What Is Light Frost: Information On The Effects Of Light Frost
By: Susan Patterson, Master Gardener
Nothing takes the smile off of a gardener’s face quicker than an early fall or a late spring frost. Even worse is the fact that it doesn’t take much of a frost to damage your prized plantings. Keep reading to find out what is light frost and plant frost info for plants affected by light frost.
Plant Frost Info
Understanding the frost dates in your gardening region is critical to maximizing your garden potential. However, there are always frosts that sneak up and catch you off guard, no matter how prepared you think you are.
Paying attention to weather forecasts in the fall and spring is essential to the health of your garden. Even a light frost can cause extreme damage to young spring plants or bring the colorful display of late summer tender plants to a screeching halt.
What is a Light Frost?
A light frost occurs when the air has dropped below freezing but the ground has not. A hard frost occurs when the air is cold and the ground is hard. Many plants can survive the occasional light frost, but more care must be taken when the weather forecast calls for a hard frost.
The effects of light frost vary from plant to plant but can include a browning or scorching effect on foliage, all the way to a complete stem collapse. Therefore, it’s usually a good idea to provide all your plants with some light frost protection.
Plants Affected by Light Frost
Tender plants can be killed by a light frost; these include tropical and subtropical varieties. When the water inside of the plant gets cold, it crystallizes. When it warms up, it cuts the inside of the plant, allowing moisture to escape and thus, killing the plant.
If the area between leaf veins appears pale brown or scorched, it may indicate frost or cold damage. Tender and tropical perennials and bulbs may turn black when hit with the first fall frost.
Light frost protection is definitely a necessity if you have tender plants in your garden. Spring frosts can cause damage to tree blossoms and young fruits. Frost-sensitive vegetables such as potatoes and tomatoes may suffer leaf scorch, browning and even death from a late spring frost.
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What to Do When Your Jalapeno Pepper Plants Get Hit by Frost
Much like fire and ice do not mix well, cold weather and frosty conditions are no friend to jalapeno pepper plants (Capsicum annuum). Jalapeno plants are heat-loving, warm-season crops that are hardy in U.S. Department of Agriculture zones 8 through 11, although they can be grown as an annual in cooler areas. Plants grow 3 to 5 feet tall, depending on the variety. Grow jalapenos in a warm, sunny area, taking measures to protect plants from frost at every stage of life. Peppers are ready for harvest within 80 to 120 days.
How to Tell If a Tomato Plant Has Frostbite
As tender annuals, tomato plants cannot tolerate frost. A frostbitten plant will quickly wilt and die. Tomatoes are usually planted out to the garden after all danger of frost has passed, but sometimes an unexpected frost may damage plants overnight. In fall, early frosts can ruin the last harvest if you don't act quickly. You may be completely unaware a frost occurred, as frost can be a localized occurrence that doesn't make it onto weather reports. Checking your plants for frostbite when you suspect a frost occurred gives you a chance to save any fruit before the plant dies.
Inspect the stem of the plant below the first set of leaves. Look for darkening and feel the stems to see if they are firm or soft. If the stems are soft and discolored the plant is frostbitten and must be disposed of.
- As tender annuals, tomato plants cannot tolerate frost.
- Checking your plants for frostbite when you suspect a frost occurred gives you a chance to save any fruit before the plant dies.
Inspect the plants leaves and the upper portion of the stem for wilting, discoloration, or softness if the lower stem is healthy. These indicate that portions of the plant are frostbitten but that the plant may survive. Remove any damaged leaves and snip off any stems that are soft to prevent rot from affecting the rest of the tomato.
Harvest all nearly ripe tomatoes or those that are green but full size from any plants suspected of frostbite. Use these tomatoes within the next one to two days, first cutting off any spots that turned soft during the freeze.
Protect plants at night if frost damage occurred but didn't kill the plants. Cover the plants with a blanket or place a cloche over the plants until the weather warms up again in spring or until you have harvested the last of the fruit in fall.
- Inspect the plants leaves and the upper portion of the stem for wilting, discoloration, or softness if the lower stem is healthy.
- Remove any damaged leaves and snip off any stems that are soft to prevent rot from affecting the rest of the tomato.
Wait until the soil has warmed up to 60 degrees F before transplanting outside. Plants are more likely to survive a light frost if the soil is warm.
If you cover plants to protect them from frost, remove the covers during the day or the plants may overheat.
Covering only protects plants from light frosts, not hard frosts which freeze the soil.
Effects of Cold Weather on Horticultural Plants in Indiana (HO-203-W) (HO-203-W)
Purdue Extension Publication
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.
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. *
* Based upon information from university of Illinois publication VC 14 a2, Vegetable Planting Guide.
1 Very 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.
2 Frost tolerant vegetables can withstand light frosts and can be planted 2 to 3 weeks before the average frost-free date.
3 Tender 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.
4 Warm 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. *
* 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.
3 Tender 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
4 Warm 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.
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.)
SERIES 30 | Episode 17
Tino Carnevale is preparing The Patch for frosty nights ahead.
Frost is caused by cold air sinking to the ground on cold, still, clear nights.
Plants are affected by frost when the water in their leaf cells freezes and expands. As the ice thaws, the cell walls collapse.
Succulent, tender leaves – such as basil (Ocimum basilicum cv.) – are more vulnerable than rigid leaves, such as cabbage (Brassica oleracea cv.), whose leaves also have a waxy coating for extra protection.
Tino shows how to make a frost protection tunnel to protect some chilli plants. Having overhead cover is enough to protect plants from light frost and the tunnel will extend plants’ growing season.
You will need:
- Supports – you can use metal stakes with poly piping forming an arch over the top, or approx. 3m-long bamboo poles, flexible enough to be bent over in an arch.
- Cross bracing – bean poles or extra bamboo
- Frost-protection fabric – an old sheet or shade cloth will do, or you can buy material specifically made for this purpose.
- Staple gun or other means of fixing the cloth
How to make it:
- Create the basic tunnel by placing arched supports every 50cm or so along the line of plants you want to protect.
- Cross-brace the tunnel by using twine to tie poles between the arches this will stabilise the structure.
- Cover the tunnel with fabric of your choice and fix in place with staple gun or safety pins.
- Cut and attach end panels to the tunnel, or allow some extra fabric that can be tied together to cover the ends.
If using an old sheet or shade cloth, it is best to only cover the plants when frost is forecast. The frost-protection fabric is designed to allow air and moisture through and has a 25-40% shade rating. The plants will still benefit from having the ends or sides lifted during the day to allow better airflow and ensure that the sun reaches the plants.
Get to Know the Vegetables That Aren’t Frost Tolerant
Which vegetables won’t survive a frost?
All of the vegetables that scream summer – tomatoes, basil, summer squash, peppers, and eggplant – will not survive low temperatures.
If it hits 32 degrees F in your garden, these plants will be brown and dead in the morning. (Note: basil can start getting frost damage at 38 degrees F.)
That’s why in the spring you need to wait until your average last frost date has passed before planting these seedlings in your garden. I also advise regularly checking the 10 day forecast around the time you want to plant hot weather vegetables.
If there’s any chance of frost you should definitely hold off on planting. As I wrote earlier in this post, I lost 35 peppers plants one Memorial Day Weekend because I was in a hurry to plant before going out of town, even though the forecast called for pretty low temperatures.
In the fall I also recommend starting to check the 10-day forecast on a regular basis as you start to get near your average first frost date.
If you see a frost warning coming up, make sure you either harvest anything you want to save before the frost or cover the plants with plastic or row cover to try to extend their life.
If you have a patchy frost, or it doesn’t quite get down to 32 F, there might be dead spots on the plant, but overall it will still be partially alive. Unfortunately, because the water in the fruit freezes as well, it’s usually not very tasty after it’s been frosted.
Personally, I’ve been found out in my garden the night before a frost gathering up any last vegetables that are harvestable.
It’s usually not worth it to me to protect the warm weather plants this late in the season because by this time they’re very diseased (tomatoes and basil) or not producing much anymore (peppers and eggplant).
Here’s a list of vegetables that won’t survive temperatures below 32 F:
Basil (frost damage occurs at 38 F)
I created a printable sheet for easy reference. You can download and print it out here: Which Vegetables Survive a Frost.