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 frost-free date.

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 makeup 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 that 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, herbicide, 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. After chilling hours have been met, 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, southwest injury, and frost cracking are similarly caused problems of trees with thin 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 that 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 riflelike 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 to 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. The white paint is not generally done in home plantings due to appearance.

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, and moisture can lead to disease, so leaving the trunk exposed during the summer is recommended.

Roots of trees and shrubs are more sensitive to cold injury than are 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. Cold hardiness of roots varies with species and rootstocks. Cold injury to roots is 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 a‚ected by the soil water content. If root damage occurs, the plant may leaf out in the spring, which is followed by defoliation due to the damaged roots not being able to supply enough water to the canopy.

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. If using organic mulch, it’s important to remove it once the spring arrives.

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

Winter desiccation 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 desiccation 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. Anti-transpirants, or anti-desiccants, 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 di‚erent species, varieties, and stages of development.

Many types of winter injury – such as sunscald, frost cracking, and root injury – discussed in the previous sections can also occur on fruit trees. Trees that 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. Even though trees remain dormant, such pruning can reduce the level of internal cold hardiness.

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 appropriately early in the spring.

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 early spring and one in early summer once trees have survived spring frosts. If they survive frosts and have a crop then go ahead and make the second application. If they lose the crop from spring frost, the second fertilizer application can be reduced or eliminated.

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, 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 pre-bloom 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 two 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, (see page 10) which was taken from Michigan State University Extension, describes 10% and 90% kill temperatures for fruit at various growth stages.

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 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 five healthy seeds (one in each carpel). Those with only two or three healthy seeds may not survive, or may be misshapen and smaller than fruit with a full complement 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 diferences 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.

Small fruit crops can also exhibit 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 unless in a favorable microclimate. 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 Purdue, Illinois, Ohio, and Kentucky Extension services are good sources for cultivar recommendations.