Controlling the Greenhouse Environment

8/2/2019 Controlling the Greenhouse Environment

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Once you have picked the right location, builta greenhouse structure that ts your needs,

developed your growing medium and pickedappropriate plant varieties that respond successullyto long day lengths, it is now essential to ocus yourattention on the control o the greenhouse environment.This is especially important i you are to be successulin getting the most out o your construction investmentand eorts. By optimizing light, temperature andhumidity, in conjunction with the proper ertilization,watering and selection o adapted varieties, an endlessarray o growing opportunities await the Alaskagreenhouse gardener and commercial producer.

LightOperating a greenhouse during the typical Alaskaoutdoor growing season (May through September)requires no light supplementation. The two mainreasons to use supplemental lighting are to increaseplant growth during low-light levels and to manipulatethe photoperiod (ratio o day length to night length) toeither initiate or delay fowering. Comparisons betweenseveral light supplementation sources are oered below(Table 1). All components o the lamp should be UL/CSA approved.

Controlling the Greenhouse

Environment

by Thomas R. Jahns, Extension Faculty, Agriculture and Horticulture, and

Jeff Smeenk, Extension Horticulture Specialist

HGA-00336

IncandescentThe incandescent light bulb type o light source maybe useul in controlling day length, but it oers littlehelp as a grow light. Since the major portion o energygoing into incandescent bulbs is released as heat ratherthan as light, conventional light bulbs are primarily usedto manipulate photoperiod rather than as supplementalighting to enhance growth.

Quartz-Halogen (Spotlights)The quartz-halogen spotlight type bulbs producea whiter light and are more electricity-ecient thanthe incandescent bulb, but because they, too, delivera point-source type o illumination they are ar lessecient than other available bulbs.

FluorescentFluorescent bulbs are the most commonly used lightsource or the home gardener. Fluorescent bulbsproduce a linear light that gives o more ecient anduniorm lighting than incandescent types. Fluorescentbulbs are available in 28225 watt congurations andcan be stacked in banks to enhance coverage areas. Inlate winter/early spring, most gardeners who start plantsrom seed utilize some type o inexpensive fuorescentshop light system as their light source.


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The 2840 watt bulbs work great or germinating seedsand growing seedlings, which do not require the moreexpensive, higher intensity, ull spectrum grow lights.The key in the use o these low-output lights is that youmust orient them to within an inch o the top o theplants to capture all the light intensity (lumens) theyhave to oer. I they are not placed close to the plants,internodal elongation will occur, producing weak,

spindly plants that will generally not perorm well.While houseplants, seedlings and low-light foweringplants can fourish under fuorescent lights, they all arshort o the light requirements needed to produce mostmature fowering and ruiting plants. White refectorsor refectors made rom aluminum oil may aid inmaximizing what little light is available rom these low-intensity bulbs (Table 2).

Fluorescent lighting is rarely used in commercial

greenhouses or producing mature fowering or ruitingplants because o their low-intensity output and thesunlight-shading eects that their xtures and refectorscreate. Their primary use in commercial operationsis in providing light or germination benches. Under

these conditions the lamps are hung very close to theseedlings. Because seedlings require much lower lightlevels than mature plants, several fuorescent lampsalone are capable o meeting their light requirements

As the seedlings enter their vegetative growth stagetheir light requirements rapidly outpace the fuorescenlamps capability o providing enough light intensityand quality to meet their demands. To sustain this rapid

growth past the seedling stage, plants need to be movedinto a greenhouse where direct sunlight or high-outpuarticial lighting is available.

High-Intensity Discharge (HID)High-intensity discharge bulbs are the most costly topurchase and operate but oer the highest quality lightoutput o all grow-light bulbs. Where plant appearanceis critical and natural sunlight limited, metal halidebulbs should be used. Due to their energy eciency

and the general quality o the light spectrum emitted(yellowish light), high-pressure sodium (HP sodium)lights are the most commonly used types ound incommercial greenhouses.

Table 1. Light Source Comparisons

Light Source Lamp Wattage Total Wattage Initial Lumens*Avg. Life(hrs.)

Incandescent 40 40 460 1,500

100 100 1,620 1,000150 150 2,850 1,000200 200 3,350 1,000

Halogen 75 75 1,000 2,000250 250 5,000 4,000

Fluorescent 40 48 3,150 7,500Cool White (CW) 75 84 6,300 12,000CWhigh output 110 138 9,200 12,000

CW Very High output215 262 15,500 9,000

Gro-lux 40 47 925 12,000

Wide Spectrum 40 47 1,700 12,000Metal Halide 250 295 20,500 10,000

400 425 31,500 20,0001,000 1,080 110,000 11,000

High-Pressure Sodium 250 300 27,500 24,000400 440 48,000 24,000

1,000 1,145 140,000 24,000*Lumens: Units of measurement of light. One lumen = one foot-candle falling on 1 square foot of area.

Source: Adapted from Greenhouse Engineering(NRAES-33); Poot, J. 1984. Application of Growlight in

Greenhouses; andHorticultural Lightingby Philips Lighting Company, Somerset, N.J.


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Standard 40W T12 FixturesDistance from Lamp (ft.) Without Reectors With Reectors0.5 500 7001.0 260 4002.0 110 180

3.0 60 1004.0 40 60*fc= foot candles.

Source: Cathey, H.M. and L.E. Campbell. 1978.Indoor Gardening Articial Lighting,

Terrariums, Hanging Baskets and Plant Selection. U.S. Department of Agriculture.

Table 2. Illumination at Various Distances from Either Cool White or Warm WhiteFluorescent Lamps (fc)*

Picture 1. HID lamp with bulb, reector and ballast/ capacitor

Depending on the crop needs, natural light availabilityand greenhouse design, there are several wattageso HID lamps available. These range rom 1501000watts. While the crop light requirements are normallydocumented and the natural lighting durationmeasurable, how the greenhouse design impactsthe choice o lamp wattage is not readily apparent.

An ecient greenhouse operation will maximize itslighting by assuring a uniorm level o light across allo its growing benches. I there are sections that receiveless light, the plants in these areas will become etiolated(leggy) and the crop will not develop the uniormheight that the market desires. In a greenhouse with alow roo it may be challenging to achieve uniorm lightwith 1000-watt xtures since the distance that theycan be moved above the crop is limited. Although it is

more costly to purchase several smaller lights than onelarge one, multiple lights allow additional fexibility inachieving uniorm light dispersal.

In addition to the wattage choices (lamp intensity) thelamp layout design necessary to achieve uniorm lightlevels also depends on the xture type, the refectorshape and the distance rom the bottom o the refectorto the top o the crop at each stage. These criteria willdetermine the distance needed between lamp xtures,which will, in turn, determine how many lamps will be

necessary to provide the desired amount o supplementallight.

HID lights have a lamp, refector, ballast and capacitor(Picture 1). They operate at high temperatures, whichcan be managed with proper ventilation to avoidpremature ailure. The design and placement o the lamprefector is also a critical actor. The refector controls theuniormity o the light pattern and the amount o lightrefected onto the crop. Unortunately, the refector also

blocks a certain amount o sunlight rom reaching thecrop. Another important refector design criteria is tostop any light rom refecting back into the bulb, whichincreases heat and can lead to premature bulb ailure.

Plant growth and photoperiod manipulation havedierent supplemental lighting requirements toaccomplish their desired goals. Photoperiod can beeasily manipulated using ordinary incandescent lightbulbs. The photoperiod, or day length, aects manyplants, and in Alaska may limit the survival o eventhe most temperature-hardy outdoor perennials. Day-length requirements vary, depending upon species(Table 3).

In the greenhouse environment, day length is easily

extended with lights and an adjustable 24-hourtimer. By stringing 60-watt incandescent bulbs (withrefectors) our eet apart down a our-oot-wide bedand supported no higher than ve eet above the plantsday length can successully be lengthened (please seelocal building codes or sae application) (Bartok2000). Reducing day length in Alaska is more dicult


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Minimum Light Levels Day lengthPlant Species Crop Stage Watts/sq. m Foot-candles* HoursAlstromeria Cultivation 3,000 120 13Anthirrhinum Propagation 9,000 370 16

Cultivation 4,500 180 24Azalea Propagation 6,000 240 18

Forcing 3,000 120 16Bedding Plants Seedlings 6,000 240 16Begonia Stock/Prop. 6,000 240 14Bromeliads Propagation 6,000 240 18

Forcing 4,500 180 24Cacteae Propagation 9,000 370 18Calceolaria Forcing 3,000 120 24Camellia Cultivation 4,500 180 14Chrysanthemum Stock 9,000 370 20

Rooting 6,000 240 20

Cut Flowers 4,500 180 18Cyclamen Propagation 6,000 240 18Gesneria Propagation 6,000 240 18

Cultivation 4,500 180 18Kalanchoe Stock 6,000 240 18

Rooting/Prop. 6,000 240 16Ferns Propagation 6,000 240 18Foliage Cuttings/Prop. 6,000 240 16Geranium Stock 7,000 650 16

Cuttings 9,000 370 16Gerbera Stock/Prop. 6,000 240 16

Gladiolus Cut Flowers 8,000 740 16Nursery Stock Rooting/Prop. 7,500 700 24Orchids Production 9,000 370 16Rose Cultivation 6,000 240 24Sinningia(gloxinia) Propagation 6,000 240 18Stephanotis Cultivation 4,500 485 18Succulents Seedlings 9,000 370 16Cucumbers Propagation 4,500 485 18

LettuceSeedlings(growth room) 25,000 2,300 24

Crop Production(GH) 7,000 650 16

Strawberries Fruit Prod. 350 30 8Tomatoes Seedlings 6,000 240 16*Foot-candles: a foot-candle is how bright the light is one foot away from the source.

Source:Energy Conservation for Commercial Greenhouses (NRAES-3). Adapted fromApplication of Growlight

in Greenhouses, PL Light Systems, St. Catherines, Ont., andPhilips Lighting Application Guide, Philips Lighting

Co., Somerset, N.J.

Table 3. Recommendations for Supplemental Greenhouse Plant Lighting


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but can be accomplished by covering plants (to keepthem in total darkness) or the required period o time.

Warning: only 12 oot candles o light will cause alight response in plants, so total darkness is requiredthroughout the entire light-reduction period to avoidreduced day-length manipulation ailures.

When the main purpose o supplemental lighting is to

enhance plant growth, several other actors must beconsidered. These include Crop light requirements Desired light intensity and quality Uniormity o light pattern (number o xtures

required) Operating costs Financial return on investment

Light selection will depend upon the plant application

to be utilized. First, the minimum light supplementationrequirements to grow a given plant species must bedetermined (Table 3). Next, energy values should beobtained or the light xtures being considered (Table4).

The number o xtures required equals the required lightlevel multiplied by the surace area to be illuminateddivided by the eective fux. Eective fux is assumedto be approximately 80 percent o the lamp fux orhigh-intensity discharge lamps. It may vary rom a low

o 50 percent to a high o 70 percent or incandescentand fuorescent lamps, depending upon refectors.

Incandescent Flourescent HP sodium Metal halide

Light Source I150W F40CW/40W 400W 400WTotal Input (W) 150 48 440 425Lamp Flux (lm) 2,850 3,150 48,000 31,500

Effective Flux (lm)* ___ ___ 38,400 25,200

Lamp Flux (mW) 11,970 9,135 110,400 88,200

Conversion Factor(mW/lm)

4.2 2.9 2.3 2.8

Effective Flux (mW)* ___ ___ 88,300 70,600*Theeffectiveuxisassumedtobeapproximately80%ofthelampuxforthetwoHIDlamps.Itcanvaryfrom

alowoflessthan50%toahighof70%forincandescentanduorescentlamps,dependingonreectors.

Source: Adapted with permission from Poot, J. 1984.Application of Growlights in Greenhouses andHorticultural

Lighting, Philips Lighting Co., Somerset, N.J.

Table 4. Energy Values for Four Illumination Sources

N= light level x surace areaeective fux

I you were going to grow lettuce seedlings in a 1.5 mx 3 m bed under lights, you would need the ollowinglight system (or equivalent) to accomplish this goal: lightlevel 25,000 mW/sq. m (Table 3) x 4.5 sq. m (1.5m x3m) = 112,500 mW total. Divide that by eective fux

o the lamps to be used (i we use 400W high-pressuresodium bulbs we would need 112,500/38,400 (Table4) = 2.93 (or three 400W sodium lamps).

Light unit PAR comparisons and conversions between HPsodium, metal halide and sunlight are oered in Table 5.

The cost o supplemental lighting should be consideredprior to the purchase o a light supplementationsystem. The HID bulbs in particular, may be extremely

expensive to operate i multiday, long-hour applicationsare needed. The ollowing ormula will help to calculateoperating costs: operating costs = xture wattage xelectricity cost x hours.

Fixture wattage = number o bulbs x (bulb wattage +xture wattage). For electricity costs reer to a recentelectric bill and divide total costs by number o kilowatt-hours o electricity used. (For example, i your monthlyelectric bill is $125 and you averaged 650 KWhr permonth, then your average cost per kilowatt hour is 20

cents ($125/650=$0.20 / KWhr), which was the 2009Homer Electric Company consumer rate or Kenai

Alaska.


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I we return to our lettuce-seedling growing exampleabove, we needed three 400W HP sodium bulbs to coverour 4.5 sq. m growing area. Each bulb xture requiresan additional 40 ballast watts (Table 1), so we woulduse a total o 400 + 40 x 3 = 1,320 watts per KWhr.Lettuce seedlings require 24 hours o daylight (Table3), so we would need to supplement approximately 12hours per day (Table 6) o supplemental daylight (i we

were to start these seedlings in mid-March).

Operating costs = 1,320 watts x $0.15/kilowatt-hour x12 hours = $2.38/day (1000 watts/kilowatt).

Shading With Alaskas intense summer sunshine and extremeday lengths, greenhouse shading may be required tomaximize plant perormance and control temperature.Table 7 oers shading suggestions or an assortmento common ornamental plants grown under northeast

United States environmental conditions. While the degreeo required shading may be higher in the Northeast,compared to Alaska, shading is important in controllinggreenhouse temperatures in Alaska, especially duringour clear, sunny days in summer. The addition o shadecloth to the outside o the greenhouse would help coolthe greenhouse during the heat o the day.

There are two distinct categories o shading materialsor the greenhouse: in one category shading compounds

Month Fairbanks KenaiJanuary 5.3 6.7February 8.5 9.1March 11.7 11.8April 15.2 14.7

May 18.8 16.8June 21.4 18.3July 19.9 18.1August 16.5 15.6September 13.0 12.9October 9.6 10.1November 6.3 7.4December 4.0 5.9

Adaptedfrom2004AstronomicalApplicationsDepartment,

U.S. Naval Observatory, Washington, D.C.

Table 6. Average Monthly Day-length Hours forFairbanks and Kenai, Alaska

Unit Type of Measurement Main useCompared with 1 foot-candle

Sunlight HP Sodium Metal Halide

Foot-candles Visible (human eye) Industry (U.S.) 1 1 1

Lux Visible (human eye)Industry(Europe)

10.76 10.76 10.76

mol.m-2 s-1

of PAR(400-700 nm)

Quanta of light in

PAR range

Horticulture

research 0.20 0.13 0.15

Moles/day(PAR)

daily light integral:accumulated PARlight during an entireday

Horticultureresearch

Foot-candles x0.00071 xhours oflight



Watts/m2(PAR)

Energy in PAR rangeEngineers,research

0.043 0.026 0.033

Watts/m2(total energy)

Total energyEngineers,research

0.101 0.074 0.089

Source:Fisher,P.,Donnelly,C.andJ.Faust.2001.Evaluating Supplemental Light for Your Greenhouse. Ohio

Florists Assn. Bulletin.

Table 5. Conversion Between Different Light Units

are directly applied to the outer glazing material; in theother category shade abrics are draped over or underthe glazing material.

There are many inside and outside installation optionsso check with a greenhouse equipment supplier or asystem tailored to your needs.

Glazing materials oer some inherent shadingcharacteristics that should not be ignored. Table 8 oerssome examples o dierent glazing materials.


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Heating and Heat LossGreenhouse heating is required in Alaska when extendingthe growing season (both early and late), and will otenenhance summer plant growth by keeping nighttimetemperatures above ambient levels, depending uponthe plant species grown. Nationally, the cost o heating

comprises 7080 percent o the annual energy dollarsspent in a commercial greenhouse operation (NaturalResource, Agriculture and Engineering Service, 2001).In Alaska, it may even be greater, and a major reasonwhy most commercial greenhouses are seasonally run.

Glazings or greenhouse coverings, cloud cover and theambient temperature determine the amount o radiantheat loss. Radiant heat is the amount o solar radiation,inrared radiation and light that enters and is absorbedby objects in the greenhouse (which warm up) and thenis reradiated back into the greenhouse or beyond. Glassor rigid plastic glazing is the best at trapping thermalradiation (greater than 96 percent, "GreenhouseEect"), while single-layer polyethylene is one o theleast ecient, allowing more than 50 percent o the

Suggested degree of shade Type of plants2535% Geraniums, chrysanthemums, snapdragons4550% Bedding plants, lilies, caladiums5055% Azaleas, begonias, gloxinias, African violets, poinsettias5560% Orchids, pachysandra, ivy, bromeliads, fcus6065% Rhododendron, dieffenbachia

7075% Fern, philodendron, dracaena7580% PalmsSource: Greenhouses for Homeowners and Gardeners (NRAES-137)

Table 7. Shading Requirements of Select Plants Grown in N.E. United States

Table 8. Greenhouse Glazing Light Transmittance ValuesGlazing Percent Light TransmittanceGlass-single 8595Glass-Factory sealed double 7075

Polyethylene-single 8090 (new)Polyethylene-double 6080Polyethylene-corrugated high density 7075Laminated acrylic/poly flm-double 87Impact modifed acrylic-double 85Fiber reinforced plastic 8590 (new)Polycarbonate-double wall rigid 83Source:Adaptedfrom:Bellows,B.2003. Solar Greenhouses Horticulture Resource List. ATTRA National

SustainableAgricultureInformationService.

thermal radiation to escape. Polyethylene, treated withinrared radiation (IR) absorbing materials, improvesthermal radiation retention by approximately 20percent (Natural Resource, Agriculture and EngineeringService, 2001). For the minimal cost dierence, the IR-Polyethylene glazing should be considered, although it

may be limited in availability (check with your localcommercial greenhouse operators). Double-layerpolyethylene glazing is commonly used commerciallyin Alaska. It oers thermal radiation retention that is 67percent more ecient than single-layer polyethylene.

In either case, polyethylene glazings should have a UVinhibitor built into them i more than one season ouse is required. I on a tight budget, check with yourlocal Alaska greenhouse operators who utilize UV-inhibited polyethylene glazings. Used glazing can otenbe purchased or a raction o the new cost and willgenerally give several years o excellent service to thegreenhouse gardener.


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Condensation orming on the underside o polyethyleneglazing can help to reduce thermal radiation escape 5075 percent (light to heavy condensation, respectively).Unortunately, this thermal radiation reduction comesat the expense o light and solar radiation reception.In Alaska, glazing should be kept clear. Normally,condensation is not an issue, since most Alaskagreenhouses use at least a 6/12 roo pitch or snow

shedding, which also helps to promote condensationruno.

Convection and inltration also infuence heat lossin Alaska greenhouses. Convection is the heat lost bycooler wind blowing over the warmer surace o theglazing. Inltration is the air exchanged between theinside and outside o the greenhouse through holes,cracks and crevices. The heat requirement doubleswhen wind speed increases rom 0 to 15 mph.

Heater SelectionGas hot-air heaters come in a wide variety ocongurations and uel options. Natural and bottledgas hot-air heaters are some o the more economicaland ecient heater types available or Alaskagreenhouses. Unit heaters, whether hanging, on-the-wall or reestanding, have proven to be ast heating,economically priced and low-maintenance heatingsources or Alaska greenhouses.

Hot water heaters and boilers that provide radiator,radiant foor and bench heat continue to gain acceptancein the greenhouse, as well as the home and garage.Both gas and electrically red, these heating sourcesprovide even heating. The boiler units should use aclosed system. Standard home hot water heaters may

Zone*A B C D E F G

-40F -30F -20F -10F -0F 15F 30FLean-to GreenhouseSingle glazing 370 330 290 250 210 175 140

Double glazing 250 220 190 160 130 100 70Freestanding GreenhouseSingle glazing 400 360 320 280 240 180 120Double glazing 250 225 200 175 150 110 85*Zone.Thetableassumesa60F inside night temperature and the following minimum temperatures for your given

area;individuallocationsmayhavetemperatureslowerthanthis.Inborderlinelocations,selectthecolderzone.

Source: Adapted from Greenhouses for Homeowners and Gardeners (NRAES-137).

Table 9. Estimated Heat Requirements in BTUs/Square Foot of Surface Area

also be used and in some cases oer a dual purpose, toproduce hot water or both heating the air and heatingaccessible tap water that can be tempered or plantwatering. This type o system must be protected romreezing until it is winterized by draining water andblowing air through all lines. While not as responsiveto rapid temperature adjustments as hot-air heatershot water baseboard and radiant foor heaters provide a

more uniorm temperature, oer multiple temperaturezone capabilities within a greenhouse and can be runat lower, uniorm temperatures in spring and all, thantheir hot-air counterparts.

Electric heaters, while more costly to run, aregenerally inexpensive to purchase and may be justright or supplementing heat or a ew pre- and post-growing season nights, or when evening temperaturesdrop below a sae margin during the growing season

Whether you choose utility heaters, baseboard heatersor inrared heaters, electric heaters work well in agreenhouse. Research at the University o Connecticutound an electrical cost savings o 25 percent whenheating an 8 x 12 greenhouse with inrared vs. utilityhot-air heaters (Bartok, 2000).

Wood heating makes both a good main and supplementaheat source. Although saety is an issue, i the greenhouseto be heated is large enough, a wood stove installationmakes an excellent heat source. Remember, a burning

wood stove should always be attended. At nighttime, agas or electric back-up system should be utilized. Heatregulation is a drawback when using a wood stove, evena thermostatically controlled one. Pellet stoves, whileexpensive, are also worth considering.


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Fuel heaters (oil, kerosene, white gas and diesel)without ventilation are not recommended orgreenhouse use. 1-K grade portable kerosene heatershave been successully used in greenhouses, i kept wellmaintained. These are especially useul in emergencysituations. Oil-red urnaces are usually too large to tinto a greenhouse structure, but may be advantageousin a situation where both the house and greenhouse

share a common heat source.

By utilizing the inormation ound in Table 9 anddetermining the square ootage o your greenhouse(rectangle or square layout: length x width) a quickestimate o heater requirements can be obtained.For example, i your reestanding greenhouse foordimensions measure 12 t. x 16 t., you have 192 sq. t.o foor space. I you are supplementing heat all winter(Zone B: -30F minimum temperature) and have a single

glazing on your greenhouse, it will require a heatercapable o (192 x 360 Btu/sq. t.) 69,120 Btu/hr. I youare supplying heat only rom April through October inSouthcentral Alaska, it will require a heater capable o

Greenhouse glazing or wall materialsU-value:(Btu/(hours x Fx square feet)

Glass - single 1.2

Plastic flm - single 1.25Fiberglass reinforced plastic - single 1.2Polycarbonate - single 1.2Glass single w/ thermal blanket 0.5Plastic flm - double 0.8Acrylic or polycarbonate - double 0.6Plastic flm double w/ thermal blanket 0.4Standard concrete blocks, 8 inches 0.5Poured concrete, 6 feet 0.75Softwood lumber, 1-inch thick 1.10Concrete block, 8 inches + 2 inches foamed

urethane board

0.07

Concrete block, 8 inches + 2 inches foamedpolystyrene board

0.10

Poured concrete, 6 inches + 2 inches foamedurethane board

0.07

Wood-framed wall with 1.5-inch thick urethaneboard

0.12

Perimeter, uninsulated 0.8Perimeter, insulated: 2-inch foam board, 24 inchesdeep

0.4

Source: Greenhouses for Homeowners and Gardeners (NRAES-137)

Table 10. Heat Transfer Coefcients

approximately (192 x 120 Btu/sq. t.) 23,040 Btu/hr(with the 120 Btu/sq. t. value coming rom Zone G.

A more accurate and complicated method or calculatingheat loss rom a greenhouse utilizes the ollowingormula: HL = SA x U x TD, where HL is heat loss; SAis surace area o greenhouse; U is the heat loss actoror the roo and wall material; and TD represents the

dierence between desired night temperature andwinter design temperature or your area.

Heat loss is a measure o the amount o heat neededto maintain the desired temperature or one hourThe heat loss actor depends on the glazing insulationproperties. The less insulative the glazing material isthe higher the value. Table 10 lists the U-value or someo the more popular glazing and wall materials used.

By matching Btu/hr. greenhouse heating requirements(heat loss) with Btu/hr. heater output, a correctly sizedheater can be obtained (when shopping or heaters makesure you compare heater output values not heater inputvalues).

By comparing the quick (Table 9)versus comprehensive (examples 1 and2 below) heater requirement ormulaswe nd that the quick methodoverestimated the reestanding 12 t. x

16 t. greenhouse heater requirementsby approximately 6,000 Btu/hr., whichis certainly ballpark. Working throughthe lean-to examples or both the quickand the comprehensive methods, youwill nd a similar overestimation romthe quick method, although againwithin acceptable levels.

Planning or the occasional, abnormallylow temperature heating requirementcan make the dierence between savingand losing a crop. An oversized heateris always better than an undersizedheater in Alaska. The importance oa heater backup system is crucial tocommercial, year-round greenhouseheating in Alaska, but is rarely an issueor the seasonal greenhouse gardenerCommercially, more than one heatingsystem is commonly used. These


10/2010

multiple heater systems are oten set up to ecientlyutilize one heater as the primary heat source, with abackup system available or extremely low outsidetemperature drops or primary heater ailures.

Compartmentalization o plant germination andgrow areas can reduce the need or heating the entiregreenhouse early season, but again is more o a

commercial greenhouse issue. Alaska home greenhousegardeners oten start their seedlings under grow lightsin the garage or basement, making the transition tothe greenhouse much later and less energy demanding

Example 1. Lean-to 12 ft. x 16 ft.Greenhouse (adapted rom Greenhouses or Homeowners andGardeners, NRAES-137).Surace area:End walls: 2 x 6 t. x 12 t. = 144 sq. t.Sidewall: 6 t. x 16 t. = 96 sq. t.

Peaks (endwalls) = 2 x 0.5 x 12 t. x 6 t. = 72 sq. t.Roo: 13.4 t. (a2 + b2 = c2) x 16 t. = 214.4 sq. t.Total: 526.4 sq. t.

Heat Loss (HL) = Surace Area (sq. t.) o greenhouse (SA) x Uactor (U) x Temperature dierence between minimum outsideand 60F inside (standard) (TD)Given:SA = 526.4 sq. t.U-value (single layer polycarbonate Table 10) = 1.2TD= 60F [desired inside temperature] + 30 (-30F) = 90Heat Loss = 526.4 x 1.2 x 90 = 56,851 Btu/hr.

Example 2. Freestanding 12 ft. x 16 ft. Greenhouse(adapted with permission rom Greenhouses orHomeowners and Gardeners, NRAES-137).Surace area:Ends: 2 x 6 t. x 12 t. = 144 sq. t.Sides: 2 x 6 t. x 16 t. = 192 sq. t.Peaks (end walls): 2 x 0.5 x 12 t. x 3 t. = 36 sq. t.Roo: 2 t. x 6.7 t. (a2 + b2 = c2) x 16 t. = 214.4 sq. t.Total: 586.4 sq. t.

Heat Loss (HL) = Surace area (sq. t.) o greenhouse (SA) x Uactor (U) x Temperature dierence between minimum outsideand 60F inside (standard) (TD)Given:SA=586.4 sq. t.

U-value (single layer polycarbonate Table 10) = 1.2TD= 60F [desired inside temperature] + 30 (-30F) = 90Heat Loss = 586.4 x 1.2 x 90 = 63,331 Btu/hr.

than or a commercial greenhouse operator. Heaterconvection tubing and orientation also may play animportant role in heat distribution within a largecommercial greenhouse, but are generally not issues orthe home greenhouse.

Solar heating is an alternative heat source derived romthe sun in the orm o solar radiation. This radiation is

captured in storage structures, purposely built into asolar greenhouse, that have the potential to reradiatethis energy back into the greenhouse at night. In aproperly built and oriented solar greenhouse, as much


11/2011

as two days o heat can be captured rom a March sun.Table 11 summarizes several o the radiation storagestructure options in common use.

The design o a solar greenhouse diers rom a normalgreenhouse design. One o the major dierences is in itsorientation to the sun. To maximize the suns availableenergy, a solar greenhouse should be oriented to within

20 o true south, with the ridge o the greenhouse east-west and a long southern exposure or optimal light andsolar radiation reception. The slope o the greenhouseroo glazing is also critical in a solar greenhouse. Arule o thumb or estimating the desired slope whentargeting the capture o a winter suns energy is to useyour latitude angle plus 15. In Alaska, the desiredwinter roo glazing angle alls somewhere between 75and 80. A shallower slope o 5060 is appropriate orcapturing a spring or all sun.

Except or the south-acing glazing, most o the rooand walls o a solar greenhouse are insulated. At night,an insulation blanket or other insulative-type materialsare unrolled or somehow attached to the inside o theglazing area to retain heat. Also, in a solar greenhousesupplemental heat may be required, especially whenthe weather is cloudy and solar-radiation limiting.

Table 11. Solar Heat Storage Assuming a 50F Minimum Greenhouse TemperatureWater/Air Temperature BTUs Stored

60F708090

1-Gallon Jug83

167250334

5-Gallon Jug417834

1,2511,668

60F708090

30-Gallon Barrel2,5005,0007,500

10,000

55-Gallon Barrel4,5909,170

13,76018,340

60F

708090

Concrete/Concrete Block224/cubic foot

448672896

Brick271

542813

1,084

600F708090

Rocks in Crate UnderBench

220/cubic foot440660880

Stone in Floor240/cubic foot

480720960

Adapted from Greenhouses for Homeowners and Gardeners (NRAES-137). Orignially published in Bartok, John

W., D.S. Ross, J. White, W.J. Roberts, C.A. Aldrich, and R.A. Parsons. 1982. Solar Greenhouses for the Home

(NRAES-2). Ithaca, N.Y.: Natural Resource, Agriculture and Engineering Service (out of print).

CoolingNeed for air movement in the greenhouseThere are two primary greenhouse needs that airfowmust address. When the temperature rises in thegreenhouse, there must be a way to get rid o the excessheat and replace the leaving hot air with cooler air. Thereis also the need to move air within the plant canopy

or proper gas and relative humidity control. In smalgreenhouses, a single system can service both needs. Inlarger greenhouses, there is usually an exhaust systemand a circulation system. It is most important to getthese systems to work together. Air movement systemsrange rom do-it-yoursel systems with parts salvagedrom many sources to computer controlled integratedsystems that need to be proessionally designed andinstalled.

Temperature Control Systems (Ventilation) As the sun warms the greenhouse, the temperaturinside can rise rapidly. Without a method o riddingthe excess heat the temperature can rise to a level thatis atal to the crops within. It is the primary unction othe ventilation system to prevent that drastic buildupo heat by replacing the hot air with cooler outside air.


12/2012

In some small greenhouses adequate ventilation canbe achieved by simply leaving a door or window openor by removing a section o the covering (Picture 2).Orient the structure to capture the normal summerwinds along the side and into the vent(s). Minimizeobstructions such as trees and buildings that can reduceor redirect the wind. For most medium- and large-sized greenhouses, air movement needs are met with a

combination o active and passive ventilation systems.

Passive ventilation systems operate on a combinationo two principles. First, as air heats up, it expands andincreases the pressure within the greenhouse. Thispressure is released through openings in the structure.Second, wind pressure pushes or pulls air through anopening in the greenhouse.

On sunny days in Alaska, the air in the greenhouse heats

up rapidly, the result o our intense spring and summersolar radiation. Air fows out o vents placed on theridge or end-walls o the greenhouse. The lost hot air isreplaced by cooler air coming in through side vents (orother uncontrolled leaks in the structure), thus coolingthe interior o the structure (Figure 1).

Wind action through an open door or window alsocauses signicant mixing o the hot air within a housewith the cooler outside air. We all have experienceda wind blowing in through a door, but signicant air

exchange also takes place when the wind is blowingin the same direction as an open, out-acing door. Themassive movement o the air in the wind can draw airout o the house. The hot air pulled out will be replacedby cooler, outside air. This wind induced cooling can besignicant.

One o the most ecient greenhouse ventilation systemsis derived rom the use o roo-ridge vents that ollowpart o or the total length o the roo ridge (Picture 3).This passive system oers outstanding ventilation whencombined with adequate intake vents. This type oventilation is oten ound in commercial greenhouses.

As greenhouse size increases, the need or roo- and/oran- assisted ventilation also increases. One o the leastexpensive greenhouse ventilation systems is the roll-upor drop-down wall design. This system utilizes plasticsheeting or wall (and in many cases roong) material.

While temporary greenhouses may utilize Visqueen-type materials, their longevity is oten seasonal at

Picture 2. Door vents and high ceilings in small greenhouses can

provide adequate ventilation.

Figure 1. Adapted with permission from Greenhouses for Hom-

eowners and Gardeners (NRAES-137).

best. UV-inhibited, commercial greenhouse-grade

lm plastics have a much greater usable liespan. Thistype o system is regularly incorporated in low-budgetgreenhouses utilizing a wooden rame, over which theplastic glazing is placed. The nice thing about this typeo design is that the walls can be rolled up or down(design dependent) to achieve desirable ventilation(Picture 4).

Mechanical ventilation systems are primarily a an, orseries o ans, a control system and a vent, or series ovents, to allow air movement. Fans used in conjunction

with adequate air-intake systems may enhancetemperature control over natural ventilation systems.

FansFans can either push resh air into a greenhouse or theycan exhaust the greenhouse air to the outside, which


13/2013

will pull resh air in through the vents. Although thereis no special preerence, most commercial operationsuse exhausting ans and inletting vents to allowreplacement air to come in rom outside.

In a reestanding greenhouse, it is customary to put theans on one end o the house and the vents on the otherend (Picture 5). This arrangement gives a good airfow

across the greenhouse crop. Locate the ans so theyexhaust with the direction o the prevailing winds. Thiswill cause the normal wind patterns to help the an.The ans would have to work much harder to orce theexhaust air against the wind pressure. Winds orientedinto vents and out o exhaust ans will yield greaterthan a 10-percent increase in eciency compared tothe opposite wind orientation.

A strategically placed door opening can enhance orhinder the ans operation. An open door in the side

Picture 3: Roof ridge vents are shown at Trinity Greenhouse in

Kenai.

Picture 4: The drop-down wall design provides inexpensive but

effective ventilation.

opposite the an encourages airfow through thegreenhouse. An open door near the an encourages airto vent through the easiest method possible and nottravel the length o the house to exit through the ventas desired.

The an capacity must match the louver or vent-fowcapability. Fan capacity the volume o air a an can

move in one minute is measured in cubic eet perminute (cm). For Alaska, the movement o 12 cubiceet o air volume per minute, per square oot o foorarea, gives an acceptable standard rom which tomeasure an output. As an example, Picture 5 shows agreenhouse measuring 16 eet x 24 eet or 384 squareeet o foor space. Multiplying 384 square eet by 12(standard or measuring an output) gives the requiredcm (4,608) to adequately ventilate the greenhouse(i a an and louvered ventilation system are used). A

cooling an system, while eective in most situations, islimited to a greenhouse length o 150 eet.

Fans are rated at dierent cm outputs under no airdrag (ree air) as well as at one or more static pressure(sp) values, which take into account air riction createdrom air movement restrictions such as air passingthrough inlet louver vents. When using these vents

Alaska greenhouse an capacity ratings o1/8 (0.125 sp)inch water static pressure should be used, rather thanthose listed under ree air values. I more restrictive

insect screening or evaporative cooling pads are usedan capacities o (0.250 sp) inch water static pressurevalues (or greater) should be used. Static pressure valuesshould be readily ound in the manuacturers technicadata. Table 12 summarize typical an perormanceunder two static pressure values.

Since ventilation needs change throughout the seasonconsider two-speed ans or variable speed ans (Picture6). A slow speed is adequate to circulate air and CO

2

through out the house while a much higher capacity oairfow will be necessary to vent the heat on a brightsunny day.

Vents Whenever you have a an, you need a vent. I youattempt to exhaust air without providing an adequatesource o replacement air, either the an will wear outaster (since it is working harder), or air pressure willcreate new air sources (also known as leaks).


14/2014

It is important that the surace area o the vent openingbe at least 1.25 times the area o the an (Table 13) or1.5 square eet o air inlet opening per 1000 cm o ancapacity (Bartok, 2000) to provide adequate air fow andventilation.

For optimum eciency, roo vents should be 1520 percento the foor area and open downwind (Figure 1) (Aldrich andBartok, 1994). While downwind venting oers optimumventilation, in Alaska it is more important to orient theposition o the greenhouse east to west to maximize sunand solar radiation reception.

The World of Industrial Fans

There are numerous types o ans available to thegreenhouse operator. Fans are rated by theircapacity to move volumes o air, measured in cubic

eet per minute (cm), against a certain amount o

pressure. Fan capacity is greatest when there is no

resistance to moving the air. As the an blows air into

a closed greenhouse, the air pressure inside increases

and a static pressure (sometimes called back pressure)

against the an develops. As the static pressure in

the house increases, the amount o air moved by the

an decreases. Companies oten list the air-moving

capacities o their ans at various static pressures.

The cm value at a static pressure o 0.125 inch (1/8

inch) o water may be the most appropriate value or

greenhouse operations.

There are many ans to choose rom and the proper

choice depends on a number o actors.

Type o an: Propeller (axial) ans are best used with

high volumes o air to move against a minimum

static pressure. They are good or circulating air and

exhausting the greenhouse. Duct ans (tube axial)

have the an mounted in a cylindrical tube or duct.

This design allows the an to operate at much higher

static pressures. Duct ans are oten used to distributeheated air rom a urnace through an air tube to the

remainder o the greenhouse.

Fan blades: As the length o the an blade increases,

the amount o air moved also increases.

Fan speed: Increasing the revolutions per minute

(RPM) o the an blades increases the amount o air

movement. Unortunately, higher an speed oten

requires larger an motors.

In addition to decisions on the an design, many ans

have motor options or 110 or 220 electrical systems

or or the amount o protection given to the motor.

For the more detailed inormation needed to properly

size large greenhouse ans, please consult your local

Heating Ventilation and Air Conditioning (HVAC)

proessional.

Source:AdaptedfromGrainger,Inc.2009. Catalog No. 400,

p.3701.

Prop diameter Air Flow (cfm)

Variable Speed 0 in. SP 0.125 in. SP

10 in. 585 285

12 in. 800 470

16 in. 1095 720

18 in. 1880 850

20 in. 2830 2255

24 in. 3240 2485

Single Speed

18 in. 2590 2190

20 in. 2955 2450

20 in. high speed 3635 3113

24 in 3270 2515

24 in. high speed 3970 3240

30 in. 6075 4195

36 in. 8225 6480

Two Speed

24 in. 3985/3760 3255/2995

Table 12: Fan Performance at Two Static Pressures

Regardless o the choice o intake or exhaust ans, thevent(s) on the opposing side o the greenhouse mustwork in conjunction with the an. Many small units arepushed open by the air pressure generated by the an.Larger vents may require motors that are synchronizedwith the ans control system. These systems allow easierairfow since the moving air does not have to hold thevents open.

Orientation to the prevailing summer winds will make asizeable dierence in cooling eciency.


15/2015

Some mechanisms use the heat expansion o certainliquids to open vents without motors and thermostats.These openers work with vents that are less than 1015pounds.

Ventilation PlacementMost ventilation systems are typically designed so

that the hot air is released high in the greenhouse andthe cool replacement air drawn into the lower part othe greenhouse. I this strategy is used in Alaska toextend the early or late growing seasons, the incomingreplacement air may be cold enough to reeze the plants.In our extended growing conditions, it is advisable tohave the cold air intake well above the crop height in thegreenhouse. This allows the cold resh air to mix withthe warmer greenhouse air beore coming in contactwith the plants. Traditional ventilation placementshould be used during the warmer parts o the growing

season to maximize cooling. For best results during oursummer months, windward wall and door vents shouldbe located at approximately plant canopy level and belarger than the area o one roo vent.

Table 13. Minimum Vent Area Requiredin Relationship to Fan SizeFan Size Minimum Vent Area

16 in. 251 sq. in.18 in. 318 sq. in.20 in. 393 sq. in.24 in. 566 sq. in.30 in. 884 sq. in.

Picture 6: A multi-speed fan helps regulate greenhouse tempera-

ture throughout the season.

Air Circulation

As the plant grows, it takes up carbon dioxide andgives o oxygen through tiny pores in the leaves calledstomates. This builds up a high concentration ooxygen and a decit o carbon dioxide in the boundarylayer between the lea surace and the air. On the openlandscape there is always enough air movement tostir the air in this boundary layer. In the greenhousea lack o air movement can cause the plants to slowtheir growth rate because they cant get enough carbondioxide to thrive.

In Alaska the outside air may be rigid, so oten we willcirculate air within the greenhouse without exchangingit with outside air. Continuously moving air keeps thetemperature more uniorm, decreases relative humidityin the canopy and on the lea suraces and maintains thecarbon dioxide level in the canopy. Locate the an andvents to get the airfow across and through the canopyrather than under the benches or across the ridgeline.

In very small (less than 300 sq. t.) greenhouses, aurnace an can be let on to provide circulation evenwhen the heat is turned o. In larger houses, dedicated8- to 16-inch circulation ans are recommendedResearch at the University o Connecticut ound thathorizontal airfow (HAF) is best or plant health, algaemanagement, condensation control and cost eciency(Pundt and Smith, 2001; Bartok, 2000). Greenhouseairfow research at the University o Kentucky oundthat air circulation rates o 40100 cm are idea(Bartok, 2000).Picture 5 Freestanding greenhouses usually have fans at one end

of the house and vents on the other.


16/2016

Fans should be installed approximately 78 eet abovethe foor and placed in a location about a quarter o theway along the length o the greenhouse. For every 50eet o greenhouse length, add another circulating an.Since these ans are rather exposed make sure that thereare good grills around the blades or worker saety. Thetotal circulation an capacity in cm should be abouta quarter o the house volume. For example, a 20 t.

x 50 t. x 8 t. greenhouse would have approximately8,000 cu. t. o volume (20 t. x 50 t. x 8 t. = 8000cu. t.); divide this by 0.25 ( house volume) and youget 2000 cm o needed air movement. At lower rateso air movement, humidity and carbon dioxide relatedproblems may develop and at higher rates (greater than150 cm), plant damage may occur. Table 14 oers ansize suggestions or circulating inside greenhouse air.

Circulation ans are not needed when exhaust ans are

running, so they should be either manually controlledor wired to turn on or o depending on exhaust anactivity (Bartok, 2000).

Ceiling ans can be eective air circulators in greenhousesranging rom 400800 square eet that have a ceilingheight o at least 10 eet. Make sure installed an-bladeheight is 8 eet or more above the foor (Bartok, 2000).

Also, the use o oscillating ans capable o moving airthrough the plant canopy can improve air circulationwithin small greenhouses.

Humidity Control A secondary role o the ventilation system is toeliminate excess humidity. I not properly vented,excess humidity condenses on the lea surace whereit can enhance disease problems. It can also condenseon the greenhouse structure where it can reduce lighttransmission and encourage rust and/or rot o the

Table 14. Air Circulation Fan RecommendationsFloor Area (sq. ft.) Fan Diameter (in.) Air Movement (cfm)

100 6 400200 8 600300 10 800400 12 1,200

500 14 1,600Adapted from Greenhouses for Homeowners and Gardeners (NRAES-137). Above recommendations provideapproximately50-100cfmcirculationratesforgreenhousesupto50feetinlength.

structure itsel. Greenhouse humidity levels can bereduced by removing the moist air around the plantsand replacing it with the cooler and drier outside air.

Cultural PracticesHigh relative humidity is undesirable or growing mostgreenhouse plants. It is one o the major contributingactors to a variety o plant diseases, including a trio

o damping-o diseases and Botrytis blight, a commonungal disease o bedding plants. Adequate plantnutrition, proper plant spacing, the morning-wateringo plants (so lea areas dry during the heat o the dayrather than go into the cool evening wet), properplanting dates and plant growth management are allcultural practices that go a long way toward reducinghumidity and subsequent oliar disease.

Ventilation and Airfow

Proper ventilation and adequate internal greenhouseairfow are critical to the success o managing relativehumidity in a greenhouse. Warm air holds more moisture

than cool air. During a warm summer day the internalgreenhouse air accumulates moisture; as the eveningoutside temperatures cool, internal air temperaturesdrop, reducing the water-holding capacity o thisinternal air until the dew point is reached, at which timewater condenses on greenhouse and plant suraces. Tominimize this problem, air exchanges must take place toremove the moisture-laden internal greenhouse air andreplace it with the drier, cooler outside air. I cool, thisresh air must next be heated (which promotes moisturecollection), which reduces internal humidity levels. Thisprocess should be repeated several times per hour atboth sundown and sunup to eectively reduce internalrelative humidity levels. Care should be exercised inkeeping heater fue gases rom being drawn into the


17/2017

greenhouse during this moisture-reduction process.Horizontal airfow helps to mix the warm and cool airtogether within a greenhouse, somewhat buering itrom dropping below the dew point (Bartok, 2000).

Control SystemsAlthough control systems can be as simple as an on-

and-o switch, more sophisticated systems such asthermostats or computer-controlled ans (controllers)ree the greenhouse operator rom having to constantlyadjust the ventilation system. It is important to locatethe control thermostat (or sensors) at the plant canopyheight in the middle o the greenhouse well away romthe end-walls. The sensor should be positioned in theshade so it reads the temperature o the air and is notbeing heated by the sunlight directly. I direct sunlightis heating the sensor the control system will think thegreenhouse is warmer than it actually is and will try

to cool the airspace more than is desired. Likewise,i the sensors are placed too close to the walls o thegreenhouse the system will think that the house iscooler than it actually is.

ThermostatsMechanical thermostats are usually cheaper thancontrollers and are commonly used in greenhouses.They operate via a temperature sensitive liquid in acoiled tube. As the temperature increases, the liquidwithin the tube expands, activating a switch that turnsa an on. As temperatures drop, the liquid withinthe thermostat cools and retracts, shutting o thean switch. In a high-low thermostat, the liquid willcontinue to retract in the coiled tube as temperaturesdrop, until it activates another low temperature switchthat can be used to turn on a heating unit within thegreenhouse. Most thermostats are accurate to within56F o switch activation. Transistorized thermostatsare more expensive than mechanical ones, but are moreaccurate, usually within 1F.

Thermostat placement must be out o direct sunlightand thus protected by a board or some other ormo insulation rom the sunlight and solar radiationentering the greenhouse. Protecting thermostats romlate aternoon sun is especially important in Alaska,as a late aternoon sun can be very intense, givingexcessively high, alse readings to a thermostat. Heatingand cooling thermostats need to be located closetogether, out o direct sunlight, preerably around plantheight and away rom back walls that heat up during

the day. Also avoid thermostat placement that receivesdirect heat rom your heater an.

Choose thermostats that are valued in two-degreeincrements. Home-type thermostats (comort zonemarkings) do not oer good control. Always place atleast one thermometer by your thermostats to determinethermostat setting accuracy. Inaccuracies should be

noted and thermostats adjusted accordingly to achievedesired temperature settings. Cooling thermostats dierrom heating thermostats. A good cooling thermostat setat 70F should turn on at 72 and o at approximately68. A good heating thermostat set at 70F will turn onat about 68 and o at about 72.

One o the biggest challenges acing Alaska greenhouseowners with heating and cooling systems is keeping yourcooling system rom running when you are heating and

conversely, keeping your heating system rom runningwhen you are cooling. This problem, oten reerred toas system overlap, can be especially dicult to controin smaller greenhouses when we try to vent withextremely cold outside air or heat with oversize heatersthat generate excessive heat. Trying to maintain a giventemperature under these two scenarios will cause yourequipment to work simultaneously i your thermostatsare adjusted too close together. Unortunately, havingthermostats adjusted too ar apart will result in erratichigh and low temperature levels. Time and setting

experimentation will be required to work out the bugsin your system and to optimize its heating and coolingcapabilities. I system overlap is a concern, the use omultiple exhaust ans controlled by separate thermostatscan oer ne-tuning options or an additional cost.

Controllers A more accurate solution is to purchase a controllein place o a thermostat. A controller is an electronicdevice capable o accurately monitoring the greenhouseenvironment. It has the ability to activate dierentpieces o equipment (ans, heaters, etc.) independentlytheoretically eliminating system overlap problemsSimple units are available to monitor temperatureand ventilation needs, while advanced models canhandle the above plus irrigation, lighting, etc. Anotheradvantage is that controllers are usually compactwaterproo units that give good service over a widevariety o environmental conditions. Installation mayalso be simpler as controller switches, relays andcontrols come wired already.


18/2018

Controllers will help, but may not eliminate systemoverlaps because the venting equipment may not actas ast as the controller, especially under Alaskas coldair conditions. Also, the wind will actor into how thecontroller and the working equipment will cycle.

Control o the greenhouse environment will make atremendous dierence in the success o greenhouses in

Alaska. By ollowing the suggestions available in thispublication, it is possible to minimize the environmentalproblems and rustrations commonly encountered ingrowing plants in greenhouses.

References Aldrich, Robert and John W. Bartok, Jr. 1994.

Greenhouse Engineering, NRAES-33. Ithaca, N.Y.:Natural Resource, Agriculture, and Engineering

Service. Astronomical Applications Dept. 2004. U.S. NavalObservatory, Washington, D.C. 20392-5420.

Bartok, John W. 2000. Greenhouses or Homeownersand Gardeners, NRAES-137. Ithaca, N.Y.: NaturalResource, Agriculture and Engineering Service.

Bellows, B. 2003. Solar Greenhouses HorticultureResource List. ATTRA National Sustainable

Agriculture Inormation Service: 1-35.

Beytes, C. (editor). 2003. Ball Redbook, Vol. 1: Greenhousesand Equipment. Chicago: Ball Publishing: 272.

Cathey, H.M. and L.E. Campbell. 1978. Indoor Gardening Artifcial Lighting, Terrariums, Hanging Baskets andPlant Selection. Washington D.C.: U.S. Department o

Agriculture.Energy Conservation or Commercial Greenhouses

NRAES-3. 2001. Ithaca, N.Y.: Natural Resource

Agriculture, and Engineering Service.Fisher, P. and E. Runkle. 2004. Lighting Up Profts

Understanding Greenhouse Lighting. Willoughby, OhioMeister Publishing Co.

Fisher, P., C. Donnelly and J. Faust. 2001. EvaluatingSupplemental Light or Your Greenhouse. Ohio Florists

Association Bulletin (May): 6.Grainger, Inc. 2009. Catalogue No. 400: 3701. Grainger

com.Horticultural Lighting. Philips Lighting Company, 200

Franklin Square Drive, P.O. Box 6800, Somerset, N.J08875-6800.Poot, J. 1984. Application o Growlight in Greenhouses

Poot Lichtenegie B.V. Westlander 42, 2636 CZSchipluiden, The Netherlands.

Pundt, L. and T. Smith. 2001. Pest Management orVegetable Bedding Plants. University o MassachusettsExtension Fact Sheet: 1-17.

AcknowledgementsThe authors would like to thank John W. Bartok, Jr., Extension proessor emeritus, Department o Natural

Resources Management and Engineering, University o Connecticut; Ron Sexton, owner, Trinity Greenhouse,Kenai, Alaska; and Proessor Meriam G. Karlsson, School o Natural Resources and Agricultural Sciencesand the Agricultural and Forestry Experiment Station, University o Alaska-Fairbanks or their input and

review in the development o this publication. We would also like to thank Natural Resource, Agriculture andEngineering Service or allowing us to reprint selected tables and fgures.


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20/20

Visit the Cooperative Extension Service website atwww.uaf.edu/ces or call 1-877-520-5211

For more information, contact your local Cooperative Extension Service ofce or Jeff Smeenk, Extension Horticulture

Specialist, at 907-746-9470 or [email protected]. Wayne Vandre, Extension Horticulture Specialist, developed this

publication in 1988 and it was substantially revised by Thomas R. Jahns and Jeff Smeenk in May 2009. All photos, artwork

and gures UAF Extension, unless otherwise noted.

America's Arctic UniversityThe University of Alaska Fairbanks Cooperative Extension Service programs are available to all, without regard to race, color, age, sex, creed, national origin, or disability

and in accordance with all applicable federal laws. Provided in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S

Department of Agriculture, Fred Schlutt, Director of Cooperative Extension Service, University of Alaska Fairbanks. The University of Alaska Fairbanks is an afrmative

action/equal opportunity employer and educational institution.

2011 University of Alaska Fairbanks. This publication may be photocopied or reprinted in its entirety for noncommercial purposes.

5-88/WV/1000 Reprinted July 201

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Controlling the Greenhouse Environment