introduction to horticulture and plant physiology · pdf fileintroduction to horticulture and...

CHAPTER 2 IDAHO MASTER GARDENER PROGRAM HANDBOOK 2 - 1

Horticulture Defined 2Climate in Horticulture 2

Macroclimate 2Microclimate 2

Role of Temperature in Horticulture 4High Temperatures 4Low Temperatures 5Temperature Modification 6

Role of Light in Horticulture 7Light Quality 7Light Intensity 8Light Duration 8

Further Reading and Resources 9

MASTER GARDENERIDAHOUNIVERSITY OF IDAHO EXTENSION

Chapter 2INTRODUCTION TO HORTICULTURE AND PLANT PHYSIOLOGY

HORTICULTURE DEFINED

Horticulture is defined by Webster’s dictionary as“the science and art of growing fruits, vegetables,and flowers.” It is the intensive commercial produc-tion of high-value and high-yielding plants. But italso includes the cultivation of garden crops andlandscape ornamentals and the interaction of scienceand art.Horticulture contributes to the economy, provides

good nutrition, and is a valuable spiritual and psy-chological therapy. Horticulture beautifies andenhances the environment. Areas of horticultureinclude the following:• Pomology. Fruit culture, including pome fruits(apple, pear, quince), stone fruits (peach, cher-ry, plum, nectarine, apricot), small fruits (blue-berry, raspberry, grape, strawberry), and nuttree fruits.

• Vegetable production. Culture of food cropsfrom vegetable plants including roots, fruits,and seeds.

• Floriculture. Growing of cut flowers, pottedplants, bedding plants, and bulbs and floraldesign.

• Environmental horticulture. Nursery produc-tion of herbaceous and woody plants for land-scape design and management.

• Postharvest physiology. Harvest, handling,and storage of horticultural crops includingflowers, fruits, and vegetables.

CLIMATE IN HORTICULTURE

MacroclimateThe term “climate” refers to the long-term weath-

er patterns of a large geographical area and is usedinterchangeably with “macroclimate.” Macroclimateis determined mainly by an area’s latitude, eleva-tion, nearness to large bodies of water, nearby ocean

and wind currents, relation to nearby forests andirrigated areas, and location in relation to topo-graphic features such as mountains. Temperature and light are two fundamental fea-

tures of climate that profoundly affect gardening.Rainfall, wind, hail, clouds, snow, and humidity alsocreate the climate of a region. Short-term variationsin rain, wind, snow, and other climatic characteris-tics are the weather.Climatologists have calculated the statistical

probabilities of certain climatic occurrences that arelikely to affect plant growth. The USDA hardinesszone map, for example, is based on an area’s aver-age minimum temperatures. The Arbor DayFoundation has produced an updated version of theclimate zone map based on the last 15 years ofwarmer temperatures (1990-2005) (figure 1).MicroclimateMicroclimates are variations in climate within a

community, yard, or other restricted area and resultfrom topographic features, soil types, aspect, orlocation of buildings, fences, and/or plantings.Different microclimates will be more or less con-ducive to different outdoor activities and will limitor enhance the success of plantings. For example, ashady northern exposure may make a better summerpatio space than the sunny south side.Gardeners can create or modify microclimates to

increase livability and diversify planting conditionson their property. Landscape features that producemicroclimates include the following:• Hills and low areas. Hillside locations are lesssubject to frost since cold air is denser thanwarm air and will flow downhill to settle inlow areas. A south-facing slope warms earlierin the spring than a north-facing slope, but willbe hotter and dryer during summer. The lee-ward side of a ridge is less subject to wind orbreeze than the windward side.

2 - 2 INTRODUCTION TO HORTICULTURE AND PLANT PHYSIOLOGY CHAPTER 2

Chapter 2

Introduction To Horticulture & Plant Physiology Jo Ann Robbins, Extension Educator, Jerome County

Susan Bell, Extension Educator, Ada County


• Structures. Structures such as buildings,fences, driveways, or sidewalks serve as heatsinks for solar radiation. Planting areas aroundthem will be warmer, especially on their south-ern sides or next to pavement. Northern sidesof buildings and fences are shady and willremain cooler and moister.

• Bodies of water. Water has a moderatingeffect on air temperatures. A lot more energy isrequired to raise the temperature of water thanthe temperature of air. Likewise, water releaseslarge amounts of heat energy when it cools.Thus, water acts as a buffer to heat or cold.Air blowing over cool water will cause adja-

cent land to warm up slower in the spring, thusdelaying bloom and growth. This can protectplants from spring frosts. In the fall, air movingover warm water keeps the surrounding landwarmer longer than areas farther away.

• Elevation. The higher the elevation, the coolerthe temperature; there is less atmosphere toretain the heat from solar radiation at high ele-vations. Each 300-foot gain in elevation resultsin an average 1°F drop in temperature.

• Raised beds. Raised beds heat quicker thansurrounding flat soil surfaces, but plants inraised beds may dry out faster and suffer rootdamage due to freezing in winter.

• Plants. Large plants create microclimates byreducing wind speed, creating shade, and rais-ing the humidity beneath them.

• Soil. Sandy soil will warm more rapidly in thespring than clay soil and can be planted earlier,resulting in a crop that will mature more rapidly.

By identifying and using microclimates to youradvantage, you can maximize the conditions forindividual plants or strategically locate garden beds,

Figure 1. Hardiness zone map of the continental United States. Courtesy of the Arbor Day Foundation.

patios, and other outdoor spaces. The right microcli-mate often will make the difference between failureand survival for some landscape plants (figure 2).

ROLE OF TEMPERATURE IN HORTICULTURE

Temperature is the climatic factor that, more thanany other, determines the kinds of plants that willgrow in an area. Photosynthesis, transpiration, andrespiration increase with rising temperature. Manyhorticultural crops thrive in warm climates such asCalifornia’s and Florida’s, but are challenged innorthern climates like Idaho’s. Cold temperaturesrestrict plant growth, freeze plants in mid-winter,and damage plants during fall and spring frosts.Surviving cold temperatures requires well-adaptedplants. Hardiness is especially important for perma-nent landscape plants such as woody ornamentalsand fruit trees.Each plant type has an optimal temperature need-

ed for growth. Some plants prefer cooler nights ordays, whereas others prefer warmer nights or days.Temperate zone vegetables and annual flowers areclassified as cool-season or warm-season crops.Cool-season crops (sweet peas, pansies, gardenpeas, onions, carrots, potatoes, lettuce, cabbage, andbroccoli) grow best in the northern portions of theUnited States, at higher elevations, or during thespring and fall in warm-climate areas.Warm-season crops (sweet corn, tomatoes, pep-

pers, melons, zinnias, and marigolds) do best duringthe warmth of summer in the north but are ideallysuited for growth over a longer season in warmer

parts of the country. Seeds of warm-season cropsrequire a soil temperature of 60°F or higher to ger-minate, whereas seeds of cool-season crops will ger-minate at a soil temperature of just 40°F.High temperaturesPlant growth is measured by the food energy pro-

duced thorough photosynthesis above that used forrespiration. Plants generally grow best at the higherend of their optimal temperature range. In the tem-perate zone, the minimum temperature for growth isabout 40°F. Photosynthesis and respiration increaseas temperatures rise until the energy used in respira-tion equals photosynthetic capacity, when growthceases. For most plants, this temperature is around96°F. For many cool-season crops, growth maycease at temperatures considerably lower than 96°F.Warm temperatures cause stored carbohydrate

reserves to be used up thorough respiration or to beconverted to starch. This affects the sweetness ofcrops such as sweet corn and peas and thus theirquality.Very high temperatures can cause physiological

damage to plants resulting in burnt leaves andslowed growth. Other high-temperature considera-tions in plant growth are discussed below.Overcoming dormancy Most temperate zone perennials need a cold peri-

od to overcome their physiological dormancy, orrest period, for either the entire plant or only fortheir flower and vegetative buds. Temperatures thatare not cold enough during the winter will keepthese plants from forming normal leaves and buds inthe spring. For example, peach cultivars for northernclimates require 700 to 1,000 hours below 45°F andabove 32°F before they break their rest period andbegin growth. If grown in the southern part of theUnited States, these peaches will not thrive becausethis requirement is not met.VernalizationSome plants require a chilling treatment to induce

flowering. This is especially common in biennialsand spring-flowering bulbs.Plant pestsPlant diseases often grow well at 96°F or higher,

increasing the chance of infection. Similarly, insectpests reproduce more rapidly during periods of hightemperatures, with resultant high pressure on plants.


Figure 2. Soil, large objects, bodies of water, andlarge plants all create microclimates.

Heat unitsPlants have a base temperature below which they

grow very little. Average temperatures above athreshold “base” temperature (40° to 50°F, depend-ing on plant type) accumulate on a seasonal basisand are called “heat units” (or “degree days”) forthat season (figure 3). Heat units are useful in esti-mating time of maturity, predicting the latest feasi-ble date for fall planting, and deciding if long-sea-son fruit cultivars will mature in a specific locality.To calculate heat units, use the following equation:

Add heat units for each day to those of the previousdays to calculate the season’s total heat units thus far.A negative number for daily heat units does notdecrease seasonal heat units, but rather leaves itunchanged.Certain sweet corn cultivars mature at 1,500 heat

units (degree days). Cool nights like we have inIdaho will slow the accumulation of heat units incomparison with areas of the country that have warmnights. This is why corn labeled “matures in 65 days”can take much longer to mature in a cooler climate!Low temperaturesMany plants are susceptible to frost and cold

temperatures. If temperatures are too cool, there willbe a lack of plant growth, a failure of seed germina-tion, and some plants will not set fruit. Species orig-inating in the tropics, for example, are injured bytemperatures below 40°F.Plants have a minimum survival temperature

below which they will be severely injured or killed.The amount of plant damage depends on many vari-ables such as the kind of plant, the plant part, thenutrients and moisture in the plant tissues, the sea-son of the year, the temperature during the freeze,the temperature after the freeze, the amount of airmovement, and the moisture level in the soil. Otherlow-temperature considerations in plant growth arediscussed below.Premature flower stalk formation (bolting)Premature flowering in plants is related to the

weather and other environmental conditions. Manybiennials will bloom in the first year if cool temper-

atures follow shortly after planting. Since manybiennial plants are grown for their roots, petioles, orleaves rather than for seed, flowering and seed for-mation make the plant inedible. (Other temperatureconditions that will cause plants to bloom early aresummer heat and fluctuating temperatures.)Development of winter hardiness or dormancyPerennial plants become more cold tolerant in

the fall after they shed their leaves. This is part ofthe “hardening” process brought on by cooler tem-peratures and shorter days. Freezing temperaturesare necessary for most plants to increase their resist-ance to cold damage, while sustained freezing tem-peratures are necessary for maximum cold tolerance.If temperatures rise for any length of time, plants

lose their tolerance to the cold. Cold tolerance willreturn with colder temperatures, but not if the budshave broken dormancy. Buds will break dormancyduring a warm spell if they have already beenexposed to chilling temperatures for the periodrequired for bud break. This type of damage is com-mon in northern climates like Idaho’s.Carbohydrate reservesPlant tissues well supplied with carbohydrates will

reach deeper dormancy and be less susceptible towinter cold. Make fertilizer applications and prunewell in advance of cool fall temperatures. Plants thatare stress free and without new growth will movecarbohydrates to the roots and other storage tissues inthe fall. Stress from insects, diseases, or other sourceswill lessen carbohydrate production and storage.


+( )High tempfor day

Low tempfor day

2Base temp = Heat units

for that day

25002008 TWFI TWIN FALLS

2000

1500

1000

500

001-01 02-01 03-01 04-01 05-01 06-01 07-01 08-01 09-01 10-01 11-01

Date

Cumu

lati

ve D

egre

e Da

ys (

50F

base

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Figure 3. Calculation of accumulated heat units(degree days) in Twin Falls, Idaho, given a base tem-perature of 50°F (the base temperature for corn).Calculated using the Oregon State University Degree-day Calculator (http:pnwpest.org/cgi-bin/ddmodel.pl)

Water status of plant tissueWinter damage can occur due to a lack of mois-

ture in the plant or plant part. If the plant goes intothe winter with little moisture in the root zone, or ifdehydration occurs while the soil is frozen, the plantwill be injured due to water stress. This is called“physiological drought.” This type of damage is par-ticularly detrimental to evergreen trees and will showup as browning needles that dry from the tips down.The windward and sunny, southwest sides of

trees are particularly at risk for browning needlesand for bark injury called “sunscald.” The windyconditions that are common in many parts of Idahowill intensify injury due to cold temperatures andphysiological drought. To prevent this damage, supply ample moisture

during the growing season. However, it is advisableto cut back on moisture in late August and earlySeptember in order to allow the plant to enter itsdormancy. Water well again in late October or earlyNovember after the plant has become completelydormant. Plant roots are still active up until theground freezes and soil temperatures drop below40°F.Frost heavingAlternate freezing and thawing can force some

plants completely out of the soil. This is called“heaving,” and young plants without a well-estab-lished root system are particularly susceptible to thistype of damage, especially when planted in the fall.Spring frostsCold temperatures will freeze tender transplants,

emerging seedlings, and opening buds in the spring.Fruit buds are easily frozen once they begin toexpand and bloom. Even expanding leaf buds canbe frozen when unusually cold spring temperaturesoccur. On still nights, when temperatures hover nearfreezing, cold air, which is heavier than warm air,will settle to the bottom of valleys and depressions.These cold spots are called “frost pockets” and mayresult in cold damage to plants in that area.Temperature modificationModifying high temperaturesYou can modify high temperatures by shading

plants with larger plants or with structures such as alath houses. Shade cloth suspended over plants willalso moderate temperature. Cooler conditions existon the shady side of a building and under trees.

Plants poorly adapted to high temperatures are notgood choices in hot, dry areas because extrememeasures must be taken to ensure their survival.Modifying low temperaturesThere are many ways to avoid or modify cold

temperatures. The most obvious are planting frost-susceptible annual crops after all frost danger is pastand selecting perennial plants that are adapted to thecold temperatures in your area. Other methods ofmodifying cold temperatures include the following:• Using covers or heat sinks. Surround theplants with medium- to large-sized rocks toabsorb heat or cover them with fabric row cov-ers, plastic sheeting, or waxed paper cloches inearly spring or when frosts are predicted.These techniques reduce outgoing stored solarradiation from soil, rocks, and plants.Depending on the type of cover, you can gain2° to 6°F of nighttime warmth. Remember thatduring sunny days it may be 20° to 25°Fwarmer under the cover, which may requireventing to keep plants from becoming too hot.

• Mulching. Use a covering of mulch to modifysoil temperature. Applied soon after the groundfreezes in early winter, mulch will keep thesoil frozen and the covered plant crowns at aconsistent cold temperature to prevent winterdamage. Applied in the spring after the soilwarms, mulch moderates soil temperatureextremes during the growing season.Certain dark or colored mulches can warm

the soil early and maintain warmer tempera-tures when the weather is still cool. To be mosteffective, an organic mulch layer should be 3to 4 inches deep. Small stones can be used asmulch to gather heat around plants.

• Using heaters or fans. Protecting tree fruitsfrom early spring frosts is done in orchardsusing heaters or large fans. This equipmentstirs the air and prevents an air “inversion,”when cooler air is trapped under a layer ofwarm air.

• Sprinkling. When liquid water changes tosolid ice, it releases heat. When water is sprin-kled on plants as they cool, the heat of freez-ing will keep the plant surface at or near 32°F.This technique is often used in orchards duringbloom time when frost or cold temperaturesare predicted.


ROLE OF LIGHT IN HORTICULTURE

Light is the part of the sun’s energy visible to thehuman eye. Solar radiation reaching the earthincludes some light near and on either side of thevisible light spectrum. Plants use mostly those lightrays that can be seen (figure 4).Light qualityWater vapor in the air acts as a prism to separate

light into its various wavelength components. Thehuman eye interprets these wavelengths as color.Beginning with the longest visible wavelength, therays become shorter through the rainbow color spec-trum: red, orange, yellow, green, blue, indigo, andviolet. Violet rays are the shortest and are slightlylonger than ultraviolet rays, which cause sunburn.The following rays are used by plants in physio-

logical processes (figure 4):• Violet. These are important for the develop-ment of red pigments in plants like apples. Athigher elevation, there is less atmosphere to

screen out the violet and ultraviolet, resultingin well-colored apples (and maybe sunburn onour skin!). Indigo and violet rays are alsoresponsible for bending flower heads and otherplant parts toward the sun (phototropism).

• Blue-violet and orange-red. These rays pro-vide the light energy for photosynthesis. Infact, plants appear green to the human eyesimply because plant pigments in leaves do notabsorb and use green light for photosynthesis;instead, it is reflected back to our eyes.

• Orange-red and far-red (longer than red).This part of the spectrum is absorbed by plantsand produces the day length response (pho-toperiodism).

Supplemental light varies in quality, with fluores-cent or cool white bulbs emitting wavelengths in theblue range. Incandescent light is high in the red andorange ranges but also emits the longer heat wavesand is too warm to be useful for plant growth. Thelight bulbs specifically designed for plants are bal-anced in the wavelengths used by plants.


Figure 4. Wavelengths and the responses of plants to the visible rays. (Reprinted from Bienz, D.R. 1980. The whyand how of home horticulture. San Francisco: W.H. Freeman and Company.)

Light intensityGardeners generally use foot-candles to measure

intensity or concentration of light, even though thefoot-candle is an older unit based on English meas-urements (amount of light falling on 1 square footfrom a candle burning 1 foot away). You will alsohear the term lux to indicate the amount of light thatfalls on a surface. Lux is a metric unit equal to 1lumen per square meter. One foot-candle is 10.76lux. Physicists use a more precise mathematicalmeasure (millimoles per square meter per second).Gardeners use foot-candles because many existinglight meters are calibrated in foot-candles.In full sunlight at noon on a summer day in the

desert, light intensity measures about 12,000 to15,000 foot-candles, possibly as high as 20,000foot-candles. Light intensity is less in the morningand late afternoon because light from the sun reach-es the earth at an oblique angle, filtered throughmore layers of atmosphere before reaching the sur-face. For the same reason, light intensity is muchlower in winter in the northern hemisphere. On aheavily overcast winter day at noon, light intensitymay be as low as 600 to 900 foot-candles in north-ern latitudes. The interior of a well-lighted homewill measure from 50 to 300 foot-candles.Tropical plants, like many of our houseplants,

thrive in nature under a jungle canopy that providesvery low light intensity. Plants not from the junglesare able to grow in and use very bright or intenselight. Most crop plants use about 1,200 foot-candlesof light, but they will grow better in light up to4,000 foot-candles because of the shading from sur-rounding leaves. Plants and leaves adapted to lowlight intensity will sunburn, wither, or die if they aresuddenly exposed to higher light intensity. Lightintensity can be decreased through shading orincreased with supplemental lights, reflective mate-rial, or white backgrounds.Insufficient light will cause plants to stretch and

become “gangly” or unusually long. Nodes will befar apart, leaves broad and thin, and the plants willhave a loose, open structure. Reduced light intensitycan also induce succulence.Light durationPlants respond to particular day lengths. Actually,

processes that occur during an uninterrupted darkperiod bring about the plant’s response, not process-

es that occur during the day, but we use day lengthas the measure. How plants respond to day length ismodified somewhat by temperature. Depending onthe plant type, blooming, for example, may bedelayed or sped up by warm or cool weather. Thebloom period can be intentionally altered with spe-cific light treatments or unintentionally altered bylights coming from streetlights or other artificialsources. Long-day plantsLong-day plants respond to day lengths longer

than a certain minimum (usually about 12 hours).Spinach, for example, is a long-day plant and, ifplanted late in the spring, it will make a flower stalkbefore producing leaves.Onion bulbing is a long-day response. Onions

produce bulbs during long days, and onion typesthat do well in northern latitudes require longer days(16 hours) compared with those adapted to moresouthern locations (11 to 12 hours). Northern-typeonions will not produce bulbs in southerly locationsbecause the days never get long enough! Southerntypes, when grown in the north, produce bulbsbefore the plant reaches a size adequate to develop agood-sized onion bulb.Short-day plantsShort-day plants respond to day lengths shorter

than a certain maximum (less than about 12 hours).Chrysanthemums are short-day plants and willbloom when day lengths range from 16 hours downto 7 hours depending on the cultivar. They grow anddevelop plant tissue and carbohydrate reserves dur-ing the spring and summer to support fall flowering.Poinsettia is another short-day plant.Day-neutral plantsDay-neutral plants do not respond to day length,

but must have sufficient growth to support flower-ing. Temperatures must also be acceptable, roughlyabove 32°F and below 96°F. Geraniums and certainstrawberry cultivars are examples of day-neutralplants.



FURTHER READING AND RESOURCES

BooksBenz, D. R. 1980. “Climate, Temperature and Light.”Chapter 7 in The Why and How of Home Horticulture.San Francisco: W.H. Freeman and Company.

Booklets and PamphletsUniversity of Idaho ExtensionPNW 221 Cold Resistance of Stone Fruit Flower BudsPNW 497 Short-Season Vegetable Gardening

Web SitesNational Weather Service, Climate Prediction Center.http://www.cpc.ncep.noaa.gov/index.php

National Oceanic and Atmospheric Administration, U.S.Department of Commerce. http://www.noaa.gov/cli-mate.html

Idaho Climate Summaries, Western Regional ClimateCenter.http://www.wrcc.dri.edu/summary/climsmid.html

Freeze/Frost Maps, National Climatic Data Center,NOAA Satellite and Information Service.http://www.ncdc.noaa.gov/oa/climate/freezefrost/frostfreemaps.html.

Online Phenology and Degree-day Models, IntegratedPlant Protection Center, Oregon State University.http://ippc2.orst.edu/cgi-bin/ddmodel.pl

Published 1993. Revised 2008.

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