Climatic Adaptability of Plants

Adaptability to the length of growing season and the intervening winters that limit the growing seasons. The notion of growing seasons is addressed, followed by a discussion of hardiness zones and plant hardiness ratings. Climatic maps make it possible to select adapted plants for any locality.

Growing Seasons

The growing season is the period of time during the year when outdoor conditions favor plant growth. Annual plants and tender perennials are killed by temperatures just a few degrees below freezing, so they can survive and grow only between the last killing frost in spring and the earliest one in autumn.

The idea of growing seasons comes from experience, but science adapted growing season information to planning crops and gardens. Since frosts do not happen on the same dates every year, climatologists calculate the average (mean) dates of frost (32°F) for all years when records have been kept, and define the average growing season as the length of time between the mean last date of occurrence of 32°F in the spring and the mean first occurrence of 32°F in autumn.

As an application, tomato plants planted in spring on the mean last date of killing frost have a 50 percent chance of being injured or killed by a later frost. Better odds can be attained by waiting patiently until they improve. In most localities, the chance of frost drops to 10 percent about two or three weeks later.

Growing season maps for the U.S. can be found in extension bulletins, garden books and sometimes even on the backs of seed packets. Some maps, usually found in extension bulletins, show growing seasons based on 28°F, or even 24°F, as well as 32°F. These can be helpful because different plants are killed by different temperatures. For example, a 28°F frost, which would kill tomatoes and geraniums, would have little effect on petunias and cabbages. Some root vegetables can be safely left in the ground for much of the winter, because there they are protected by the heat of the earth.

Other climatic factors regulate the quality of the growing season. Temperature, as it relates to optimum temperatures for different plants, the amount of rainfall, and how much of it is absorbed by the soil rather than running off in a “gully-washer,” and wind, all have an effect on the amount and quality of plant growth. Growing season maps are not precise enough to take into account local conditions different from those elsewhere in the same general area, so it is a good idea to consult others who have greater experience in the locality.

The discussion to this point relates primarily to the Northern states. In many parts of the South there are two growing seasons: spring and fall, separated by a summer too long and hot for many plants. In the Deep South, the growing season may extend through winter. It is always a good idea to consult local sources of information.

Hardiness Zones

Trees, shrubs and other perennial plants expected to live for years may fail to do so if they are not cold hardy enough to withstand the lowest winter temperatures in the locality where they are growing. To select plants that will live, it is necessary to know about growing seasons and also the severity of the winters between them. A plant hardiness zone map is a tool that can help determine what to expect from a local winter climate.

Plant hardiness zone maps are based on the average annual minimum temperature (AAMT) for each map point. This is the lowest temperature recorded at a site each year, averaged for all the years for which records exist, for different locations (weather stations) in the map area. Areas with the same AAMT are connected by isotherm lines that serve as boundaries between hardiness zones. In the USDA Plant Hardiness Zone Map, for example, southern Iowa and northern Missouri are in Zone 5, where the AAMT ranges between -10° and -20°F, while southern Oklahoma and North Central Texas are in Zone 7, with an AAMT range of +10° to 0°F.

Plant Hardiness Ratings

In many reference books and nursery catalogs, plants are assigned hardiness zone ratings, which show the coldest zones in which they can be expected to be cold hardy. Sometimes they are also given the numbers of the southernmost zones where they will perform well. By matching one’s hardiness zone of a locality, as shown on the map, with the hardiness zone rating of a particular plant, it is possible to predict whether the plant will succeed in that locality. Nothing about plants and climate is perfectly predictable. The hardiness zone does not tell the whole story, for several reasons.

First, in all hardiness zones there are microclimates. Plants in an area exposed to the full force of a northwest wind may not survive even if their hardiness zone rating suggests that they should be safe there. Conversely, some plants that would not be expected to survive may do so in a site sheltered from winter wind and sun, or covered by an insulating blanket of deep snow.

Comparisons of Arnold Arboretum and USDA Hardiness Zones¹

Second, many plants are more likely to be damaged or killed by cold in spring or autumn than in winter. For example, in a warm, moist late summer and early fall, some shrubs, such as evergreen azaleas, may be stimulated to continue growing well into autumn. Since growth must stop before plants can begin the process of cold-hardening, plants actively growing this late do not harden soon enough to withstand the first hard freeze, even when that freeze would not have been severe enough to injure them had they hardened earlier. Other plants, such as raspberries, peaches, and the flower buds of many early-flowering shrubs, can be activated prematurely by unseasonably warm weather in late winter, predisposing them to injury from severe freezes yet to come during spring.

Third, some evergreen shrubs and trees, such as certain arborvitaes, rhododendrons and yews, are prone to injury from “fast freezing.” During sunny but cold winter days, their leaves are often warmed by heat rays from the sun to temperatures 30° to 40°F higher than that of the surrounding air. Then, as the sun’s rays are interrupted by the shadow of a cloud or building, the leaves are chilled rapidly back to the air temperature—so fast that their cells cannot make the adjustments that would protect them from slower freezing—and so are killed even though they might easily withstand the same low temperature, applied slowly. In short, just as winter injury does not always happen in winter, it also may have little to do with minimum temperature.

Fourth, many plants are killed or injured by winter conditions not directly related to temperature. Such is the lethal drying experienced by some evergreens exposed to winter sun and wind, especially when their roots are frozen in the ground and unable to absorb water to replace the water that is lost from the leaves by transpiration (evaporation from the leaf surfaces).

In spite of all this, hardiness zones are still the best single predictors of plant hardiness in a given locality, and they offer a good way to begin when selecting plants. The prediction may be improved, however, by checking with others who have more experience in the locality.

A note of caution: not all plant hardiness zone maps are the same. The first hardiness zone map in the United States was published in 1938 in Donald Wyman’s book, Hedges, Screens and Windbreaks. This map, called the Arnold Arboretum Hardiness Zone Map, has been republished since in several other books by Wyman and others. In 1960, the U.S. National Arboretum staff published a new plant hardiness zone map, now called the USDA Plant Hardiness Zone Map. This map is an improvement over the Arnold Arboretum map in that it has zones of equal size (10°F, with 5° subzones) rather than zones of variable size. Otherwise, the two maps are so similar that readers may assume they are identical, but different enough to cause serious problems, as shown in the table. For example, the AAMT span of -10° to -20°F, called Zone 4 on the Arnold Arboretum map, corresponds to Zone 5 on the USDA map. If a person living in USDA Zone 4 were to select flowering dogwood (Cornus florida) from a source that lists it as hardy to Arnold Arboretum Zone 4 (USDA Zone 5), the decision probably would be fatal for the dogwood. It does not matter which map is used as long as the plant ratings published for that specific map are used; mixing maps and ratings can court disaster.

In 1967, a new and different plant hardiness map was published by the Canadian government in the book, Ornamental Shrubs for Canada, by L.C. Sherk and A.R. Buckley of the Canadian Plant Research Institute. This map is based only partly on AAMT, in a climatic model that takes into consideration other climatic elements as well. This map works very well if one uses the plant hardiness zone ratings intended for use with the Canadian map, such as are used in the book Ornamental Shrubs for Canada.

A revised version of the USDA map was published by the National Arboretum in 1990, with the rationale that, since the 15 years or so prior to publication had included a number of severe winters, at least for most of the Eastern U.S., a new map was needed. Instead of using data for all years in which records had been kept, a sample of 13 years (1974 to 1986) was selected, and the resulting map was strikingly different from the 1960 map. This was apparently done in the belief that the climate was cooling in the long run, even though at the same time scientists were beginning to find evidence of global warming. A return to milder-than-normal winters in most of the Eastern U.S. in the decade following (1986 to 1995) demonstrated that the 1990 USDA map probably is too conservative a predictor of future winters for at least the Central and Southeastern U.S.

Extreme Minimum Temperatures

The USDA map shows residents of southern Michigan, away from the lakes, that they are in Zone 5, where the AAMT ranges from -10° to -20°F. This may surprise older residents of this area who can remember clearly a cold snap to -25° or -30°F! One must remember, however, that the extreme minimum temperature (EMT) for any given locality, that is, the lowest temperature ever recorded, is usually 10° to 20°F lower than the AAMT, averaging about 14°F lower for many different locations.

Plant hardiness zone ratings automatically take into account the difference between AAMT and EMT. They are based on actual plant performance in the different zones. Because of this it does not matter that EMTs much lower than AAMTs can be expected.

— Harrison L. Flint

See also Agronomy; Horticulture; Temperate Fruit Industry; Vegetable Industry; Weather

References

• Flint, Harrison L. Landscape Plants for Eastern North America: Exclusive of Florida and the Immediate Gulf Cost, 2nd edition. New York, NY: Wiley-Interscience, 1997. 
• Sherk, Lawrence C. and Buckley, Arthur R. Ornamental Shrubs for Canada. Ottawa: Canada Department of Agriculture, 1968. 
• U.S. Department of Agriculture. Yearbook of Agriculture: Climate and Man, Washington, DC: U.S. Government Printing Office, 1941. 
• U.S. Department of Agriculture. USDA Plant Hardiness Map, Agricultural Research Service. Washington, DC: U.S. Government Printing Office, 1990. Also available online at: http://www.usna.usda.gov/Hardzone/ushzmap.html. 
• U.S. Department of Commerce, Environmental Data Service. Weather Atlas of the United States. Detroit, MI: Gale Research Co., 1975. 
• Wyman, Donald. Trees for American Gardens, 3rd edition. New York, NY: Macmillan, 1990. 
• Wyman, Donald, and Harrison L. Flint. “Plant Hardiness Zone Maps.” Arnoldia 45 (1985): 32-34.