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Chapter 7: weather and climate.


bolting electromagnetic spectrum etiolation frost-sensitive frost-tender frost-tolerant growing degree days lake effect macroclimate microclimate photon phototropism phytochrome vernalization

The growth rate, flowering, and fruit and seed development of plants are affected by the environmental conditions in which they are grown. Environmental conditions that are consistent on a larger scale are macroclimates. Those that are restricted to a smaller area, such as around a house or yard or in a neighborhood, low area, and so on, are referred to as microclimates. Environmental conditions are determined largely by the weather in a given area, and weather is determined by several factors including type and amount of precipitation, latitude, prevailing winds, light intensity, and daily temperatures as well as annual high and low temperatures. Climate refers to the average condition of weather in a given region over a long period of time. Weather can change from one season to another and from one year to another. Yet, weather forecasters and climatologists make predictions based on statistical data. Agriculturists and horticulturists base decisions on when to plant and harvest on these predictions.

Plants have evolved in certain environments, under climatic conditions that have played an important role in their survival. For example, deciduous plants in temperate climates have evolved to drop their leaves when days become short and temperatures drop from optimal growing temperatures. Another example of plants that have evolved under climatic conditions are woodland species, such as many ferns, and wildflowers that tolerate low levels of light but become burned and may even be killed by intense light. Some flowering plants are able to grow, bloom, and set seed under low light conditions. Many of our houseplants evolved in the understory of tropical rainforests where low light levels prevail. The leaves are particularly efficient in photosynthesizing with low levels of light. That explains why they do so well inside our homes and offices. On the other hand, many plants require higher levels of light, and flowering, in particular, often requires a great deal of light energy. Thus, many plants that are grown indoors or even in shady areas of the landscape will fail to bloom, may not even grow, and will perhaps die because of a lack of light energy.

It is useful to know what kinds of plants can grow in a certain climate to determine whether they can be successfully cultivated somewhere else. Many of our horticulturally useful plants originated in other places around the globe. Most of the plants on earth grow in two climatic zones: tropical and temperate. Not only are tropical areas warmer year-round, but they also experience less fluctuation in the length of days and nights. To understand the climatic conditions in which plants originally evolved is to better understand the conditions one should provide for them to be the most successful gardener.


Temperature and light are the most important environmental factors for plant growth. Many chemical processes taking place within plant cells depend on specific temperatures. The processes of photosynthesis and respiration, for example, occur more quickly, more slowly, or not at all, depending on the temperature (Fig. 7-1). Two aspects of the effect of temperature on plants are growing degree days and how plants survive freezing.

The growth rate of plants depends on the amount of heat the plant receives. This is a phenomenon known as growing degree days (see box).

Temperate, Subtropical, Tropical

Global climatic zones are determined by the position of the earth as it relates to the sun. Tropical climates exist between latitudes of 23.5[degrees]N and 23.5[degrees]S. The northern point is just off the southern tip of Florida (the Tropic of Cancer) and extends south to the Tropic of Capricorn, which runs through southern Brazil and northern South Africa. The only U.S. state that is located in the tropics is Hawaii. Tropical temperatures and lengths of days are relatively constant, although temperatures will differ, even in the tropics because of elevational differences. Thus, microclimates help determine what plants will grow in an area. Although they do not experience four seasons per se, tropical areas are commonly known to have dry and wet seasons that affect plant growth and flowering.

The temperate zone lies to the north and south of the tropical zone and continues to the Arctic and Antarctic Circles (latitudes of 66.5[degrees]N and 66.5[degrees]S). Cold and warm seasons occur, and these vary by latitude within the climate zone. Great temperature fluctuations exist in the temperate zone, both from one season to another and also from night to day. For example, in Illinois, nights are commonly 20[degrees]F or more cooler than days, and they may be as much as 50[degrees]F cooler. The southern portion of the temperate zone is sometimes referred to as being subtropical, although this is not an official designation. Some areas of the temperate zone experience rainy and dry seasons. The arctic zone lies within the Arctic and Antarctic Circles. It is characterized by freezing temperatures and by having the longest and shortest days and nights.

Winter Low Temperatures

Plants are affected by temperatures in different ways throughout their life cycle and throughout the seasons of the year. In areas where temperatures do not fluctuate significantly, plants tend not to respond in ways such as deciduousness or chilling requirements for flower development. But in areas with distinct cold and warm seasons, these and other plant adaptations have evolved. For examples, herbaceous tissue is susceptible to freezing, and thus woody plants drop their leaves in fall when temperatures begin to drop. In the spring, when temperatures begin to warm, many woody plants start blooming before their leaves emerge. When there is a late spring frost, many blooms are lost overnight. This is a disadvantage to the sexual reproduction of these plants. But many woody spring-flowering plants produce such an abundance of flowers and these flowers open over a period of 1 or 2 weeks, so the chances of some of them surviving the spring frost are greatly improved.


Cool temperatures during storage of vegetables and flowers result in reduced respiration rates, causing the plant to retain more of its sugars. Fruits stored this way will remain sweeter for a longer period of time, and flowers kept cool after harvest will maintain better quality for a longer period of time.

Plants can be classified in relation to their ability to withstand cold temperatures. This characteristic is mainly of concern with vegetable crops to determine when it will be safe to plant and grow them under ambient conditions. Plants that can tolerate a hard freeze are classified as frost-tolerant plants. Those that cannot tolerate a hard freeze are frost-tender or frost-sensitive.

VERNALIZATION. Vernalization is a term that describes the requirement of some plants for cold temperatures for floral development. Plants that are vernalized receive the required cold temperatures (usually 35 to 41[degrees]F) and when warm spring temperatures arrive, they are ready to bloom. Lack of adequate vernalization prevents, reduces, or delays flowering, whereas artificial imposition of such requirements may reduce the time to flowering. Plants vary in their requirements for vernalization, particularly in the time the cold temperatures are required more so than in the actual temperatures required.

CHILLING. Lack of chilling in some areas prevents flowering in apples, peaches, and other plants that require chilling. Ornamental trees that are valued for their flowers, if not for their fruit, will also fail to provide their colorful display if they do not receive the required chilling units during the dormant season.

USDA Hardiness Zones

The U.S. Department of Agriculture (USDA) has developed a map based on the average annual minimum temperatures divided into 10[degrees] increments (Fig. 7-2). There are 11 zones altogether in the United States and Canada. Small numbers (e.g., 1, 2, and 3) indicate colder minimum temperatures, whereas higher numbers (e.g., 8, 9, and 10) indicate warmer temperatures. Plants are rated for their cold hardiness, which is an indication of the coldest temperature they can be subjected to without dying. For plants that are hardy in your area, find the zone in your location, and then look up plant information to identify those that are hardy in that zone. Some references list plant hardiness by a single number, indicating the coldest zone the plant will be hardy in. Others give a range of zones.


Frost-heaving occurs when soil and the roots in it are pushed upward during freeze and thaw cycles in winter. Shallow-rooted plants such as strawberries are affected by this force. Thus, strawberries are covered with a mulch, such as straw, to reduce the fluctuations in temperature and the resultant upheaval of soil in the root zone. Other plants can also be affected and should be watched for this problem.

What to Do about Cold Temperatures

Although a sheltered spot can sometimes be provided for plants that are marginally hardy in an area, this is usually not possible on a large scale. The best practice is to select plants that are hardy in your region. In the case of woody plants, buy from nurseries that are located in your area or in a colder region rather than from someone in a warmer region. Check, if possible, to see whether the seed or vegetative stock for the tree or shrub came from the same or from a colder region. This will help ensure that the plant has the genetic makeup to allow it to survive winters in your area.

For planting of spring annuals, use the technique of hardening-off described in chapter 3 to ensure that the plants are acclimated to the colder night temperatures they will surely be subjected to when they are planted outdoors. Other activities you can use to help transplants in spring include encouraging carbohydrate accumulation by providing high light intensity before planting them outdoors, mulching or otherwise protecting the plants after planting them, irrigating them to prevent freezing, and providing wind protection to prevent desiccation.

Summer High Temperatures

Warmer temperatures act as a signal to cause flowering in plants. This is apparent in spinach and lettuce, which are cool-season crops. When grown in the spring they produce vegetative growth, and their leaves may be harvested over a period of weeks. However, as temperatures warm up, a flower stalk begins to develop. Flowers emerge on a stalk that arises from the center of the basal rosette of leaves. This is a process known as bolting.

Temperatures directly affect life cycles and the development of insect pests of ornamental, fruit, and vegetable crops. As temperatures increase above a base temperature, insect development speeds up (see box). High temperatures can cause plants to stop growing. The critical temperature at which this happens for many plants is 86[degrees]F.


AHS HEAT ZONES. A system of measuring the amount of time a region experiences temperatures over 86[degrees]F has been developed by Dr. H. Marc Cathey, the American Horticultural Society (AHS), and the Meteorological Evaluation Service Co. Inc. The culmination of this work was the development of the AHS Plant Heat-Zone Map (Fig. 7-3). The 12-zone map indicates the average number of days each year that a given region experiences heat days--those days with temperatures over 86[degrees]F and the point at which plants experience damage to cellular proteins. The zones range from Zone 1 (with no heat days) to Zone 12 (210 or more heat days). This system is complementary to the USDA Hardiness Zone Map and aids in establishing a plant's likelihood for survival with extremes in temperature. Many horticultural plants have been evaluated for their ability to survive in the various heat zones. This information is regularly available on plant information tags sold with many plants.

Other Climate Zone Systems

Other climate zone systems have been developed, although these are not much used in relation to plant cultivation. These include the Sunset gardening regions that were first developed for the western United States and later expanded to the rest of the country. These regions are used in gardening books published by Sunset. Another climate zone system incorporates areas called biomes. These were developed by incorporating temperature, precipitation, latitude, and altitude. Biomes apply more generally to noncultivated plants and are more useful in biological or botanical studies.



Plants are composed mainly of water. Some plants, such as cacti and succulents, have developed the ability to store water in their tissues to enable them to survive long periods of heat and drought. The biomes that have been developed divide the world into zones of precipitation, from rainforests, receiving 200 or more inches of rain per year, to deserts, receiving 10 or fewer inches per year. The welwitschia plant (Welwitschia mirabilis) found in the Namib Desert receives virtually no rain at all but survives on moisture provided by the mist of fog that moves through the area regularly.

Rainfall is the most common form of precipitation, although other forms occur regularly. These include snow, hail, fog, and mist. Humidity occurs in some areas more than in others. Areas of low humidity are more common in the southwestern United States. Areas of high humidity experience increased incidence of pathogens that thrive under moist or humid conditions.

Inside the home, air conditioners and furnaces create relatively dry air. Some plants will not do as well under such conditions. A humidifier can be beneficial to these plants.

Snow is a winter phenomenon in the higher latitudes and altitudes. Snow can be heavy and wet or light and dry. Heavy wet snow is more damaging to plants, especially to evergreen trees and shrubs whose branches can become burdened with heavy wet snow to the breaking point.

Because snow is crystallized water, it takes up more space than a drop of water would. It has been estimated that 10 inches of snow might provide only an inch of water once it has melted. Melting snow can be a good source of water for plants in winter. If little snow falls during the winter, it may be necessary to water your evergreen plants on occasion to prevent excessive drying. Snow can provide the benefit of cold temperature protection. This is particularly true when temperatures are cold enough to kill dormant flower and leaf buds. A good snow cover has been shown to protect forsythia flowers during such damaging temperatures as shown in spring, when all the flowers that were below the previous snow line bloom and those that were above it do not.

Dew is moisture that condenses on objects when the object is warmer than the surrounding air. The cooler air cannot hold as much moisture as the object, and so the moisture precipitates out of the air. The dew point temperature is that temperature at which the air can no longer hold the amount of moisture in it. This is when dew forms on grass and other objects. When dew point temperatures are high, for example, 70[degrees]F or higher, fungal invasion is a problem, especially on golf courses and other turf areas.


Microclimates are small areas of altered or modified temperatures and/or moisture. For example, under the eaves of a house, there is a dry area that may be in shade all year long. In the corner of a brick wall, plants may be more sheltered than they would be in the middle of the yard. Houses themselves may provide sheltered areas for plants where they are protected from wind or where a south-facing wall reflects light and heat to nearby plants. If you want to grow a plant that is not hardy in your area, it is sometimes possible to modify an area in which to grow that plant. This is usually possible only for plants that are hardy in one-half zone warmer or approximately 5[degrees]F warmer minimum temperature.

Elevation, Terrain, Topography

The elevation and terrain of an area have a variety of effects on temperature and moisture. For example, as you move upward on a slope, hill, or mountain, the air becomes cooler at a rate of about 1[degrees]F per 300 feet (Fig. 7-4). This temperature change is negligible on elevation changes of less than 300 feet, and some other forces come into play, such as heavier, cold air draining down-slope.


Although it is counter-intuitive, since the air is cooler as you move up a mountainside, on a slope cold air drains and hot air rises. Therefore, if you have a sloped area in your yard, you will probably be more successful growing plants at the top or middle rather than the bottom of the slope (Fig. 7-5). This fact is particularly true if you live in an area where frost occurs. For example, in spring, when fruit trees are blooming, a late frost can kill the flower buds. Such trees are more susceptible at the bottom of a slope. They also will be exposed to freezing temperatures more often and colder temperatures in general throughout winter. The side of a slope is not particularly desirable for growing trees because of problems with moisture and drainage. South-facing slopes are warmest, followed by west, east, and north. In addition to being warmer throughout the year, south-facing slopes warm up earlier in spring and thus may offer an advantage for growing longer-season crops or having earlier blooms in spring.

Frost Pocket

Frost pockets form in depressions as a result of cold air sinking (Fig. 7-6). As the cold air accumulates and is trapped in the depression, frost forms if dew is present and temperatures drop below freezing. In general, these areas should be avoided for crop production.


Rain Shadows

As air approaches geographical features of elevation, such as escarpments or mountains, it is forced to rise (Fig. 7-7). As the air rises, it cools, and moisture in the air condenses, as cool air holds less moisture than warm air does. At some point, the air cannot hold all the moisture in it, so the excess is released. This may be in the form of rain or snow or mist. As the air continues to move to the other side of the mountain, it has lost its moisture, creating a dry region known as a rain shadow.

Bodies of Water

Bodies of water such as oceans and lakes have a moderating influence on the air temperatures around them. As a result of this effect, they cool the air around them in summer and warm it in winter. In the United States, prevailing air movement is generally from west to east. The coast along the east side of the Pacific Ocean, as well as coastal areas east of large and small lakes and other bodies of water, benefit from the cooler air as it moves across the water in summer. In winter, these areas are slightly warmer. Areas in the Great Lakes region may be one-half to one full hardiness zone warmer than surrounding areas, allowing excellent vegetable and fruit production in some of these areas, as well as growing of more diverse ornamental plants than can be grown in nearby areas. Foggy mornings prevail along the Pacific Coast. The warmer temperatures in winter and the additional moisture have beneficial effects on the plants and crops grown in that narrow region. Plants that would freeze farther inland survive and thrive there.



Cold air masses that move across relatively warm bodies of water, such as lakes, in winter pick up heat and moisture as they pass; the air mass then cools and condenses as it passes over land, becoming a source of snow that precipitates out when the air mass hits land. This is sometimes referred to as lake effect snow (Fig. 7-8). The phenomenon is common in the Great Lakes region and over the Great Salt Lake, Cape Cod, Chesapeake Bay, and Long Island Sound.


Plants need light for photosynthesis, which provides the fuel for growth. The effects of light on plants extend further than growth, however. Plant responses to light include amount and direction of growth and development as well as timing and amount of flowering, color development in fruit, and plant movements, or phototropisms.

Plant Responses to Light

Plant responses to light may be attributed to specific biochemical activity at the cellular level. Some of these responses are well understood, whereas the exact modes of operations of others still intrigue scientists. For example, photosynthesis is known to occur as a chemical reaction of the molecule chlorophyll at specific wavelengths on the electromagnetic spectrum (Figs. 7-9 and 7-10). Phytochrome is another light-sensitive, or photosensitive, molecule in plants that is involved in the flowering response.


Light is visible to the human eye within a certain range of wavelengths on the electromagnetic spectrum (Fig. 7-11). The range of wavelengths exceeds those that are visible to the human eye. Other wavelengths on the electromagnetic spectrum are radio waves, microwaves, x-rays, gamma rays, and ultraviolet waves. Our eyes can only perceive certain wavelengths, whereas the eyes of other animals including insects, can perceive wavelengths in other ranges of the electromagnetic spectrum. For example, bees and butterflies can see in the ultraviolet range. Because some flower petals emit colors in that range, bees and butterflies sometimes see flowers as brightly glowing, ultraviolet objects, much as we might see when using a special black light. Sometimes markings on petals of flowers are brightly displayed in ultraviolet colors, pointing the way to the nectar.

Have you ever noticed how plants tend to grow toward light? For example, if you have ever tried to grow seedlings on a windowsill, you may have noticed how they sometimes bend their stems towards the window. Or, maybe you have heard that sunflowers turn to face the sun as it moves across the sky throughout the day. This is due to a tropism, or plant movement, that is controlled by exposure to light. Phototropisms may be either positive, in the case of stems, or negative, in the case of roots. They are thought to be due to a change in the concentration of the plant growth hormone auxin in the stem tip. Auxin stimulates cell elongation.

If plants do not receive enough light, they will become stretched and weak. They lack chlorophyll and become very light green. This stretched condition is known as etiolation. This also happens to plants grown under a high proportion of red wavelengths, such as those given off by incandescent lamps (Fig. 7-12). Plants that are grown under ordinary house lighting display this phenomenon, usually being stretched and weak.



Quantity: Duration and Intensity

Light can be measured, both in the length of time it is available and in the amount of energy emitted. The length of time is usually measured as daylength, or photoperiod. The energy emitted is measured in photons. However, the history of light measurements has its roots in measuring light given off by household light bulbs, using a term known as footcandles. At their origin lightbulbs were compared to candles, much in the way that car engines were compared to horse power. This method of measurement is misleading for plants, because chlorophyll is responsible for plant growth, and it is responsive to wavelengths different from those measured by footcandles. Currently, the more accurate method is to measure light in photons. The internationally accepted unit for expressing photons is micromoles (one millionth of a mole) per square meter per second ([micro]mol/[m.sup.2]/sec). Because the unit is given per second and light accumulates over the number of hours in a photoperiod, or day, one can state the quantity of light in 1 day. This amount is referred to as the daily light integral. Therefore, a light source that provides 350 [micro]mol/[m.sup.2]/sec, over a 16-hour photoperiod, provides a daily light integral of just over 20 moles. This number can be multiplied by 5 to get the approximate number of footcandles.



The amount of sunlight emitted on a sunny day at noon is about 2,000 mol/[m.sup.2]/ sec (10,000 footcandles). This amount is more than enough for many plants. On a sunny day, light is not a limiting factor to plant growth. The other factors required for plant growth are carbon dioxide, water, the correct temperature range, and the essential nutrients.

The angle of the sun changes throughout the year (Fig. 7-13). When you are planning landscaping or gardening, it is important to consider the objectives for placing plants and decide whether they will be met with respect to the sun's angle. Further discussion on energy-efficient landscaping is found in chapter 18. The changing angle of the sun is also important in greenhouse design and orientation. At latitudes at or above 40[degrees] the length of the greenhouse should run east and west to maximize the sun's light, especially in winter, when the sun is low in the sky.


Quantity and Quality: Photoperiod

Photoperiod is the term used to describe the length of time plants receive light in a 24-hour period. This is an important component of light quantity and plays an important role in the amount of growth the plant will experience in a 24-hour period. However, there is another function of photoperiod that is a special feature of plants: a response system in some plants that results in flowering. This is called a photoperiodic response. Originally scientists thought that photoperiodic plants bloomed in response to daylength. This concept was held for many years, but later research demonstrated that the length of the dark period actually triggered the flowering response (Fig. 7-14).

Photoperiodic plants may be either long-day (LD) plants or short-day (SD) plants. LD plants will bloom when days are longer than nights, whereas SD plants will bloom when days are shorter than nights. Table 7-1 shows photoperiodic responses of selected plants.




Light quality refers to the wavelength, or spectral distribution, of light (see Fig. 7-11). Humans perceive light that is emitted in the wavelength range between about 400 and 700 [micro]m (micrometers). This is referred to as visible light. Plants perceive light in very different ways than do humans. Yet, there is a similarity. Plants perceive light with photosensitive pigments, just as humans do. However, our photosensitive pigments are restricted to our eyes, whereas plants have such pigments primarily in their leaves. The most famous photosensitive pigment in plants, of course, is chlorophyll. At certain wavelengths, chlorophyll is activated in photosynthesis, whereas at slightly different wavelengths, it is activated to manufacture more chlorophyll (see Fig. 7-10). As light levels decrease in autumn, chlorophyll production decreases until it finally comes to a halt. At the same time, chlorophyll breakdown continues at a steady rate until all the chlorophyll is gone, and other pigments are visible. Anthocyanins, producing a red color, and carotenoids, producing yellow, are most familiar in fall displays of many temperate trees and shrubs.

If we understand how photosensitive pigments respond to specific wavelengths of light, we will better understand how to improve or change the light quality and thus affect growth and flowering. This understanding is especially important when plants are grown under artificial conditions, such as indoors or in greenhouses. Flowering crops are more valuable when they are in bloom, and all plants are more valuable when they are healthy and growing well.

Increasing Light

In a landscape, one can optimize the amount of light a plant receives by correct placement. Consideration is given to the amount of sun or shade in various locations in the garden, and the plant is placed accordingly. Likewise, inside the home, a plant should be placed according to the amount of light it requires and the amount available (see Tables 12-1 and 12-3). Some plants are placed near windows, whereas others require indirect light. Many plants cannot be grown indoors because of inadequate light levels. However, special lamps may be purchased to provide adequate amounts of light in the proper wavelengths. Furthermore, greenhouse producers use special lamps to grow crops in low-light regions of the country or during winter when the photoperiod is too short to supply an ample quantity of light for production.

Artificial Light Sources

If you want to improve lighting in your home to sustain plant growth, you should provide adequate quantity and appropriate quality. In other words, increase intensity in the correct wavelengths. The types of lamps that are available for home use include special grow-lamps and common household fluorescent light bulbs. The former are available from a number of manufacturers. If you decide to use fluorescent light bulbs, use cool-white bulbs that emit more energy in the blue spectrum. This light will facilitate chlorophyll activity. There is a direct relationship between proximity of light to the plant and the energy the plant receives: the closer to the plant the bulbs are placed, the more light the plant will receive. Fluorescent bulbs will emit some heat that could be detrimental to plants, but the ballast is generally the hottest part of the fixture, so choose a location where the heat will not adversely affect the plant.


Weather is determined by precipitation, light, and temperatures and comprises the type of climate in a given region. Light and temperature affect many important processes in plants, including photosynthesis, respiration, and transpiration. They also affect flowering in many plants. Growing degree days can be calculated to predict when certain developmental events will occur in plants for which this information is available.

Hardiness of plants determines how well they will survive low winter temperatures in a region. Precipitation levels can determine which plants will grow in an area or in what manner plants will develop specialized structures to adapt to low or high amounts of water.

Elevation, terrain, and topography all affect temperatures and precipitation and even light levels. Bodies of water moderate temperatures around them.

Light is of great importance, not only for the amount of light available for plant growth but also for the quality of light available. Light quality affects flowering in many plants and also affects production and activity of chlorophyll. Photoperiod affects flowering in some species and can also contribute to plant growth.

A thorough understanding of how temperature, light, and precipitation affect plant growth will aid in decisions of where to plant, what plants to grow, and how to produce high-quality plants in a given climate. It can also provide insight into methods for supplementing those inputs when they are in short supply or in reducing them when they are excessive. Before selecting plants for a garden, one must be familiar with the climate and weather in one's region and use that information to pick appropriate plants.


* Record high and low temperatures for your area for a month. Plot the numbers on a graph. Calculate the average high and low temperatures for the month.

* Obtain a light meter and measure and record light levels in different areas: indoors by a window, indoors away from windows, outside under a tree, and outside in full sun. Categorize low, medium, and high light areas and create a zone map identifying these areas.

* Observe an area in your yard for shade received during the day. Use your observations to estimate the number of hours of sunlight received in an area to identify areas of full sun, partial sun/partial shade, and full shade.


1. Environmental conditions that occur over a large region are--, whereas those that are restricted to smaller areas are--.

2. The most important environmental factors for plant growth are--and--.

3. The zone between 23.5[[degrees]S and 23.5[degrees]N is called the--. The zone between 23.5[degrees]S and 66.5[degrees]S and 23.5[degrees]N and 66.5[degrees]N is called--.

4. Temperature and daylength are relatively constant in the--zone.

5. Growing degree days are used to predict the--.

6. Discuss chilling units.

7. The USDA hardiness zones are based on--.

8. What form of precipitation forms on plants as air around them cools and moisture condenses?

9. On a slope, cold air moves--, and warm air moves--.

10. Rain shadows are associated with which geographical features?


Dennis, J., & Wolff, G. (1992). It's raining frogs and fishes. New York: Harper Collins.

Reed, D. W. (2005). Horticulture: science and practices. Retrieved July, 27, 2005, from lectsupl/Temp/temp.html#page36.

Sloane, E. (1952). Eric Sloane's weather book. Boston: Duell, Sloane, and Pearce--Little, Brown.

U.S. National Arboretum. (2003). USDA Plant Hardiness Zone Map (USDA Miscellaneous Publication No. 1475, Issued January 1990). Retrieved June, 1, 2005, from

Growing Degree Days and Degree Days

The term growing degree days (GDDs) refers to the amount of heat required for different amounts of growth in a plant. Plant growth is a response to the temperature as long as no other limiting factors exist. For example, models predicting stage of growth for a corn crop have been developed. These help seed companies calculate when pollination will occur and thus when detasseling for controlled pollination should begin. Threshold high and low temperatures are set at 50[degrees]F and 86[degrees]F because growth is assumed not to occur at temperatures colder or warmer than these. Each corn cultivar has specific GDD requirements. The formula for GDD is to add the high temperature and low temperature for 1 day (24-hour period), divide by 2, and subtract 50 (to get to the base temperature of 50[degrees]F). If the high temperature is greater than 86[degrees]F, then 86 is used. GDDs are widely used by corn growers to determine when harvest should occur.

GDDS affect all sorts of plants in the spring and summer, including both food and ornamental species, but models have not been developed for all of them. The concept does help to explain, though, why plant growth is further along some years than for the same time in other years or why some flowers bloom earlier or later in different years.

Insect development also depends on warm temperatures, and a baseline of 50[degrees]F is commonly used to calculate those degree days, too. Emergence of insects has been calculated for many crop pests based on degree days. This useful tool can help predict when to spray for a particular pest. This information is a great improvement over past spray schedules that were hit or miss, with sprays often being applied indiscriminately. Degree days are usually calculated beginning when temperatures warm up and vary depending on the prevailing local climatic conditions.

Dr. Marietta Loehrlein currently teaches horticulture classes at Western Illinois University in Macomb, Illinois. She earned both her bachelor's degree in Agronomy and her master's degree in Plant Genetics at The University of Arizona. Her master's research project was concerned with germination problems associated with triploid seeds, from which seedless watermelons grow. Following that she worked for 5 years in a breeding and research program for Sunworld, International near Bakersfield, California. She worked with peaches, nectarines, plums, apricots, and cherries. Then she returned to school to earn her Ph.D. in Horticultural Genetics at The Pennsylvania State University. Her Ph.D. research focused on flowering processes in regal pelargonium (also called Martha Washington geraniums). While at The Pennsylvania State University, she bred a new cultivar of regal pelargonium, "Camelot." At Western Illinois University, Dr. Loehrlein teaches nine courses: Greenhouse and Nursery Management, Introductory Horticulture, Landscape Design, Landscape Management, Home Horticulture, Plant Propagation, Turf Management, and two courses in Plant Identification.

Photoperiodic Response of Selected Plants

                                              LONG DAY
NAME                SPECIES                   OR SHORT DAY

Achillea, yarrow    Achillea filipendula,     LD
                      Achillea millefolium

Carnation           Dianthus caryophyllus     LD

Chrysanthemum       Dendranthema              SD

Gayfeather          Liatris spicata           LD

New England         Aster nova-angliae        SD

Poinsettia          Euphorbia pulcherrima     SD

Sedum               Sedum spectabilis         SD

Spinach             Spinacea oleracea         LD

Tuberous begonia    Begonia tuberosa          LD
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Author:Loehrlein, Marietta M.
Publication:Home Horticulture: Principles and Practices
Date:Jan 1, 2008
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