Cold Protection Methods

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Cold Protection Methods
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Jackson, L.K.
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FactSheetHS-121 June1994ColdProtectionMethods1 L.K.JacksonandL.R.Parsons2INTRODUCTIONTOYOUNGTREECOLD PROTECTIONProtectingyoungtreesfromcolddamageisa difficulttaskwhichhasbeencomplicatedbyseveral factorsinthelastdecade.Thesecomplications includeasignificantincreaseinthenumberofyoung treesplantedoverthelastseveralyears,increasesin costoffuel,equipmentandlaborandanincreasein thenumberandseverityoffreezes.Theproblemsof youngtreecare,ashortageoftreesandincreasingly frequentfreezeshavegeneratedanewinterestin protectingyoungcitrustreesfrompossibledamageby cold.Sinceyoungtreesaresmallandoccupya relativelysmallpercentageofaplantedgroveacre, protectionbymostactivemeansisnotparticularly effective.Thisisespeciallytrueofheatingwithfossil fuelsourceswhicharenowquiteexpensivein additiontobeinginefficientforyoungtreeprotection. Windmachinescouldbeconsideredforprotection, buttheiruseislimitedtocalmnightswith temperatureinversionsandthecostofacquisitionand operationofthisequipmentcouldnotbe economicallyjustifiedfornon-bearinggroves. Irrigationforcoldprotectionisapossibilityandis nowwidelyusedinmanyyounggroveswhere properlydesignedandmaintainedmicrosprinkler systemsareinplace.Suchsystemsrequire uninteruptiblepowersourcestoavoidproblemsof electricalblackouts.Manyyoungcitrustreesare placedinasituationwhereactivecoldprotection measuresaredifficult,ifnotimpossible,andgrowers havetorelyuponpassivemeansofcoldprotection. Someofthemoreimportantpassivecoldprotection measuresincludecultivarandrootstockselection,site selection,cleancultivation,pre-freezeirrigationand theuseofbanksandwraps.COMPARISONOFMETHODSHighfuelcosthasmadegroveheatingduring freezenightsprohibitivelyexpensiveexceptforhigh valuecrops.Windmachinesareeffectiveundersome conditions,buttheyrequiremaintenanceandneeda strongtemperatureinversionforoptimum effectiveness.Fogcanprovideprotection,butlight windscanblowthefogawayfromthegroveand obscurenearbyroadways.Highvolumeoverhead sprinklerirrigationhasbeenusedeffectivelyonlimes andavocadoesinsouthFloridawheretemperatures donotnormallygofarbelowfreezing.Incentraland northFlorida,wheretemperaturesareusuallycolder, overheadsprinklersshouldnotbeusedonlargecitrus treesbecausetheweightoftheiceformedcanbreak offlimbsandcausetreecollapse.Withoverhead systems,allleavesarewettedandsusceptibleto damagingevaporativecoolingduringlowhumidityor windyfreezes.Manytreeswerekilledinthewindy 1962freezewhenoverheadsprinklerswereused. Becauseofthecostoffuel,microsprinklerirrigation 1.ThisdocumentisFactSheetHS-121,aseriesoftheHorticulturalSciencesDepartment,FloridaCooperativeExtensionService,Institute ofFoodandAgriculturalSciences,UniversityofFlorida.Revised:June1993.Publicationdate:June1992.Revised:June1994. 2.L.K.Jackson,professor,extensionhorticulturist,DepartmentofFruitCrops;L.R.Parsons,professor,extensionwaterresourcespecialist, DepartmentofHorticulturalSciences,CitrusResearchandEducationCenter,LakeAlfred,Florida,abranchcampusoftheUniversityof Florida,Gainesville,Florida. Tradenames,whereused,aregivenforthepurposeofprovidingspecificinformation.Theydonotconstituteanendorsementorguaranteeof productsnamed,nordoesitimplycriticismofproductsnotnamed. TheInstituteofFoodandAgriculturalSciencesisanequalopportunity/affirmativeactionemployerauthorizedtoprovideresearch,educational informationandotherservicesonlytoindividualsandinstitutionsthatfunctionwithoutregardtorace,color,sex,age,handicap,ornational origin.Forinformationonobtainingotherextensionpublications,contactyourcountyCooperativeExtensionServiceoffice. FloridaCooperativeExtensionService/InstituteofFoodandAgriculturalSciences/UniversityofFlorida/ChristineTaylorStephens,Dean

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ColdProtectionMethods Page2israpidlybecomingthepreferredmethodfor providingcoldprotection.Thistypeofirrigation worksparticularlywellforresetsandthelowertrunk andbranchesofyoungtrees.However,once microsprinklerirrigationhasbegun,itmustbe continueduntilthegrovetemperaturerisesabove freezing.Ifirrigationstopsbeforethen,thetreeswill likelybemoredamagedthaniftheirrigation continued.Bankingveryeffectivelyprovidescold protectiontothetrunksofyoungtrees.However, banksaretimeconsumingtoerectandcanproduce somepestandculturalproblems.Thegrowercan avoidsomeoftheseproblemsbyusingtreewraps, whichcanbeleftonforanextendedperiodoftime onceinstalled.Whiletreewrapsasawholeare effectiveforcoldprotection,theyarenotaseffective asbanking.Protectionvariesgreatlydependingonthe typeoftreewrapused.Table1comparesenergy requirementsforthevariousmethodsofcold protectiondiscussedhere.SOILBANKINGSoilbanking(Figure1)consistsofplacinga moundofsoilaroundthetree'strunktoprotectthe budunionandtrunkfromcold.Itisoneofthemost efficientcoldprotectionmethodsforyoungtreesand hasbeenusedwithsuccessformanyyears. Figure1. SoilbankBankingprinciplesSincethesoilstoresheatfromthesunduringthe dayandreleasesitatnightheatdeepinthesoil movesuptothesurfacebyconductionandislostto theairbyradiation.Bymoundingsoilaroundthe trunkofatree(banking),heatisconductedthrough thesoilandintotheprotectedareaoftheyoungtree. Thus,bankingprotectsbyconductionandinsulation aswell.WhentobankAdefiniteanswertothequestionofwhentobank hasnotbeenderived.Itwouldbemostefficientif treeswerebankedthedaybeforeafreeze,butthe stateoftheartofweatherforecastingdoesnotpermit thisluxury.GrowersinmuchofsouthFloridadonot bankatallsincethatareahassuchalowcold damageprobability.However,growersinthenorth andmuchofcentralFloridarealizethehigh probabilityofcolddamageandroutinelybankyoung treesinthefallasaregularproductionpractice.A goodruleofthumbistotrytohavealltreesbanked byNovember15for thenorthernareasandnolaterthanmid-December fortherestofthestate.MakingtheBankBankscanbeconstructedwithashovelorhoe,a bladeonatractororsimilartool,orwithabanking machine.Buildthemashighasreasonable,upinto thescaffoldlimbswheneverpossible.Higherbanks affordmoreprotection,buttheyalsorequiremore laborandexpensetobuild.Useonlysoilwhichisfree ofweeds,sticks,bagsorothertrashasthesewill invitedamagefrominsectsanddisease.Watchbanks carefullyduringthewintersincewindandrainmay erodethem.Rapidrecoveryoffreeze-damagedtrees willbethepayoffforagoodbankingjob.UnbankingTreescanbesafelyunbankedassoonasthe dangerofcoldweatherhaspassed.Inmostareasthis willbeinmidorlateFebruary.Ifbanksremainon thetreestoolonginwarmweather,diseaseandinsect problemsincreaseandthereisdangerofa

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ColdProtectionMethods Page3physiologicalbarksloughingdisorder(sweating)which canquicklykilltheyoungtree.Unbankingshouldbe supervisedjustascloselyasbankingtopreventtree damagefromcarelessequipmentoperation.Also, caremustbetakentoensurethebankisremoved completelyandthesoilcarefullyleveledaroundthe youngtree.Leavingtoomuchsoilaroundthetree trunkmayencouragefootrotinthesusceptiblescion portionofthetree.BankingHazards TreeDamageCarelessoperationofequipmentmaybreaklimbs, skintrunksandevendestroytrees.Equipment operatorsmustbeconscientiousandwell-trainedif theoperationistobeasuccess.Brokenlimbsand skinnedtrunksshouldbetreatedwithagood water-repellentpruningpaintorfungicidebefore beingcoveredwithsoil.Somemechanicalequipment usedforbankingremovesconsiderablesoilfroma relativelysmallarea,resultingindamagetoroots nearthesoilsurface.Theuseofsuchequipment shouldbeavoidedorcareshouldbetakentomake suredamageisminimized.DiseasesandInsectsFungaldiseasecansometimesbeaproblemwhen treesarebanked.Placingsoilonthesusceptiblescion portionoftheyoungcitrustreemaypredisposethe planttofootrotifconditionsareoptimalfor developmentofthefungus.Applicationofasuitable fungicidebeforebankingwillhelpreducethe incidenceoffootrot.Antsandtermitesmay sometimesbecomeaprobleminbanks,particularlyif thereistrashinthesoilusedtoconstructthebanks. Problemssuchasthesecanbedealtwithasthey occurorapreventiveinsecticidecanbesprayedatthe timeofbanking.Manygrowersroutinelyspraytrees withasuitableinsecticide-fungicidemixturejust beforebankingasaninsurancemeasure.One hundredgallonsofsprayshouldtreat400to600trees ifproperlyapplied.BankingConsiderationsThesefactorsshouldbetakenintoconsideration beforechoosingbankingasacoldprotectionmethod: 1.Soilbanksmustbeputupbeforedangerofcold andremovedassoonaspossibleafterthethreat ofcoldhaspassed. 2.Labortobuildbanksisexpensive. 3.Hotperiodsduringwintermonthsmaynecessitate earlyremovalofatleastaportionofthebank beforethedangerofcoldisover. 4.Constructionofbanksisoftenhinderedbyweeds orinthecaseoflargertrees,overhanginglimbs.SoilBankSummary Advantages1.Excellentinsulatingvalue(12-15aboveair temperatureinmostcases) 2.Sproutinhibitor 3.Conformswelltolargeorirregularlyshapedtrees 4.Nocostformaterial,onlylaborDisadvantages1.Mustbeconstructedandtakendownseasonally 2.Difficulttomaintain 3.Occasionalproblemswithbarksloughingandfoot rot 4.Moderateinsectanddiseaseproblems 5.Mustberemovedafterfreezedamagetoallow regrowth 6.LaborcostisexpensiveTREEWRAPS TheoryofColdProtectionTreewrapsaremostusefulinprotectingyoung citrustreesduringmildtomoderatefreezesorin traditionallywarmerlocationswithinthestate.Tree wrapsprotectonlythetrunk,andconsequentlyleaf losscanoccurduringmoderateorseverefreezes. Wrapsworkbydelaying,butnotpreventing,heatloss fromthetreetrunkasairtemperaturesdecrease. Temperaturesundertreewrapsgenerallyare0to 6Fhigherthanairtemperatures,dependingonthe typeofwrap.However,thetreeproducesandstores verylittleheat,andduringseverefreezesoflong durationsthetemperaturesundermostwrapswill approachairtemperatures.Wrapsaremosteffective duringfreezesofshortdurationswheretemperatures droprapidly.Theyarelesseffective,however,during freezeswheretemperaturesdecreaseslowlyand remainlowforprotractedperiods.Theeffectiveness ofthewrapisrelatedtotheinsulatingvalueofthe wrapmaterial.Consequently,wrappingtrunkswith

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ColdProtectionMethods Page4thin-walledmaterialsisineffectivefortemperature control,whilethickerinsulatingmaterialsaremore effective.WrappingMosttreewraps,unlikesoilbanks,canbe attachedanytimeduringtheyearandleftonthetree throughouttheyearorevenforseveralyears. However,sometypesofwraps,likethosemadeof poorinsulatingmaterialsorclearplastic,maydamage orevenkillthetreeduetoexcessivedaytimetrunk temperaturesduringthesummer. Whenfreezedamageoccurs,wrapsshouldbe removedorpusheddowntoallowforgrowthofnew shoots.Wrapsshouldbeproperlypositionedand fastenedaroundthetrunkforbestresults.Itis importanttocovertheentirelowertrunk,especially atthebase.HeatingEffectsInsulatingmaterialsareusedextensivelyinmost treewrapstoprovidecoldprotection.Sinceinsulation holdsheatin,protectionisprovidedbyslowingdown thelossofheatfromyoungtreetrunks,thusmaking themwarmer.Howeversincethereisverylittleheat storedinthetrunkofayoungcitrustree,wraps utilizinginsulationalonehavelimitedeffectiveness.DormancyEffectsThedegreeofdormancyofyoungcitrustreesis afunctionofenvironment,andmeasurestoslowthe growthoftreesusuallyresultsindormancyanda betterabilitytotoleratelowtemperatures.Insulating materialsinsomecasesmayhelptokeeptreetrunks coolduringdaylighthoursresultingingreater dormancyandanincreasedtolerancetolow temperatures.Thoughnotsubstantiatedbyresearch, theprincipleisconfirmedbyobservation.Possible effectsoflightontreedormancyisspeculativebut observationssupportthetheory.Treeswrappedwith opaquematerialsrarelysproutundersuchwraps becauselightisexcluded.Sproutingisevidenceof growthandlackofdormancy,somaterialswhich blocklightmayhelptocontributetotreedormancy.UseofLiquidsSomewrapsutilizepouchesofliquid(usuallywith anicenucleatorinsolution)tofurnishadditionalheat insidethewrap,nexttothetreetrunk.Whenliquids freeze,heatoffusionisreleasedwhichcangenerate considerableheat.Whenthisheatisreleasedwithin theconfinesofaninsulatingmaterial,andnexttothe treetrunk,itcanbequiteeffective.TypesofTreeWrapsSelectionofthepropertreewrapforaparticular grovedependsonanumberoffactorsincludingcost, easeofinstallationandprobabilityoffreezedamage. Forexample,growersinnorthernregionsofthestate shouldchoosewrapswithgoodinsulatingqualities, whilegrowersinwarmersouthernlocationsmayopt forlesscostly,thinnerwraps.Treewrapsalsoinhibit sproutsandprotecttrunksfromherbicideand mechanicaldamage.Consequently,noonewrapis bestforallsituations.FiberglassWrapTheadvantagesanddisadvantagesoffiberglass wrap(Figure2)arediscussedbelow. Figure2. Fiberglasswrap

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ColdProtectionMethods Page5Advantages1.Highinsulatingvalue(3-6aboveair temperature) 2.Moderatelydurable 3.SproutInhibitor 4.Canbepusheddowntoallowforregrowth followingafreeze 5.Inert,willnotholdwaterforlongperiodsoftime, rarelycausesfootrotproblems 6.Moderatelyinexpensive 7.Conformswelltolargeorirregularlyshaped trunksDisadvantages1.Moredifficulttoinstallandhandlethansome otherwraps 2.ModerateantproblemsPolyurethanefoamTheadvantagesanddisadvantagesof polyurethanewrap(Figure3)arelistedbelow. Figure3.PolyurethanefoamwrapAdvantages1.Highinsulatingvalue(3-6aboveair temperature) 2.Moderatelydurable 3.SproutInhibitor 4.Moderatelyinexpensive 5.Moderatelyeasytohandleandinstall 6.Conformswelltolargeorirregularlyshaped trunksDisadvantages1.Maybecomewaterlogged,particularlyifusedwith irrigation 2.Sunlightdeterioratessomewraps 3.Footrotisanoccasionalproblem 4.Mustberemovedafterfreezedamagetoallow regrowthRigidPolystyreneFoam(Thick-Walled)Listedbelowaretheadvantagesand disadvantagesofthick-walledrigidpolystyrenefoam (Figure4).

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ColdProtectionMethods Page6 Figure4. Thick-walledrigidpolystyrenefoamAdvantages1.Veryhighinsulatingvalue(4-8aboveair temperature) 2.Verydurable 3.Moderatesproutinhibitor 4.Willnotholdwater,rarelyfootrotproblems 5.EasytohandleandinstallDisadvantages1.Expensive 2.Moderateantproblems 3.Mustberemovedafterfreezedamagetoallow forgrowth 4.Subjecttolooseningbyanimals,mayfitpoorlyon irregularlyshapedtrunksRigidPolystyreneFoam(Thin-Walled)Belowarelistedtheadvantagesanddisadvantages ofthin-walledrigidpolystyrenefoamwrap(Figure5). Figure5. Thin-walledrigidpolystyrenefoamAdvantages1.Lowtomoderatedurability 2.Sproutinhibitor 3.Willnotholdwater,nofootrotproblems 4.Inexpensive 5.ModeratelyeasytohandleandinstallDisadvantages1.Lowinsulatingvalue(0-2aboveair temperature) 2.Moderatetosevereantproblems 3.Mustberemovedafterfreezedamagetoallow regrowth 4.Notsuitedforlarge,rapidlygrowingtrees,mayfit poorlyonirregularlyshapedtrunksClosedCellPolyethyleneFoamDiscussedbelowaretheadvantagesand disadvantagesofclosed-cellpolyethylenefoam (Figure6).

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ColdProtectionMethods Page7Advantages Figure6. Closed-cellpolyethylenefoam1.Moderateinsulatingvalue(2-4aboveair temperature) 2.Moderatelydurable 3.Sproutinhibitor 4.Inert,willnotholdwater,rarelycausesfootrot problems. 5.Easytohandleandinstall 6.Somemodelsuseirrigationwatersupplytube insideforextraprotection.Disadvantages1.Moderatelyexpensive 2.Antproblemsaresevereinsomeareas 3.Mustberemovedafterfreezedamagetoallow regrowth 4.Maycausebarksloughingandfitpoorlyonlarge orirregularly-shapedtrunksMICROSPRINKLERIRRIGATIONOverhead,high-volumesprinklershavebeenused successfullyincitrusnurseriesforyearsasameansof coldprotection.Recently,therehasbeeninterestin usinglow-volumemicrosprinklerstoprotectyoung treesinthefield;however,successvarieswiththe typeofsystem,applicationrates,typeoffreeze (advectivevs.radiative),andseverityofthefreeze.TheoryofProtectionWaterprotectsyoungtreesbytransferringheatto thetreeandtheenvironment.Theheatisprovided fromtwosources,sensibleheatandthelatentheatof fusion.Mostirrigationwatercomesoutofthe groundat68to72F,dependingonthedepthofthe well.Infact,someartesianwellsprovidewaterof 80Formore.Asthewaterissprayedintotheair,it releasesthisstored(sensible)heat.However,bythe timethewaterreachesthetreeithaslostmostofits energy,particularlyforlowvolumemicrosprinkler systems.Consequently,themajorsourceofheatfrom irrigationisprovidedwhenthewaterchangestoice (latentheatoffusion).Aslongaswaterisconstantly changingtoicethetemperatureoftheice-water mixturewillremainat32F.Thehighertherateof waterapplicationtoagivenarea,thegreaterthe amountofheatenergythatisapplied. Themajorproblemsintheuseofirrigationfor coldprotectionoccurwheninadequateamountsof waterareappliedorunderwindy(advective) conditions.Evaporativecooling,whichremoves7.5 timestheenergyaddedbyheatoffusion,maycause severereductionsintemperatureunderwindy conditions,particularlywheninadequateamountsof waterareused.Inaddition,mostirrigationsystems willnotprotecttheupperportionofthecanopy.

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ColdProtectionMethods Page8TypesofMicrosprinklerSystemsAnumberoflow-volumemicrosprinklerswhich canbeusedforcoldprotectionofyoungcitrustrees arecurrentlyavailable.Aswithtreewraps,noone systemisbestforagivengrovesituation.Remember thatmicrosprinklerirrigationisprimarilyusedto irrigatetrees,andpracticalirrigationdesignsmaynot necessarilyprovideoptimumcoldprotection.Again, cost,easeofoperation,andespeciallyprobabilityof freezedamageshouldbeconsideredwhenselecting anirrigationsystem.However,thekeytosuccessful coldprotectionusinganymicrosprinklersystemis providingacontinuousandadequatevolumeofwater directlytothetrunkofthetree.Thisisparticularly trueduringadvectivefreezeswherewatermaybe blownawayfromthetrunk. Itisgenerallyadvisabletoplacetheemitter northwestofthetree,approximately1yardorless fromthetrunk.Emittersshouldbeattachedtorisers forgreatesttreetrunkprotection.Improper placementorinadequatespraycoveragewillgreatly lessentheeffectivenessoftheirrigation.A90spray patternwhichconcentratesthewateronthetrunk andlowerlimbsgivescoldprotectionsuperiortoa 360or180pattern.Invertedconesprinklers positionedabovethewrapinthetreealsogive adequateprotection.Thevolumeofwaterapplied dependsontheamountofcoldprotectionrequired. Generally,10gallonsperhour(gph)applieddirectly tothetrunkina90patternwillprovideadequate coldprotectionduringmostfreezes.WrapsPlusIrrigationThiscombinationofcoldprotectionmeasures providesprotectionbyinsulationplusheatoffusion fromwaterfreezingonthewrap,andinsomecases, wateractuallybeingpipedthroughthewrapto provideevenmoreprotection.Sprayingwateron wrapsinsufficientvolumeandwithoutinterruption willtheoreticallynotallowtemperaturestofallbelow 32F.Furthermore,ifgroundwaterispipedthrough thewrappriortosprayingitexternally,additional protectioncouldbeprovided. Whenusedincombinationwithadequate irrigationmosttreewrapsprovidecoldprotectionto thetrunk.However,onlywrapswithhighinsulating characteristicsprovideprotectionwhenirrigationis discontinuedduetoapoweroutageorbreakinthe irrigationlines.Acombinationoftreewrapsand microsprinklerirrigationprovideslowcostinsurance againstsuchproblems.COLDPROTECTIONUSINGHEATERSThegreatlyincreasedcostoffuelhaspractically eliminatedheatersfromthegrowerscoldprotection strategy.However,heaterscanstillbecosteffective whenusedtoprotecthigh-valuecitruscultivars.UsingHeatersOrchardheatersprovideheatbydirectradiation andconvection.Stackheatersgiveout25-30percent radiantheat,whichmovesalongastraightlinefrom theheatertothetrees.Airaroundtheimmediate areaoftheheaterisheatedbyconvection;someof thisheatislostifitrisesabovethelevelofthe orchard.Becauseoftheneedforfuel-burning efficiencyandpollutionreduction,orchardheaters haveevolvedtotheuprightstackdesign.Vaporizing pot-typestackheat(forexample,jumboconesand returnstacks)havetheadvantageoflowinitialcost, maneuverability,andversatility.However,fuelcanbe lostduetospillage,leakage,andboilingoffuelleftin theheatersaftertheyareextinguished.Labor requirementsforlightingandrefuelingheatersare high,andanadditionalcrewisfrequentlyneededto refuelheatersifseveralnightsoffreezeprotectionare required.Comparedtoindividualstackheaters, centralizedpressurefuelsystemsburningdieselfuel andliquidpropanearemorefuel-efficientandoffer considerablelaborsavings.Fuelstorageforany heatingsystemisabigexpenseandenvironmental liability.EnergySavingTips1.Maintainheatersingoodworkingorder. Periodicallycleanthestacksformostefficient burningoffuelandtokeepemissionswithinthe standardsspecifiedbyairpollutionlaws. 2.Havesufficientthermographsorthermometers throughoutthegrovearea. 3.Largegrovescangenerallybeheatedmore efficientlythansmallgroves.Toprotectgrove borders,additionalheatersmustbeplacedalong theedgesofthegrove,especiallyonnorthand westsides. 4.Calculatetemperaturedropvs.timethroughout thenighttobetterdeterminewhenheatingshould bestarted. 5.Itisimportanttolightheatersonetotwodegrees abovethelethaltemperatureofleavesor

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ColdProtectionMethods Page9blossomsandbuds.Iffruitistobeprotected, beginprotectiononeortwohoursafterthe criticalfreezingtemperatureoffruithasbeen reached,sincethefruithasmoremassthanbuds andcoolsmoreslowlyoruseathermometerto determinetheinternaltemperatureofthefruit. 6.Itisfrequentlypossibletostabilizetemperatures duringtheinitialphaseofprotectionbylighting everyotherrowofheatersorbylightingcentral systemsandthenturningthepressuredown. Additionalheaterscanthenbelitorlinepressure canberaisedslightlytomaintainthetemperature inthegroveastemperaturesdropoutsidethe heatedarea. 7.Manysmallheatersgenerallyprovidemore efficientheatdistributionthanafewlargeones. Thispointbecameparticularlyimportantwith higherfuelcosts.Theadditionalcapitaloutlayof agreaternumberofheaterscouldbereturned throughmoreefficientorchardheating. 8.Befamiliarwithcoldareasinyourgrovesothat heatersinthoseareascanbelitfirst.MinimizingHeatingRequirementsSelectingthepropertemperatureforlighting heatersorstartinganysystemofcoldprotectioncan affectfuelsavings.Forexample,usingclimaticdata forBartow,Florida,protectingagrovenineoutoften yearsat28F.wouldrequireatleast26hoursof heatingperwinter.However,ifthecropwould tolerate24F.,thegrowerwouldonlyhavetoheat fivehours,usingone-fifthasmuchfuel.Citrusfruit willwithstandtemperaturesof28Fforapproximately twohours.Butleavesandtwigs(fruitingsurface)will oftenwithstand24Forlower.Withtheuncertain futureoffuelsupplies,growersmayseriouslyconsider onlyprotectingthefruitingsurfaceofthetreeand allowingthefruittofreeze.Thefruitmaystillbeused forprocessingifitisharvestedwithinaweektoten daysfollowingthefreeze.Leaffreezingpointsarea goodestimateofthetemperatureatwhichleaves twigsandwoodfreeze.Often,twigsandleaveswill freezeatornear24Fintheearlyfall,butmay withstand22Forslightlylowertemperaturesduring mid-winter.WINDMACHINESWindmachinesoffersomeexcellentadvantages incoldprotectionbecausetheyminimizelabor requirements,consumelessfuelperacreprotected andrequirelessfuelstoragethanheaters.Theyare permanentlylocatedinthegroveandhavealow operationalcostperacre.Fuelrequirementsforwind machinesareabout10gal/hror1gal/acre/hr comparedto10-35gal/acre/hrwithheaters.These advantagesmustbeweighedagainstthedisadvantages ofratherhighcapitalcostsandthefailureofthewind machinetoprovideadequatecoldprotectionunder allconditions.Windmachinesaredependenton havinganinversion--thatis,warmerairat approximately40-50feetabovetheorchard.A temperatureinversionofatleast5differenceis necessaryandaninversionof10-15Fmakesthewind machineveryeffective.Theyaremostbeneficial whenlocatedinlowpocketswheretheymixcold, heavyair,whichsettlesthere,withwarmerairabove. Ingeneral,onecanusetheruleofthumbthat10 horsepowerisrequiredtoprotectoneacre.Usually, onewindmachineisrequiredforeach10acreblock. However,theincreaseintemperaturesarehighest nearestthemachineanddecreasetowardtheedgeof areaprotection.Heaterscanfrequentlybeusednear theedgeoftheareaprotectedtoremedythis situation.Startwindmachineswhentemperaturesare twotothreedegreesabovethelethaltemperature. Becauseofthelowcostofrunningawindmachine, plusthefactthatitcanonlyraisethetemperaturea fewdegrees,itisnecessarytostartthewindmachine early.Itisveryimportantthatwindmachinesberun

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ColdProtectionMethods Page10attherpmspecifiedbythemanufacturer,sincethey provideconsiderablylessprotectionwhenoperatedat alowerspeed. Helicoptersaresometimesusedasa cold-protectiondevice,iftheyarestationednearby. Otherwise,theyaretooexpensive.Theyareutilized asalarge,movingwindmachine.Whenhelicopters areusedeffectively,anumberoftemperature monitorsarerequiredinthegrovetodeterminethe coldestareasandthefrequencyofpassesthe helicoptermustmake.Monitorsshouldturnonalight whentemperaturesreachacriticalvalue.Rapid refuelingormorethanonehelicoptermaybe necessarysinceprotectioncannotbehaltedonce temperaturesarebelowthecriticalpoint. Heatinginconjunctionwithwindmachines providebetterprotectionatlowercostthanheaters alone.Forexample,anorchardrequiring35heaters peracrewithouttheuseofwindmachineswould require15heatersperacrewithwindmachines. Heaterspluswindmachinesandgoodairtemperature inversionswouldpermitheaterstobeusedlessthan halfthetime,whichwouldreducefuelconsumption andincreasetheheater'slifespan. Table1. Energyrequirementsofvariouscoldprotectionmethodsforyoungcitrustrees FuelConsumption Method gal/hr/acre BTUs/hr/acre(inthousands) Heaters 20-40 2,800-5,600 Windmachines 0.5-1.5 70-210 Highvolumesprinklers 0.25-0.75 35-105 Lowvolumesprinklers 0.10-0.25 14-35 Source:T.R.Mee