Cold Protection of Ornamental Plants 1

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1 ENH1 Cold Protection of Ornamental Plants 1 Dewayne L. Ingram and Thomas H. Yeager 2 Winter temperatures in Florida are frequently low enough to cause cold injury to tropical, subtropical, and occasionally temperate plants not adapted to Florida climatic conditions. Tropical plants and summer annuals do not adapt or harden to withstand temperatures below freezing and many are injured by temperatures below 50 F (10 C). Subtropical plants can harden or acclimate (become accustomed to a new climate) to withstand freezing temperatures and properly conditioned temperate plants can withstand temperatures substantially below freezing. Freezing conditions occur annually in north and central Florida, while below freezing temperatures are rare for south Florida. Freeze probabilities for various locations in Florida are published in IFAS Bulletin 777, Freeze Probabilities in Florida. Freezes can be characterized as radiational or advective. Radiational freezes or frosts occur on calm, clear nights when heat radiates from the surfaces of objects into the environment. These surfaces can become colder than the air above them due to this rapid loss of heat or long wave radiation. When the air is moist, a radiant freeze results in deposits of ice or frost on surfaces. Dry radiational freezes leave no ice deposits but can cause freeze damage. Plant damage from a radiational freeze can be minimized by reducing radiant heat loss from plant and soil surfaces. Advective freezes occur when cold air masses move from northern regions causing a sudden drop in temperature. Windy conditions are normal during advective freezes. Although radiant heat loss occurs during an advective freeze, the conditions are quite different from a radiational freeze. Plant protection during advective freezes is more difficult. The ability of plants to withstand freezing temperatures is affected by temperature fluctuations and day lengths prior to a freeze. A gradual decrease in temperature over a period of time increases the ability of plants or plant parts to withstand cold temperatures. A sudden decrease in temperature in late fall or early winter usually results in more damage than the same low temperature in January of February. Short durations of warmer temperatures in midwinter can deacclimate some plants resulting in bud break or flowering. Deacclimated plants are more prone to freeze injury. Preconditioning of tropical plants to withstand chilling temperatures has not been well documented. 1. This document is ENH1, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date June Reviewed October Visit the EDIS Web Site at 2. Dewayne L. Ingram, professor and former extension horticulturist; Thomas H. Yeager, associate professor and extension woody horticulturist, Environmental Horticulture Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean

2 Cold Protection of Ornamental Plants 2 Cold injury can occur to the entire plant or to plant parts such as fruits, flowers, buds, leaves, trunks, stems or roots. Many plant parts can adapt to tolerate cold, but fruits and roots have little ability to acclimate or develop cold tolerance. Cold injury to roots of plants in exposed containers is a common occurrence and usually is not evident until the plant is stressed by higher temperatures. Leaf and stem tissue will not survive ice formation inside the cells (result of rapid freeze) but many plants can adapt to tolerate ice formation between cells. One type of winter injury is plant desiccation or drying out. This is characterized by marginal or leaf tip burn in mild cases and totally brown leaves in severe cases. Desiccation occurs when dry winds and solar radiation result in the loss of more water from the leaves than can be absorbed and/or transported by a cold or frozen root system. Root systems in the landscape are seldom frozen in Florida, but potting media in small containers in north Florida can be frozen for several consecutive hours. WHAT TO DO BEFORE THE FREEZE Homeowners can take steps to help acclimate plants to cold temperatures and to protect plants from temperature extremes. These steps range from selection of a proper planting site to alteration of cultural practices. Planting Site Selection The microclimate of a location is determined by factors such as elevation, landform, surface reflectivity, soil properties, degree of canopy cover, proximity of structures or plants, and the general solar heat exchange model. Temperature fluctuation can differ from one location to another, even within a residential landscape. Thus, existing microclimates and/or possible modifications of microclimates should be considered when choosing the planting site for cold sensitive plants. Tender plants should be planted in a site with good air drainage, and not in a low area where cold air settles. Arranging plantings, fences, or other barriers to protect tender plants from cold winds improves cold protection, especially from advective freezes. Poorly drained soils result in weak, shallow roots which are susceptible to cold injury. Proper Plant Nutrition Plants grown with optimal levels and balance of nutrients will tolerate cold temperatures better and recover from injury faster than plants grown with suboptimal or imbalanced nutrition. Late fall fertilization of nutrient deficient plants or fertilization before unseasonably warm periods can result in a late flush of growth which is more susceptible to cold injury. Plants in Florida landscapes should be fertilized four times per year. Landscape plants in north and north central Florida should be fertilized in March, June, September, and December. Plants in south and south central Florida should be fertilized in February, May, August, and November. One to 1 1/2 pounds (454 to 681 grams) of or 8-8-8, or 1/2 pound (227 grams) of or should be applied per 100 square feet (9 square meters) of planting area for the first three applications per year. A decrease in the amount of fertilizer applied in the fall is necessary because plant nutrient consumption declines during the colder season. Plants grown in colder portions of the state require one-third to one-half the standard fertilization rate in the fall, and two-thirds the standard rate should be applied in the warmer sections of Florida. Shading Tree canopy covers can reduce cold injury caused by radiational freezes. Plants in shaded locations usually go dormant earlier in the fall and remain dormant later in the spring. Tree canopies elevate minimum night temperature under them by reducing radiant heat loss from the ground to the atmosphere. Shading from early morning sun may decrease bark splitting of some woody plants. Plants that thrive in light shade usually display less winter desiccation than plants in full sun. But plants requiring sunlight that are grown in shade will be unhealthy, sparsely foliated, and less tolerant of cold temperatures. Windbreaks Fences, buildings, and temporary coverings, as well as adjacent plantings, can protect plants from

3 Cold Protection of Ornamental Plants 3 cold winds. Windbreaks are especially helpful in reducing the effects of short-lived advective freezes and their accompanying winds. Injury due to radiational freezes is influenced little by windbreaks. The height, density, and location of a windbreak will affect the degree of wind speed reduction at a given site. Water Relations Watering landscape plants before a freeze can help protect plants. A well watered soil will absorb more solar radiation than dry soil and will reradiate heat during the night. This practice elevated minimum night temperatures in the canopy of citrus trees by as much as 2 F (1 C). However, prolonged saturated soil conditions damage the root systems of most plants. Other Cultural Practices Avoid late summer or early fall pruning which can alter the plant hormonal balance resulting in lateral vegetative budbreak and a flush of growth. This new growth is more susceptible to cold injury. Healthy plants are more resistant to cold than plants weakened by disease, insect damage, or nematode damage. Routine inspection for pests and implementation of necessary control measures are essential. Contact your County Extension Office for information on pest identification and recommended controls. Methods of Protection Plants in containers can be moved into protective structures where heat can be supplied and/or trapped. Containers that must be left outdoors should be protected by mulches and pushed together before a freeze to reduce heat loss from container sidewalls. Leaves of large canopy plants may be damaged if crowded together for extended periods. Heat radiating from soil surfaces warms the air above the soil or is carried away by air currents. Radiant heat from the soil protects low growing plants on calm cold nights, while tall, open plants receive little benefit. Radiant heat loss is reduced by mulches placed around plants to protect the roots. For perennials, the root system is all that needs to be protected since the plants die back to the ground annually. Coverings protect more from frost than from extreme cold. Covers that extend to the ground and are not in contact with plant foliage can lessen cold injury by reducing radiant heat loss from the plant and the ground. Foliage in contact with the cover is often injured because of heat transfer from the foliage to the colder cover. Some examples of coverings are: cloth sheets, quilts or black plastic. It is necessary to remove plastic covers during a sunny day or provide ventilation of trapped solar radiation. A light bulb under a cover is a simple method of providing heat to ornamental plants in the landscape. WHAT TO DO DURING A FREEZE Ornamental plants can be protected during a freeze by sprinkling the plants with water. Sprinkling for cold protection helps keep leaf surface temperatures near 32 F (0 C) because sprinkling utilizes latent heat released when water changes from a liquid to a solid state. Sprinkling must begin as freezing temperatures are reached and continue until thawing is completed. Water must be evenly distributed and supplied in ample quantity to maintain a film of liquid water on the foliage surfaces. Irrigation for several days may water soak the soil resulting in damaged root systems and/or plant breakage due to ice build up. Consult Extension Circular 348, Sprinkler Irrigation for Cold Protection, for more technical information on this subject. WHAT TO DO AFTER THE FREEZE Water Needs Plant water needs should be checked after a freeze. The foliage could be transpiring (losing water vapor) on a sunny day after a freeze while water in the soil or container medium is frozen. Apply water to thaw the soil and provide available water for the plant. Soils or media with high soluble salts should not be allowed to dry because salts would be concentrated into a small volume of water and can burn plant roots.

4 Cold Protection of Ornamental Plants 4 Pruning Severe pruning should be delayed until new growth appears to ensure that live wood is not removed. Dead, unsightly leaves may be removed as soon as they turn brown after a freeze if a high level of maintenance is desired. Cold injury may appear as a lack of spring bud break on a portion or all of the plant, or as an overall weak appearance. Branch tips may be damaged while older wood is free of injury. Cold injured wood can be identified by examining the cambium layer (food conducting tissue) under the bark for black or brown coloration. Prune these branches behind the point of discoloration. Florida homeowners enjoy a vast array of plant materials and often desire a tropical or semitropical appearance to their landscapes. Plants are often planted past their northern limit in Florida, although microclimates differ dramatically. Tropical and subtropical plants can be used effectively in the landscape, but they must be protected or replaced when necessary. A combination of tender and hardy plants should be planted in order to prevent total devastation of the landscape by extremely cold weather.

5 ENH80 Low Temperature Damage to Turf 1 L.E. Trenholm 2 Injury to warm-season turfgrasses often occurs when temperatures drop below 20 F (-6.7 C). In general, major winter injury to turfgrass is caused by the following: 1) tissue desiccation, 2) direct low temperature kill, 3) diseases, and 4) traffic effects. For example, damage from the freeze can probably be attributed to poor cultural practices which weakened turf and made it more susceptible to injury or death from low temperatures. Subsequent damage may also have resulted from effects of traffic on frozen turf. Reasons for Temperature Damage Most warm-season grasses have very poor cold tolerance ratings when compared to cool-season grasses. Due to lower fall temperatures and reduced daylengths, warm-season grasses enter a state of dormancy, evidenced by brown, dead shoot tissue, which is maintained throughout the winter in north Florida. In central Florida, a growth and metabolism reduction, rather than an actual dormancy, may be seen. This death of shoot tissue or lack of growth does not generally indicate that the grass is not going to recover; instead, this is a natural state and provides protection for the grass when faced with cold temperatures. In cases of severe freezing temperatures, some grasses may suffer irreversible damage, and use of these grasses should be limited to warmer climates. For instance, St. Augustinegrass, which generally exhibits poor cold tolerance, is not used as extensively in north Florida as other grasses, and is used less as you progress into northern Georgia. Cultural factors that tend to promote cold injury include: poor drainage (soil compaction), excessive thatch, reduced lighting, excessive fall nitrogen fertilization, and a close mowing height. The weather pattern preceding a severe and sudden cold wave also influences a turf's low temperature tolerance. In general, if turf has had several frosts prior to a drastic temperature drop, it has been better `conditioned' to survive. The cold snap in much of north and central Florida was preceded by three to five frosts. These helped increase carbohydrates and proteins in plants that enabled crown tissue to withstand cold temperatures without severe membrane disruption. The freezes that occurred in the early 1980s did not have these preconditioning periods, resulting in severe damage. In this case, grasses were still green, and protective crown tissue was succulent and therefore susceptible to cold temperatures. Shaded areas may suffer more intense cold damage. Shade (low light intensity) prevents normal daytime soil warming; therefore, these areas stay colder for longer periods of time, and more low 1. This document is Fact Sheet ENH-80, a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First published: May Revised: May Please visit the EDIS web site at 2. L.E. Trenholm, Assistant Professor, Extension Turfgrass Specialist, Environmental Horticulture Department, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Christine Taylor Waddill, Dean.

6 Low Temperature Damage to Turf 2 temperature damage may occur. Shade also reduces the plant's ability to produce carbohydrates needed for increased cold tolerance. Traffic (foot or vehicular) may further increase injury to cold damaged turf. Traffic should not be allowed on frozen turf until the soil and plants have completely thawed. Syringing the area lightly prior to allowing traffic on it will help reduce frozen turf injury associated with traffic. Assessing the Extent of Injury Symptoms of direct low temperature damage includes leaves that initially appear wilted. They may subsequently take on a water-soaked look, turning whitish brown and then progressing to a dark brown. Damaged leaves are not turgid and tend to mat over the soil, often emitting a distinct putrid odor. Areas hardest hit are usually poorly drained ones such as soil depressions. If you suspect your grass has experienced cold damage, take several 4 to 5 inch diameter plugs from suspected areas and place them in a warm area for regrowth. A greenhouse or warm windowsill should suffice. Observe these for 30 days or until growth resumes. If good regrowth occurs, then little damage is assumed. If regrowth is absent or sporadic, then some degree of damage was sustained. Selection of Cold-Hardy Grasses Within the warm-season grasses, the most cold-hardy species is zoysiagrass, followed in descending order by bermudagrass, bahiagrass, centipedegrass, seashore paspalum, carpetgrass, and St. Augustinegrass. Within these species, there are different degrees of cold tolerance between cultivars. For instance, centipedegrass cultivars 'Oklawn,' 'Tifblair,' and especially 'TennTurf' have good cold tolerance. In St. Augustinegrasses, 'Raleigh,' 'Bitterblue,' 'Seville,' and 'Jade' generally exhibit the best cold tolerance, while 'Floratam,' 'Floralawn,' and 'Floratine' are more susceptible to cold temperatures. Management Practices to Minimize Cold Damage Regardless of turfgrass species selected, the following management practices can help minimize cold temperature damage. Recently planted (sprigged, sodded, or seeded) grasses can expect to be more severely damaged by cold. Because roots are less developed and shoot tissue more tender, overall stress tolerance is reduced in just-planted grasses. To minimize cold damage, particularly in north Florida, delay fall planting of grasses until spring or early summer. In south Florida, grasses may be planted year-round, but care should be taken to protect immature turf from occasional cold temperatures. Fertility can also influence cold tolerance. Late season (late September in north Florida, after mid October in the central and southern regions) application of nitrogen will promote shoot growth in the fall, when the grass growth and metabolism are slowing down. Fall shoot growth will deplete carbohydrate reserves, which help the grass regrow from any stress, and tender shoots are less able to tolerate adverse conditions such as cold. Therefore, late-season application of nitrogen is not recommended. Potassium fertility in the fall has been shown to enhance cold tolerance and promote earlier spring greenup of grass. Application of potassium at the rate of 1/2 to 1 lb. per 1000 square feet is recommended for the last fertilization of the year. Effects of shade can increase cold damage. Because shaded areas do not become as warm as areas in full sun, injury in these areas may be more severe. Compacted soils also remain cooler than well-drained areas, which increases the probability of cold temperature damage. See Environmental Horticulture and Soil and Water Science Department factsheets for more information on relieving soil compaction. Increasing mowing height can reduce cold injury in a number of ways. First, it will promote deeper rooting, which is one factor always associated with greater stress tolerance. It will also allow for production and storage of more carbohydrates late in

7 Low Temperature Damage to Turf 3 the summer. In addition, higher mowing heights can create a warmer micro-environment due to extra canopy cover provided by longer leaf tissue. Because cold damage may initially resemble drought stress, people sometimes feel that additional water may be needed. Overall, correct irrigation practices can alleviate many stresses faced by turf, but as the grass goes into dormancy, water needs are reduced. Spring Greenup Unless your turfgrass has been subjected to unusually cold or freezing temperatures for long periods, or your management practices have augmented the effects of the temperatures, your grass should begin to green up as temperatures and day lengths increase in the spring. At this time, recommended fertility, irrigation, and mowing practices should be resumed for the best health of your lawn all season.

8 ENH-92 Treating Cold-Damaged Palms 1 Alan W. Meerow 2 Cold weather slows down the growth of palms, reduces the activity of the roots, and often weakens the plant to the point where a disease can become active and kill the palm. Severe cold damage from frost or freezing temperatures destroy plant tissues and may severely reduce water conduction in the trunk for years. Often the only above-ground portion of a cold-damaged palm that is still alive is the protected bud. As warmer weather returns, primary or secondary plant pathogens often attack weakened plants through damaged tissue. Possible Preventative Action The secondary plant pathogens that cause death of the bud soon after freeze damage are, in most cases, bacteria that are present on healthy palm tissue at low levels, but become a problem only after the damage is received. Consequently, there may be value in applying a preventative spray of fungicidal copper before freezing temperatures are reached in order to reduce these bacteria populations to the lowest levels possible. This strategy has not been tested, however, under controlled conditions. It is evident that palm tissue deficient in one or more essential plant nutrients is less tolerant of exposure to freezing temperatures. Thus, it is important that palms receive a balanced fertilization in late summer or early fall to insure that foliar nutrient levels are near optimum as winter approaches. Protecting the Damaged Palm While Waiting for Warm Weather Figure 1. Severe cold damage. To avoid attacks by primary or secondary plant pathogens, it is important that steps be taken to insure protection of the healthy bud until active growth resumes. 1. This document is ENH-92, one of a series of the Environmental Horticulture Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date December Reviewed October Visit the EDIS Web Site at 2. Alan W. Meerow, associate professor, Palm and Tropical Ornamentals Specialist, Environmental Horticulture Department, Ft. Lauderdale Agricultural Research and Education Center (AREC), Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean

9 Treating Cold-Damaged Palms 2 Remove the cold-damaged portion of the leaves. Leaves should not be completely removed if they are green (even if they are spotted from the cold). The green intact portions of the palm are important to assure adequate photosynthesis during the recovery stage. Disease Control 1. Immediately after pruning, spray the palms with a fungicide containing copper at the recommended rate. The use of fungicide is recommended only for palms not bearing edible fruit. Include a spreader sticker. 2. Repeat the copper spray 10 days after the first treatment or use another broad spectrum fungicide. Contact your county agent for current fungicide recommendations. In all cases, these sprays must cover the damaged tissue and healthy bud thoroughly. Copper sprays should not be repeated more than twice because of the possibility of copper phytotoxicity. 3. Palms growing in containers may benefit from a soil drench of fungicides that suppress root diseases. Contact your county agent for current recommendations of available fungicide formulations. 4. Occasionally, cold damage is so severe or disease has already progressed to the point where the spear leaf becomes loose and pulls out easily. With these palms, there is still a chance of recovery if the meristem (growth point) is alive. To treat these palms, remove as much dead and decaying material from around the bud as possible so it can dry out. Drench with a copper fungicide in the bud using the force of the sprayer to clean out the bud as much as possible. Follow up ten days later. Nutrients Warmer weather promotes rapid growth and this helps the palms recover. After the two initial sprays, a monthly application of soluble nutrients should be applied to the leaves. The following formulation has been tested by the Institute of Food and Agricultural Sciences. Other products containing similar nutrients should work equally as well: 1/4 to 1/2 teaspoon per gallon S.T.E.M. (Peter's Soluble Trace Element Mix) Spreader sticker Cryptic Cold Damage Palms that were severely damaged during the winter should be watched carefully during the subsequent spring and summer seasons. Damage to embryonic leaves within the bud may not show up until those leaves emerge (as much as six months to one year after the freeze). If leaves emerging during the spring and summer months appear deformed, partially, browned or otherwise abnormal, this may be indicative of this type of damage. In most cases, the palm will grow out of this later in the season. Figure 2. deformed, partially, browned or otherwise abnormal. Freeze damage to conducting tissue in the trunk may limit the ability of the palm to supply water to the canopy of leaves. Unlike typical broad-leafed trees, palms have no ability to regenerate conducting tissue in the trunk. Sudden collapse of some (or even all) of the leaves in the crown during the first periods of high temperature in the spring or summer after a winter freeze may indicate that this type of trunk damage has occurred. Unfortunately, there is nothing that can be done to remedy this, and loss of the palm will be inevitable. Conclusion The above steps will help reduce loss from cold damage and speed up recovery. Nutrient sprays should continue into the summer if the plants are young or newly established in the landscape. Older palms will benefit from a soil application of a

10 Treating Cold-Damaged Palms 3 granular palm fertilizer in the spring that is repeated every three to four months.