendar and AY-231, Determining Spring and Fall Frost-Freeze Risks in Indiana.

Proper management is needed to protect vegetable and flower crops from freezes. Delay field planting as late as possible in the spring to avoid a late frost. Plant fall crops as early as possible to allow time for maturation before the first frost. Use HO-186 as a guideline.

Delay nitrogen applications on early spring planted vegetables until the danger of cold injury has passed. Apply nitrogen early in the season to fall planted crops, so that as plants approach maturity, the tissue will be in a slight declining growth state rather than a flush stage of succulent growth.

Table 1. Symptoms of freezing injury on some vegetables.

---------------------------------------------------------------------------
Artichoke: Epidermis be comes detached and forms whitish to light tan
blisters.  When blisters are broken, underlying tissue turns brown.

Asparagus: Tip becomes limp and dark; the rest of the spear is
water-soaked.  Thawed spears become mushy.

Beet: External and internal water-soaking; sometimes blackening of
conducting tissue.

Broccoli: The youngest florets in the center of the curd are most
sensitive to freezing injury. They turn brown and give off strong
odors upon thawing.

Cabbage: Leaves become water-soaked, translucent, and limp upon
thawing; epidermis separates.

Carrot: Blistered ap pearance, jag ed length-wise cracks.  Interior
becomes water-soaked and darkened upon thawing.

Cauliflower: Curds turn brown and have a strong off-odor when cooked.

Celery: Leaves and petioles appear wilted and water-soaked upon
thawing.  Petioles freeze more readily than leaves.

Garlic: Thawed cloves ap pear grayish-yellow and water-soaked.

Lettuce: Blistering; dead cells of the separated epidermis on outer
leaves be come tan; increased sus- ceptibility to physical damage
and decay.

Onion: Thawed bulbs are soft, grayish-yellow, and water-soaked in
cross section; often limited to individual scales.

Pepper, bell: Dead, water-soaked tissue in part or all of pericarp
surface; pitting, shriveling, and decay follow thawing.

Potato: Freezing injury may not be externally evident, but shows as
gray or bluish-gray atches beneath the sink. Thawed tubers become soft
and watery.

Radish: Thawed tissues appear translucent; roots soften and shrivel.

Sweet potato: A yellowish-brown discoloration of the vascular ring,
and a yellowish-green water-soaked appearance of other tissues. Roots
soften and become very susceptible to decay.

Tomato: Water soaked and soft upon thawing. In partially frozen fruits,
the margin between healthy and dead tissue is distinct, especially in
green fruits.

Turnip: Small water-soaked spots or pitting on the surface.  Injured
tissues appear tan or gray and give off an objectionable odor.
--------------------------------------------------------------------------

Plastic mulches have been shown to increase soil temperature and hasten early plant development. During the day, sunlight warms the soil. At night, the plastic traps the heat, keeping the warmth in the soil. Clear plastic allows greater soil warming than dark colored (black, brown, gray) plastic (10°F to 20°F warming for clear, 5°F to 10°F for dark). This can increase the earliness of many crops, especially melons, by speeding up germination and early growth. However, the clear plastic allows light through, which can create a significant weed problem. Dark colored mulches block most or all of the light, which reduces weed growth and the amount of weed control needed.

Row coverings, which are often used in conjunction with plastic mulch, are specifically designed to promote early crop growth while reducing heat loss at night. There are many types of materials used, and many types of cover designs, including row tunnels, where the cover is supported by wire hoops, and floating row covers, where the material is allowed to lie directly on the crop. Several states in the Midwest, especially Illinois and Indiana, are examining the different materials to determine which is the best and most economical to use. More on this topic can be found later under specific cold protection measures.

Table 2. Frost resistance of vegetables.*

very hardy1  Frost tolerant2   Tender3      Warm loving4
-----------------------------------------------------------
Asparagus    Beet              Snap bean    Lima bean
Collards     Broccoli          Sweet corn   Cucumber
Endive       Brussels sprout   Tomato       Eggplant
Kale         Cabbage                        Muskmelon
Kohlrabi     Carrot                         Okra
Lettuce      Cauliflower                    Pepper
Mustard      Celeriac                       Pumpkin
Onion (sets  Celery                         Squash,
 and seeds)  Chard                           summer
Pea          Chinese cabbage                Squash,
Potato       Jerusalem                       winter
Rhubarb       artichoke                     Sweet
Rutabaga     Onion (plants)                  potato
Salsify      Parsnip                        Watermelon
Spinach      Radish
Turnip
-----------------------------------------------------------
*Based upon information from university of Illinois publication
VC 14 a2, Vegetable Planting Guide.

1very hardy vegetables can withstand freezing temperatures
and hard frosts for short periods without injury. They may be planted
as soon as the ground can be prepared. usually 4 to 6 weeks before the
average frost-free date.

2Frost tolerant vegetables can withstand light frosts and
can be planted 2 to 3 weeks before the average frost-free date.

3Tender vegetables are injured or killed by frost, and
their seeds do not germinate well in cold soil. They are usually
planted on or after the average frost-free date.

4Warm loving vegetables cannot tolerate cold. They require
warm soils for germination and good growth, and should be planted 1 to
2 weeks after the average frost-free date.

Vegetable and flower transplants should be hardened off before they are planted in the field. This slows the growth of the plants, decreasing the chance of injury. The plants should be gradually exposed to the lower temperatures and higher levels of sunlight found in the field for about two weeks before planting. A cold frame can be used for this. The plants can also be placed on wagons, which are brought outdoors during the day and returned to the barn at night. Row covers may also help protect young transplants.

Sprinkler irrigation is sometimes used to protect vegetables. Saturating the soil early in the day may help protect plants, since the water will warm up during the day and release the heat slowly during the night. Sprinkling the plants during frosty nights can also help prevent injury. See the section under specific cold protection measures for more information.

Chemical frost protectants, including surfactants and combinations of fungicides and bactericides, are being examined as possible methods of protecting crops. These products help prevent the formation of ice crystals, by destroying the bacteria that help cause ice crystals to form (called "ice-nucleating bacteria"). These products will provide some protection, at least for a few degrees below freezing. However, killing the bacteria will not prevent ice crystal formation caused by dust and other materials. These products should be used in conjunction with other protection measures, and should not be the only preventative measures used.

Table 3. Frost resistance of annual flowers.*

Very hardy1  Frost tolerant2    Tender3      Warm loving4
--------------------------------------------------------------
Cornflower   Bells of Ireland   Aster        Argeratum
Ornamental    (Molucella)       Nicotinia    Balsam
 cabbage     Black-eyed Susan   Petunia      Begonia
Pansy         (Rudbeckia)       Scabiosa     Cockscomb
Primrose     Coreopsis          Statice       (Celosia)
Violet       Pinks (Dianthus)   Sweet        Cosmos
             Pot Marigold        Alyssum     Impatiens
              (Calendula)       Verbena      Lobelia
             Snapdragon                      Marigold
             Stock (Matthiola                Moss Rose
              incana)                         (Portulaca)
             Sweet pea                       Periwinkle
             Torenia                          (Vinca)
                                            Phlox, annual
                                            Salpiglossis
                                            Salvia
                                            Zinnia
--------------------------------------------------------------  
* Based upon information from Purdue University publication HO-14.
Starting Seeds Indoors.

1 Very hardy flowers can withstand freezing temperatures
and hard frosts for short periods without injury. They may be planted
as soon as the ground can be prepared, usually 4 to 6 weeks before the
average frost-free date.

2 Frost tolerant flowers can withstand light frosts and can
be planted 2 to 3 weeks before the average frost-free date.

3Tender flowers are injured or killed by frost. Transplants
lack vigor in cold soil. and may need to be replaced for desired
floral display.  They are usually planted on or after the average
frost-free date.

4Warm loving flowers cannot tolerate cold. They require
warm soils for transplants to survive and become established. They
should be planted 1 to 2 weeks after the average frost-free date.

Woody Plants and Perennials

Woody trees and shrubs (both fruit and landscape) and many herbaceous perennials (including strawberries) can tolerate very low temperatures if they are allowed to harden off and go dormant in the fall. Hardening off is triggered by the shorter days of late summer and fall, which cause the plant to stop growing. At this time, overwintering buds are matured. These buds are often covered by protective bud scales which protect the bud from water loss and physical damage.

The second step for a plant to harden off and become dormant is exposure to low temperatures, at or below freezing, for at least part of the daily cycle. This causes changes in the plant's metabolism, which causes changes in the quantity, location, and make-up of sugars, proteins, moisture, and other plant chemicals. In deciduous plants, this is seen by the change in leaf color and leaf drop. All of this promotes resistance to freezing to develop. If cold weather occurs before the hardening process is complete, injury can occur.

Many factors can affect a plant's ability to harden off before cold weather. Late summer or early fall nitrogen fertilization can stimulate the production of new growth, which will be too lush and tender to survive. By withholding nitrogen applications in late summer, or reducing the amount applied so that stimulation does not occur, the plant's carbohydrate (sugar) reserves can go into storage, allowing the plant tissues to withstand cold temperatures better (sugars accumulate in the tissues and act like an antifreeze, lowering the temperature needed for the water in the tissues to freeze). Heavy fruit load can deplete these reserves; therefore, it is important to maintain healthy foliage after the crop has been harvested, so that accumulation of carbohydrates in the tree can occur.

Late summer pruning, or a wet fall following a dry summer, can also stimulate new growth, which will not be able to tolerate colder temperatures later. A tree weakened by drought, insect injury (especially girdling caused by borers and defoliation caused by caterpillars and beetles), disease, or mechanical injury to the trunk or roots, will be more susceptible to cold weather. Hardiness can also be affected by the duration and intensity of sunlight, length of growing season, amount and timing of rainfall, soil type and drainage, wind exposure, and cultural practices.

Hardiness is also affected by the return of warm temperatures. A few days of warm weather in mid or late winter can reduce plant cold hardiness significantly. Once cold hardiness is lost from mid or late winter warming, the plant cannot return to the same level of hardiness. If mild winter temperatures prevail, damage is unlikely. However, should severe temperatures occur, the tree will likely be damaged.

Different plant tissues have different degrees of hardiness. For example, flower buds are more sensitive to cold than leaf buds. A frost may damage the flower buds of a bulb or fruit tree without harming subsequent foliar growth.

Sunscald and frost cracking are similarly caused problems of trees with thin, dark bark, such as peach or silver maple. They occur when the bark and underlying cambium, usually on the south or southwest side of the tree, heat up on cold, bright days. When the sun sets or is blocked by a cloud, the bark and cambium quickly return to air temperature, which can cause physical and physiological damage.

Frost cracks, which are longitudinal splits in the bark, are an example of the physical damage which can occur. The bark and the wood underneath contract at different rates as they cool, causing mechanical stress. Eventually the bark splits, sometimes violently enough to produce a rifle-like noise. The cracks may heal over the following season, but are likely to split again the following winter. In the meantime, wood-decaying organisms and insects have an entry site. Sunscald is an example of the physiological damage caused by extreme temperature fluctuations. The elevated temperature of the trunk causes the cambium to lose its hardiness and become active. The drop in temperature kills the non hardy cambial tissues. Sometimes physical damage also occurs, and the scalded bark may split, forming an entry point for decay-causing organisms. Many cankers on trees result from sunscald.

Several measures can be taken which will prevent the sun from over-warming the trunk and limbs of trees susceptible to frost cracking and sunscald. Many commercial fruit growers will use white exterior latex paint to reflect the sunlight and keep the bark temperature from rising. The paint does not protect directly against extremely low temperatures; it will, however, reduce the wide fluctuations in temperatures. Do not use oil based paint, as it may kill trees. Apply the paint in late fall or early winter to the entire trunk, from the ground to the main crotch. Paint when the temperature is above 50°F and when dry weather is expected for several days.

Physical barriers can also be used to block the sunlight. These would include plastic or paper tree wraps. These wraps must be removed annually, to prevent girdling of the trunk. Also, insects tend to hide under the wrap, so leaving the trunk exposed during the summer is recommended.

Roots of trees and shrubs are more sensitive to cold injury than the stem tissues. In the landscape or orchard, the roots are not commonly injured because the soil and snow cover protect them from exposure to freezing air temperatures. Containerized plants in nurseries are very susceptible to freezing of the roots, since they are more exposed (see HO-157, Overwintering of Nursery Plants, for more information). Cold hardiness of roots varies with species and rootstocks. Cold injury to roots appears to be greater in sandy soils than in clay, since cold temperatures penetrate deeper into soils with lots of air spaces. For the same reason, injury is more likely in dry soils than in moist.

Another type of winter root injury is caused by "frost heaving." The repeated freezing and thawing of the soil forces plants, especially smaller ones (strawberries, shrubs, young trees), to move upwards in the soil, sometimes pushing them out of the soil altogether. This can break many of the fine feeder roots. Injury or death usually follows if roots are broken or the shoots and exposed roots become dried out. Frost heaving is most common in heavy soils; it is also affected by the soil water content.

Both frost heaving and freezing injury to the roots can be controlled in similar ways. Proper care during the growing season (irrigation, fertilization, and pest control) will promote healthier, hardier plants with deeper, more extensive root systems. Planting trees and shrubs at the proper depth and in well-drained soil will also prevent problems. Snow cover or an organic mulch, such as wood chips or sawdust, will help insulate the soil, preventing rapid fluctuations in temperature.

Winter dessication is a serious problem with narrow and broad leaf evergreens, such as pine and rhododendron. Containerized nursery stock, whose small, above ground root balls freeze easily, and newly planted bare root or balled-and-burlapped plants with their reduced root systems, are also very vulnerable.

Winter dessication injury occurs when the absorption of water by the roots cannot keep up with the amount of moisture lost by the foliage (transpiration). This occurs mostly on sunny days, especially when it is windy and when the soil water is frozen and the plant cannot absorb it, or if water is in short supply. Injury appears as brown leaf margins or needle tips at the onset of the first period of warm weather. In severe cases, all of the leaves and buds are killed More commonly, though, the leaves alone are injured or killed, but the buds survive.

Winter dessication can be prevented by making sure the plant is well supplied with water in the fall and early winter. Irrigation of 1/2 to 1 inch of water per week should continue up until the ground freezes. Screens and windbreaks can be used to shelter susceptible evergreens. Antitranspirants, or antidessicants, can also provide some protection by reducing the amount of moisture lost by the plant. Be sure to follow all label directions carefully.

Fruit Crops

Cold injury is a common cause of economic loss in fruit crops. Fruit plants can be affected either by winter injury, which occurs when the trees are dormant; and spring frost injury, which occurs when the trees are no longer dormant, but in various stages of flower, fruit, and/or leaf development. Both types of injury occur when temperatures drop below certain threshold levels. The injury threshold temperature is lower for dormant than nondormant tissues, and varies for different species, varieties, and stages of development.

Many of the types of winter injury discussed in the previous sections can also occur on fruit trees, such as sunscald, frost cracking, and root injury. Trees which go into dormancy in a weakened condition, due to overcropping, drought, poor nutrition, pests, etc, are more susceptible to winter injury.

Proper management includes pruning trees for optimal growth. Pruning should be done in late winter or early spring. Pruning from October to January stimulates trees during a period when low temperatures can injure the tissues around the pruning wound.

A large crop of fruit will reduce the tree's ability to accumulate carbohydrate reserves, resulting in problems in hardening off, as mentioned earlier. Therefore, growers should thin their fruit load early in the spring.

Select rootstocks and interstems carefully if winter injury is considered to be a problem in your area. MARK, M9, M26, and apples with interstems are more susceptible to winter injury.

Site selection for an orchard is important. Fruit trees should not be planted in poorly drained soils, frost pockets, or other undesirable areas.

Trees need sufficient moisture and nutrient reserves to survive the winter. Irrigate in the summer and fall if drought conditions prevail. Split nitrogen applications are recommended for peach trees: one in the spring and one in the fall after leaf fall to increase late season reserves.

Commercial fruit growers, especially of stone fruits, should protect their trees from sunscald and frost cracks as previously discussed. Again, remember to use white exterior latex paint, not oil based paint, when painting trunks.

Spring frosts and freezes are an annual threat to the buds of many fruit crops. As the weather warms up, the buds begin to come out of dormancy, losing their hardiness as they do. The further developed the buds are, the more susceptible they are to injury if the temperature should drop. Also, the critical temperature at which injury can be expected depends on the stage of bud development, as well as the length of time the temperature stays at or below the critical temperature.

Not all the buds in an orchard, or even on the same plant, develop at the same rate. The stage of development of the buds depends on species, cultivar, location on the shoot, orchard site, and management practices. Therefore, it's rare that all of the buds in a field are at the same level of hardiness. If a freeze hits, the most advanced buds may be injured, while the less developed ones may survive. However, if critical temperatures occur after the 100 percent bloom stage, then all fruit and flowers are essentially equally susceptible to damage.

If injured sufficiently during the prebloom or bloom stages, the buds will dry up and eventually drop. The period between injury and drop varies with stage of development, temperature and rainfall, but usually occurs within 2 weeks. Growers need to quickly know the extent of damage, as they must make important thinning, fertilization, pruning, and pest control decisions.

Flower buds can be examined for freezing injury by cutting into them with a sharp knife or razor blade. Be sure the buds cut are flower buds, and be sure to cut through the reproductive organs. Brown discoloration indicates injury. A healthy bud will be creamy white to pale green. Flowers already in bloom can be assessed for damage by cutting crosswise through the ovary (the tiny developing fruit at the base of the petals). Again, brown discoloration indicates injury. These symptoms are usually visible after several days, although warm temperatures hasten the process. If the style is damaged before pollination, fertilization will not occur and a fruit will not form.

Table 4, which was taken from the University of Kentucky (Extension Publication ID-37, Commercial Fruit Spray Guide) illustrates the developmental stages of flower buds for several tree fruits. Two temperatures are given for each stage of development. The temperature that causes 10 percent kill is listed on the left; the temperature that causes 90 percent kill is on the right.

Table 4. Floral development stages for fruit crops critical temperatures for flower bud kill.

(from University of Kentucky Extension publications ID-37, Commercial Fruit Spray Schedule 1988-89.)

Table 5. Critical temperatures and cold injury evaluations for small fruit crops.*

-----------------------------------------------------------------------
* Strawberries
Critical Temperatures:
   Plants, mid-winter: 0°F to 10°F
   Plants, late fall with cool preconditioning: 22°F
   Open flowers: 28°F

Evaluation of Cold Injury:

   Much of strawberry crop (at least 30 percent) is produced on the
first (king) blooms; loss of these blooms can be serious. Killed
blooms develop a dark center.

Freeze Injury Prevention:

 * Site selection.

 * Plant late-blooming cultivars.

 * Overhead irrigation for freeze protection is highly recommended.

 * Floating row covers.

 * Cover dormant plants with straw (1 to 3 inches).

*Grapes
Critical Temperatures:
   New growth: 30°F
   Woody vine:
   French hybrids: -5°F to -10°F
   American: -8°F to -18°F

 Evaluation of Cold Injury:
   After being frozen back, grapes will often regrow, bloom,
and produce about half a crop, depending upon cultivar.
Frozen canes or vines may die in late winter or early spring.

 Freeze Injury Prevention:

 * Site selection.

 * Plant types adapted to your region.

 * Plant cold hardy cultivars.

 * Use overhead irrigation for spring freeze protection (ice
   buildup can break trellis).

 * Have bare, packed soil on vineyard floor.

 * Keep vines healthy with proper fertilization and pest control.

*Blackberries
Critical Temperatures:
   Dormant thorny plant: -10°F
  Dormant thornless plant: 0°F
  Open flowers: 28°F

Evaluation of Cold Injury:
   The center of the flower will become dark after the ovaries
and pistils are killed.  Injured canes have a dry pith, rather
than a moist green pith; cambium is brown.

Freeze Injury Prevention:

 * Site selection.

 * Use overhead irrigation for spring freeze protection.

 * Grow cold hardy thorny types; avoid sensitive thornless
   plants.

 * Have bare, packed soil on orchard floor.

* Blueberries
Critical Temperatures:
   Dormant flower buds: -17°F to -20°F
   Open flower buds: 28°F

Evaluation of Cold Injury:
   Open flowers killed by freezes will remain on bush for
weeks or months in a brown, dried state; may be mistaken for
diseases like flower blight or mummy berry. Damaged fruit
may abort or be reduced in size. Dead immature seeds will be
brown.

Freeze Injury Prevention:

 * Site selection.

 * Grow cold tolerant highbush berries, not sensitive rabbit eyes. Grow
cultivars recommended for your area.

 * Use overhead irrigation for spring freeze protection.

   Have bare, packed soil on orchard floor.
-----------------------------------------------------------------------
* Based upon University of Georgia Extension publication Cold Weather
Injury and Horticultural Crops in Georgia; Effects and Protective
Measures.

Heavy bud set usually means that there is the potential for an excessive crop load, and that growers may need to do some thinning. At early stages of flower development, a wide range exists in temperatures that will cause slight or severe injury; this range narrows later in development. A grower who needs to thin may decide not to use frost protection techniques if a light freeze is predicted early in development, although this gets riskier as time goes on. Frost protection techniques will be discussed later.

Young fruit that undergo a freeze can also be injured. During a severe freeze, the fruit can be damaged internally. The fruit tissue can be injured, appearing mushy and watersoaked. The developing seeds can also be damaged. Damaged seed will often appear brown; however, stone fruit seed may have only the embryo killed, with the cotyledons continuing to appear normal. Apples and pears normally have up to 10 seeds per fruit; the fruit should still develop normally if there are at least 5 healthy seeds (one in each carpel). Those with only 2 or 3 healthy seeds may not survive, or may be misshapen and smaller than fruit with a full compliment of seed. The presence of such fruit complicates thinning decisions as well as assessments of crop loads and production levels, because at the time the thinning decision must be made, no externally visible differences may be seen.

Sometimes a frost may not be severe enough to cause internal damage, but it will cause cosmetic external damage. Apples can form patches or rings of light brown russetty tissue on the blossom end of the fruit (sometimes called "frost rings"). Peaches and other stone fruit may ooze gum from the fruit during the growing season; they may also become distorted, or develop longitudinal cuts or grooves, called "cat scratching."

Small fruit crops can also receive significant amounts of frost injury in the spring. The critical temperatures, ways to evaluate cold injury, and recommended protective measures for use on several small fruit crops commonly grown in Indiana were outlined by Gerard Krewer of the University of Georgia, and are listed in Table 5.

Proper selection of species and cultivars will decrease the potential for crop loss. Certain fruit crops, like peaches, nectarines, and apricots, are not hardy enough to grow in cold climates reliably, and therefore should not be planted in northern and central Indiana. Early blooming cultivars are also more at risk than later blooming ones. For growers in an area where late spring frosts are common, selection of late blossoming cultivars may be the best chance for success. Publications from the Purdue, Illinois, Ohio, and Kentucky Extension Services are good sources for cultivar recommendations.

Specific Cold Protection Measures

Sprinklers

While there are several techniques for using irrigation to prevent frost injury, the most common method is overhead sprinkling. This method has been tested in nearly every major fruit growing area worldwide. It is a practical protective technique, but it's also an exacting one.

Frost protection with sprinklers succeeds because of an important physical property of water: when water cools, it gives up a fixed amount of heat for each degree of temperature loss. One B.t.u. of heat is removed from each pound of water as it cools 1°F (one kilocalorie of heat is removed from each kilogram of water for each 1°C reduction). Heat is given up until the temperature of the water reaches 32° F. It then gives off 144 B.t.u. of heat per pound (79 kilocalories per kilogram) as it turns to ice. This heat energy, called the "latent heat of fusion," is available to prevent the plant tissue from dropping below 31.5° F. As long as a film of water is maintained by continuous application, the temperature of the plant material will remain at or above 31.5° F, even though a layer of ice is steadily being formed. If the water source fails, the ice and plant can become colder than the surrounding air because of evaporative cooling, creating more injury than if the sprinkling had never been attempted.

The installation of an overhead sprinkling system for frost protection should be carefully engineered for even distribution. A system designed for irrigation may not be adequate for frost injury prevention. Operating the entire system at one time for frost control requires larger mainline pipes, pump, and motor capacity than when partial areas are being irrigated. Growers who are considering overhead sprinkling should contact reliable dealers or consultants to assist in the engineering layout. Some specifications to consider are shown in Tables 6, 7, and 8.

The water film must be maintained continuously as long as temperatures are low enough to freeze, or until the ice starts to melt rapidly. Inadequate application rate or poor distribution can cause a buildup of ice. Under long periods of freezing, this can result in excessive weight, which can cause limb breakage. To assure proper application, the sprinkler heads should rotate at least once per minute; they should also be designed to prevent ice buildup around the activator spring.

Sprinkler setup and spacing is dictated by the site: tree spacing and direction, traffic patterns in the orchard, and wind velocity and direction. Use Table 6 to calculate the application rate needed to provide protection at the expected wind speed and minimum temperature. Then use Table 7 to decide which sprinkler spacing and nozzle to use to achieve the desired rate of application. The sprinklers should be setup to provide about 30 percent overlap to cover for a gentle breeze.

Table 6. Application rate recommended for cold protection under different wind and temperature conditions.*

 Minimum
temperature      wind speed (miles/hour)
 expected    0 to 1      2 to 4      5 to 8
---------------------------------------------
  °F       Application rate (inches/hour)
   27        0.10        0.10        0.10
   26        0.10        0.10        0.14
   24        0.10        0.16        0.30
   22        0.12        0.24        0.50
   20        0.16        0.30        0.60
   18        0.20        0.40        0.70
   15        0.26        0.50        0.90
---------------------------------------------
 From University of Florida Agricultural Extension Circular 287.

Table 7. Precipitation rate (inches/hour) for selected nozzle capacity and sprinkler spacings.*

 Sprinkler           Gallons per minute/sprinkler
spacing (ft.) 2   3   4   5  6   8   10  12  15
---------------------------------------------------
30 x 30      .21 .32
30 x 40      .16 .24 .32
40 x 40          .18 .24 .30
40 x 50          .14 .19 .24 .29
40 x 60          .12 .16 .20 .24 .32
50 x 50          .12 .15 .19 .23 .31
50 x 60              .13 .16 .19 .26 .32
60 x 60                  .13 .16 .21 .27 .32
60 x 70                      .14 .18 .23 .28 .34
---------------------------------------------------
*From University of Georgia Extension Publication Cold Weather
and Horticultural Crops in Georgia: Effects and Protective Measures.

Table 8. Approximate pump capacities required for different application rates.*

                          Acres Irrigated
		--------------------------------
Application     1     2    4     8      10    20
  rate		--------------------------------
                       Pump capacity required
---------------------------------------------------
ln./hr.                   Gallons/minute
  0.10         45    90   180   360    450   900
  0.15         68   135   270   540    675  1350
  0.20         90   180   360   720    900  1800
  0.25        113   225   450   900   1125  2250
  0.30        135   270   540  1080   1350  2700
  0.35        158   315   630  1260   1580  3160
---------------------------------------------------
*From University of Georgia Extension publication Cold Weather and
Horticultural Crops in Georgia: Effects and Protective Measures.

Since it is not economically feasible to engineer a flexible-rate sprinkling system, it is recommended that the system be set up to apply between 0.15 and 0.20 inches per hour. This will protect fruit buds and blossoms down to 20°F at a low dewpoint (very little moisture in the air) and under nearly windless conditions. If a grower has experienced more severe weather conditions during bloom before, he may want to design his system accordingly.

An application rate of 0.15 inches per hour requires 68 gallons of water per acre per minute, or 4,038 gallons per acre per hour. Sprinklers need to run until the danger of frost has passed, about 10 hours on the average, so a total of 40,380 gallons of water will be needed for each acre being protected. Since some arctic air masses have been known to require three successive nights of protection, more than 120,000 gallons of water per acre will be needed for each cold snap. Many growers will need to have special wells or holding ponds available to meet this demand for water.

A point to consider is that at 0.15 inches per hour, a 10-hour run will apply 1 1/2 inches of water to a field. Several cold nights in a row could saturate heavy or poorly drained soils, leading to root rot and plant death.

Sprinkling should begin when the temperature reaches 32°F or 33°F when the dewpoint is high; some growers begin sprinkling at higher temperatures when the dewpoint is extremely low, since dry air cools faster than moist air. This provides enough time to get complete bud wetting before critical temperatures are reached. It will also help prevent ice crystals from forming and clogging pipes and sprinkler nozzles. It is safe to stop sprinkling when the air temperature reaches 33°F if it is not windy, or when free water runs between the ice and twigs. It is not necessary to keep sprinkling until all of the ice has melted.

Row Covers

Row covers are used to modify the environment around the plant in order to protect it from cold temperatures and to increase earliness and yield. Row covers work by trapping radiant heat during the day and retarding its loss at night. They also block the wind, which can accelerate cold damage.

Several types of row covers are available; they are listed in Table 9, along with some comments on their effectiveness. Many universities are doing research on different row cover materials and methods of use, including Purdue University and the University of Illinois.

Table 9. Row covers and frost protection.*

                        Frost
Type of covering      protection   Comments
-----------------------------------------------------
Clear polyethylene      Fair       Inexpensive
 (hooped)                          Labor intensive
Clear polyethylene      Fair       Excessive heat
 (floating)                          buildup
Slitted polyethylene    Fair       Allows heat escape
                                   Hard to install
Perforated polyethylene Fair       Excessive heat
                                    buildup
Spunbonded polyester    Good       Possibly abrasive
 (floating)                        High cost
Spundbonded poly-
 propylene (floating)   Good       High cost
Extruded polypropylene  Poor       Inexpensive
 (floating)                        Tears easily
-----------------------------------------------------
* From University of Georgia Extension Publication Cold Weather
and Horticultural Crops in Georgia: Effects and Protective Measures.

The two major ways to apply row covers are the traditional supported system, where metal hoops hold the material up, and the new floating row cover system, where the material simply lies, or floats, on top of the crop. As the crop grows under a floating row cover, the material is lifted up; supported covers do not come into contact with the plants. With both systems, the edges of the cover must be anchored down, usually with soil. They can be left in place for several weeks. Generally, row covers reduce natural light levels by only 10 to 20 percent. Most of the materials are porous enough to allow irrigation and pesticides to pass through.

In newly established vegetable crops, row covers protect plants from temperatures 2°F to 5°F below freezing for short durations. They reduce damage, but don't offer complete protection, when the temperature drops below 26° F. In strawberries, nearly 50 percent more blooms survived 2 days at 26° F under row covers, according to an Illinois study.

Slitted row covers protect crops from 30° F to 31°F frosts only. The floating row covers protect against frosts of 28° F to 29° F. Research in Florida has shown that sprinkler irrigation used in combination with row covers can extend frost protection to around 21° F. However, row covers in general cannot be expected to reliably protect crops below about 28° F, and then only for short durations.

The cost per acre of using row covers varies, depending on the material used, how it is used (hooped or floating), row spacing, and labor. With high value crops, where achieving an early market is important, these materials quickly pay for themselves.

Heating

Outdoor heaters are still an important frost control method in some areas of the country; they are often used in combination with wind machines. High fuel costs have reduced the use of this technique, though.

The number of heaters needed per acre depends upon the orchard. Cold spots and borders on the up-wind side of the orchard will require more heaters. The number of heaters needed will depend on your past experiences in the orchard during frosts, the type of heater used, and its B.t.u. output. Remember, several small fires provide better protection than a few large fires.

Heaters should be lit when the temperature reaches the critical point for your orchard's stage of flower bud development. Heaters in frost pockets and on the upwind side of the orchard should be lit first, moving toward the naturally warmer areas next.

Heaters that produce excessive amounts of smoke should be avoided. A common misconception is that smoke retards heat loss from the orchard. Smoke particles, although visually dense, are not of the appropriate size to reflect thermal radiation. Furthermore, ordinances often prohibit the use of heaters and fuels which produce lots of smoke.

Because of both the high cost of fuel and the environmental concerns, heaters are not commonly used in Indiana. Wind machines that bring warm air down into an orchard from higher elevations during a temperature inversion and also stir the air to prevent cold air from settling into frost pockets are more practical. Wind machines that are often used are stationary, power-driven fans and helicopters.

Acknowledgments

The author would like to acknowledge and thank Drs. Jim Simon and Richard Hayden from Purdue University; Drs. John Gerber and Jeffrey Dawson from the University of Illinois; Dr. Gerald Brown from the University of Kentucky; Dr. M.E. (Butch) Feree of the University of Georgia; all the Extension staff from these Universities for their help and expertise; and the Warrick County, Indiana, Extension staff for help with composition.

Bibliography


New 12/88

Cooperative Extension Work in Agriculture and Home Economics, State of Indiana, Purdue University and U.S. Department of Agriculture Cooperating. H.A. Wadsworth, Director, West Lafayette, IN. Issued in furtherance of the Acts of May 8 and June 30, 1914. It is the policy of the Cooperative Extension Service of Purdue University that all persons shall have equal opportunity and access to our programs and facilities.