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TheProblemofWeedControlinNewlyEstablishedNative
Plantings
BenRobinson,DanielBurgin,ElysiaHarcombe,KeaganHopper,SamanthaMogford
GEOG30912thOctober2020
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TableofContents
ExecutiveSummary................................................................................................................2
1:Introduction.......................................................................................................................3
2:Methods.............................................................................................................................32.1:ManaWhenuaEngagement.......................................................................................................4
3:LiteratureReview...............................................................................................................53.1:PlantingPlanningandSitePreparation......................................................................................53.2:NaturalProcesses......................................................................................................................53.3:MechanicalControl....................................................................................................................63.4:MulchingandMatting................................................................................................................63.5:ChemicalControl.......................................................................................................................63.6:LiteratureKnowledgeGaps........................................................................................................7
4:InterviewResultsandDiscussion........................................................................................74.1:PlantingPlanningandSitePreparation......................................................................................74.2:NaturalProcesses......................................................................................................................84.3:MechanicalControl....................................................................................................................84.4:MulchesandMatting:................................................................................................................94.5:ChemicalControl.....................................................................................................................104.6:FinancesandKeyFindings........................................................................................................10
5:SummaryandConcludingComments.................................................................................11
Acknowledgements...............................................................................................................11
AppendixA:InterviewQuestions..........................................................................................15
AppendixB:SpeciesList........................................................................................................16
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ExecutiveSummary
• WithEarth’swarmingclimate,landscaperevegetationiskeytoreducingCO2emissions,increasingbiodiversity,andpreservingculturalimportance.Effectiveweedcontroliskeytothesuccessoftheserevegetationefforts.
• ThisreportaimstoevaluatethemosteffectiveweedcontroloptionswhenplantingnativespeciesintoexoticgrasswithinCanterbury.Fivemethodswereexplored:mechanical,chemical,naturalprocesses,mulchesandmatting,andsitepreparation.Effectiveness,cost,andlabourwereconsidered.
• Thisreportreviewsexistingliteratureandtheresultsfrominterviewswithexperts.Weusedasemi-structuredinterviewasafastwaytolearnfromexperts’experiencewhichallowselaborationofideasandtopicsnotexpected.Sitevisitscheckedthesefindingsagainstfieldobservations.
• Themainfindingsfromtheresearchshowedchemicalstobethemosteffectiveandcheapestweedcontroloption.However,hand-weedingandmattingandmulchescanbemoreviableoptionsonsomesites.Financingcontroliscriticalbutcostsvarygreatly.Goodsitepreparationandecologicaltechniquescanreducefutureweedingefforts.
• Duetotimeconstraints,fieldexperimentscouldnotbeconducted.Thiswouldhaveallowedaquantitativecomparisonofthedifferenttechniquesandtheircosts.Sometopics(suchasmattingandmulches)hadlittleinformationspecifictoNewZealandnativeplantings,sothefindingsintheliteraturereviewmaybelessrelevant.
• Furtherresearchshouldconductfieldexperimentstocompareweedcontroltechniques.Otherknowledgegaps,suchasthebestsitepreparation,safetyofbiologicalcontrolinNewZealand,andhowtominimiseenvironmentalimpactsfromchemicalcontrol,shouldberesearched.
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1:IntroductionEarth’sclimatehaswarmedanaverageof1°CsincetheindustrialrevolutionbutreachingnetzeroCO2emissionsandreducingotheremissionswouldbesufficienttoslow,andeventuallystop,anthropogenicwarming(IPCC,2018).Carbonsequestrationbylandscaperevegetationiskeytotheachievementofnet-zeroCO2(IPCC,2019).
Plantingtreesremovesatmosphericcarbonandincreaseslocalbiodiversity.Thisincreasesresiliencebyoffsettingcarbonemissionsandincreasingecosystemfunctionalredundancy(IPCC,2019).Plantingnativetreesisespeciallybeneficialsincetheyrelease2.5timeslesscarbondioxidefromthesoilintotheatmospherethansimilarexoticspeciesduetoslowermicrobialdecomposition(Walleretal.,2020).Vegetationalsoreduceserosionandhelpstrapsediment,nitrogen,phosphorusandpathogensbeforetheyenterwaterways,improvingwaterwayhealth(Burrelletal.,2014;Prosseretal.,2016;Wright,2012).Plantsdirectlycoolthesurroundingareathroughincreasingshadingandevapotranspiration(Kongetal.,2016).Indigenousforestsalsohavesignificantculturalimportance(Forbesetal.,2020).TheimportanceofplantingmorenativespecieshasbeenrecognisedinNewZealandthroughnumerousnationalandlocalrevegetationprograms.
Theseplantingsneedthreetofiveyearsofregularweedcontroltominimisemortalityandachievesuccessfulestablishment(Landscapearchitect,personalcommunication,September17,2020).Althoughmanyweedcontrolguidesexist,therearenocomprehensiveguidelinesforCanterburythatcombineconventionalweedcontroltechniqueswithecologicalprocesses,sitepreparationandcostcomparisons.Creatingregion-specificweedcontrolguidelinesiskeyasCanterbury’smostproblematicweedspeciesdifferfromnationallists(Meurketal.,unpublishedresearch).Additionally,acommoncauseofinvasiveweedcontrolfailureisaninadequateevaluationofthecostsandbenefitsofdifferentcontroloptions(Kettenring&Adams,2011).Asweedcontrolrequiresalotofresourcessuchastime,labourandmoney,itisimportanttocreatecomprehensiveguidelinestoeffectivelymanageweedcontrolinnativeplantings.
ThisreportaimstoevaluatewhateffectiveweedcontroloptionsexistwhenplantingnativespeciesintoexoticgrassinCanterbury.Theresearchisfocusedonfivemethods:sitepreparation,chemicalsprays,mattingandmulches,mechanicalcontrols,anduseofnaturalprocesses,andconsidersfinancialcosts.Thisreportsummariseskeypointsfromliterature,outlinesappliedmethodology,reviewskeyfindingsfromtheinterviewsandmakesrecommendationsforfutureresearch.
2:MethodsThemethodsusedtogatherdataforthisreportcomprisedofassessmentofexistingliterature,interviewswithexpertsandsitevisits.
Areviewofpeer-reviewedliteratureandexistingweedcontrolguidelinesprovidedabasisofinformationwhichidentifiedexistingknowledgegapsandstructuredinterviewquestions.TheliteraturereviewisprovidedinSection2.
AsrecommendedbyFrancesetal.(2009),semi-structureddiscoveryinterviewswerechosenasafastwaytolearnfromexperts’experience.Thisinterviewstructuremeansintervieweescoulddivergefrom
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thepreparedquestionstoelaborateoncertaindetails,tellstoriesandprovidenewinformation(Francesetal.,2009).ThequestionsforintervieweesareprovidedinAppendixA.Theinterviewcontactswereprimarilysourcedfromtheresearchcommunitypartner,DiLucas,andwereemailedaparticipationinvite.Insomecases,thecontactpassedourrequesttosomeoneelsewhoweinterviewedinstead.Wecontactedfourteenpeoplewhoseexperienceencompassedecology,botany,landscapearchitecture,nurseryworkandcouncilworkrequestinginterviews,andwereabletoconductfive.Theinterviewswereamixtureofin-personandonlineandconsistedofquestionsabouttheparticipant’sexperienceusingweedcontrolandopinionsonvarioustechniques’effectivenessandcosts.
Nativeplantingvisitswereusedtocross-checkthesecondaryinterviewdataagainstpersonalobservations.ThesesitesincludedarestorationsiteinTempletonontheCanterburyPlains,thefoothillsofthePortHillsandsurroundingarea,andtheHeathcoteandChristchurchAdventureParkPlantingsites.Observationsweremadeaboututilisedweedcontroltechniquesandhoweffectivelyweedsweresuppressed.
Dataanalysisconsistedofextractingkeypointsfromtheinterviewnotesandcompilingthemintothemesonanotherdocument,whichwasusedtoidentifyimportantfindingsandcommonpointsforthisreport.Theliteraturereviewsectionwassummarisedbythegroupmemberwhoconductedthereviewonthattopic,andthekeyfindingswerecheckedagainstsitevisitobservationstoconfirmreal-lifeapplicabilitybeforeentryintothisreport.
2.1:ManaWhenuaEngagementThesignificanceofthisresearchtomanawhenuawasidentifiedinamanawhenuaengagementreport.Nativerestorationiscloselylinkedwithkaitiakitanga,animportantMāoriphilosophy(Walkeretal.,2019).ThereareobligationsundertheRMA(Sections6-8)toconsiderMāorivaluesandtheTreatyofWaitangiwhenalteringtheenvironment.TheNgāiTahuIwiManagementPlanoutlinesthatnativerestorationisaformofculturalwellbeingrestoration(NgāPapatipuRūnangaWorkingGroup,DyannaJollyConsulting,2013).Therefore,weedcontrolinnativeplantingshassignificancetomanawhenuaasithasaroleinkaitiakitanga,obligationsunderthelawandsignificancetotheiwioftheresearchfocusarea.ThemanawhenuaengagementreportwasanalysedbyDrAbbySuszko,theCollegeofScienceKaiārahiattheUniversityofCanterbury.DrSuszkoadvisedtheresearchteamthatfurtherengagementwithmanawhenuawouldbebeneficialtotheresearchthroughtheMāoriconsultationprocessattheUniversityofCanterbury.
TheNgāiTahuConsultationandEngagementGroup(NTCEG)Māoriconsultationprocesswasthenundertaken.ThisinvolvedprovidingaresearchproposalandabriefreportconsideringhowvariousaspectsoftheresearchcouldhaveanimpactonMāori.TheNTCEGrespondedwiththeadvicethatfurtherengagementwithMāoriwasnotneeded.
Toanswertheresearchquestioneffectively,theculturalimpactsofweedcontrolmethodsneedtobeconsidered.Anattemptwasmadetoorganiseinterviewswithtwomanawhenuacontactstoestablishtheculturalimpactsofdifferentweedcontrolmethods,despitebeingadvisedbyNTCEGthatfurtherconsultationwithMāoriwasnotneeded.However,noreplywasreceivedfromeithercontact,noliteratureonthetopicwasfoundandtherewasnospecificfurtherinformationinNgāiTahu’sIwiManagementPlan.Therefore,theculturalimpactsoftheweedcontrolmethodscouldnotbeestablished.
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3:LiteratureReviewThissectionsummarisestheliteraturereviewonthetopicofweedcontrolinnativeplantings.
3.1:PlantingPlanningandSitePreparationRapidachievementofadensecanopyisvitalasitreducessunlightforweedgrowth(Davis&Meurk,2001;Sullivanetal.,2009),andincreasesnativeplantgrowthratesbyprovidingwindshelterandagoodmicroclimate(Pratt,1996;Sullivanetal.,2009).Plantingdenselywasthekeymethodthatallowedacanopytoformquickly(Davis&Meurk,2001;Sullivanetal.,2009).
Theonlysoilcharacteristicaddressedintheliteraturewassoilcompaction.Studiesfocussedonspecificspeciesandshowedthatthereisvariationamongstnativeplantsintheirtolerancetosoilcompaction(Antonetal.,2015;Bassettetal.,2005;Sullivanetal.,2009).Despitethis,Sullivanetal.(2009)andDavis&Meurk(2001)bothconcludedthatplantinginrippedsoilswas,ingeneral,agoodmethodforsitepreparationfornativeplants.Theloosesoilencouragesrootdevelopmentandcanincreasegrowthrateswhichallowsforaquickercanopycreation.
Therewasclearconsensusintheliteraturethatvegetationaroundthesiteshouldbeclearedpriortoplanting(Antonetal.,2015;Davis&Meurk,2001;Sullivanetal.,2009).Clearingofgrassandotherweedsaroundplantsreducesthecompetitionforwaterandlight,decreasingthemortalityratesofnewnativeplants(Antonetal.,2015;Davis&Meurk,2001).
3.2:NaturalProcessesTheroleofnurseplantsandclassicalbiologicalcontrolwasexamined.Firesareusedoverseas(Fuhlendorf&Engle,2004)butinsomeNewZealandecosystems,fireandgrazingcanbeasdetrimentalasweeds(Holdawayetal.,2014).Althoughgrazingcanbeusedtosuccessfullycontrolsomeweeds(Bellingham&Coomes,2003;Popay&Field,1996),itislikelynotviablefornativeplantingsbecauseofhighpalatability.
Nurseplantsarespecieswhichprovideshelterforyoungseedlings,andScotchbroomandgorsehavebeensuccessfullyusedforthisinNewZealand(Burrowsetal.,2015;Wilsonetal.,2017).HinewaiReserveonBanksPeninsulaisanexampleofsuccessfullyusingnurseplantsforweedsuppressionandnativeseedingshelter(Wilsonetal.,2017).However,gorseandscotchbroomaresomeofCanterbury’sworstinvasiveweeds;theyreplacepasture,displacenativespeciesandincreasenitrogenleaching(Isern,2007;Magesanetal.,2012).
Classicalbiologicalcontrolinvolvestheintroductionofaspecies’naturalenemy.Thesuccessofbiologicalcontrolonweedsisdependentontheweed’straits;theintroductionofsevenbiologicalcontrolagentshashadlittleeffectongorse(Isern,2007)buttwoweevilswouldbesufficienttoreducenoddingthistlepopulationsinCanterburyby20%peryear(Shea&Kelly,2004).TherearealsoNewZealandexampleswherebiologicalcontrolhastargetedeightnon-targetspecies(seeBarrattetal.,2007).However,overall,biologicalcontrolisunlikelytohavenon-targeteffectsandthesearemostlikelytooccuroncactiandthistles(Suckling&Sforza,2014).NewZealandhasnonativeandfewcommerciallyvaluablespecies(Crippsetal.,2013),sodetrimentalhost-shiftsmaybelesslikely.Thismeta-analysiswasnotNewZealandspecificandNewZealandhasmanyunusualfloraandflorasotheapplicabilityofthisreportislimited.Additionally,becauseoftheriskofhostshifts,intensivescreening
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ofcontrolagentsisrequiredwhichcantake16to25years(Ogden,2020).Biologicalcontrolofspecieswhichalsohaveeconomicvaluecanalsobecontroversial(Isern,2007)andmightnotberecommendedforspeciessuchasgorse,broomandpasturegrasses.
3.3:MechanicalControlTheliteratureregardingmechanicalcontrolprimarilyfocussedonmowingandhandpulling,whichwasfoundtobeefficientbutlabour-intensiveandcostly.MacDonaldetal.(2013)reportedthatfouryearsofhandpullingreducedthecoverofspottedknapweedinalocalareatoonly1%.Threeyearsofhandpullingcausedlargereductionsinthebiomassanddensityofadultknapweedincombinationwithmowing.Handpullingalonereducedthecoverofknapweedfrom10%to2%afteroneyear,butincreasedcoverreturnedwiththeabsenceofcontrol.Handpullingneedstobecarriedoutregularlyonalong-termbasisorbecombinedwithmechanicalmowingtoseeproperresults.Handpullingiseffective,butlabourcostsarestillanissueforlongtermcontrol.
MacDonald(2013)foundthattheonlycombinationsoftreatmentsthatreducedtheknapweedseedbankwashandpullingfollowingmowing.Withoutthecontinuingsuppressionoftheseedproduction,theseedbankdensitieswillpossiblyreturntothe400m2thatwerefoundonuntreatedareaswithinthestudy(MacDonald,2013),emphasizingtheneedforlong-termmanagement.
3.4:MulchingandMattingPlasticmattingischeapbuthascostlylabourandthereforenonetsavings(Schonbeck,1999),butitdidreduceweedsbetween24-68%inuncoveredvs.coveredplots(Cirujedaetal.,2012).Plasticwasfoundtobemoreresilientthanpapermulchbecauseitwouldnottearfrompegsaseasilyinhighwinds(Harrington&Bedford,2004).Waterrun-offisasignificantissuewithplasticoptions(Schonbeck,1999).Klett(2014)deemedpapermulch(newspaper)tobeareadilyavailableoption,butitonlyhadalifespanofabout6months,wasletdownbypoorwindresilienceandissomewhatunattractive(Harrington&Bedford,2004).Carpetwasarelativelypopularoptioninsmallnativeplantings,butwasmorelabourintensivetoapplythanmulch,andsomecarpetscanleachchemicals(Smailletal.,2011).Bark,woodchips,andotherwoodoptionsweresomewhatpopularintheliteratureandarerelativelyaccessible,aswellasnotbeingeasilyblownaway.Theseoptionscancausenitrogendeficienciesifincorporatedintosoil,andaresomewhatflammable(Klett,2014).Lightmulchoptionssuchasgrassandstrawwerenotfoundtobeusefulbecauseofrapidbreakdownandeasyweedpermeation,buttheycanbeusefulforshorterperiodweedcontrolwhenlayeredandrequireminimalmanuallabour(Schonbeck,1999).
3.5:ChemicalControlHerbicideusecanmeanthedifferencebetweensurvivalandfailureofnativeplantings(Porteous,1993),asthesmallstatureofanativespeciesmaypredisposethemtobeingoutcompetedbyweeds(Champion,1998).Chemicalspraysarethemostusedweedcontrolduetoitstime-andcost-effectiveness,withglyphosatebeingthemostcommonchemicalapplied;however,glyphosatecannotcontrolallweeds(Porteous,1993;Rolandoetal.,2013).Haloxyfopisgoodfortargetedcontrolofgrassyweeds(Champion,1998).Broom,gorse,PinusradiataandvariousgrassspecieswerefoundtobethemostproblematicweedstocontrolwithspraysinNewZealand(Rolandoetal.,2013).Otherdifficulties
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stemfrompasturescontainingamixtureofdifferentweedspecieswhichwillreactdifferentlytotheherbicideused(Bourdôtetal.,2007),whichhighlightstheneedforspecies-targetedspraycontrol.
3.6:LiteratureKnowledgeGapsSignificantknowledgegapswerefoundduringtheliteraturereview.Muchoftheliteratureonsitepreparationfocusedonindividualspecies.Therewasnoliteratureonsoilcharacteristicsotherthancompactionanditseffectsonnativeplants.TheliteraturewasunclearonhowsafeclassicalbiologicalcontrolistouseinNewZealand.Mostoftheliteratureonmulchesandmattingwaslimitedindetailandprimarilyrelatedtousageforvegetablecropsandforeignplants,makingitdifficulttoapplytonativeplants.Mechanicalcontrolliteraturelargelyfocusedonspecificsiteswithuniquephysicalcharacteristicsandweeds,meaningthatlimitedinformationcouldbedrawnoutandappliedtothisresearch.Themostsignificantliteraturegapwashowherbicides’environmentaleffectscanbemitigatedorpreventedandtheculturalimplicationsofweedcontrol.
4:InterviewResultsandDiscussionOverall,thefindingsweresimilartoourliteraturereview.Novelfindingswillbediscussedintheirrelevantsections.Onenewandpreviouslyunconsideredfindingwasthedebatearoundtheroleofcommunityengagementinweedcontrol.Someintervieweesbelievedhand-weedingwastoolabour-intensiveandtimeconsumingcomparedtospraysandmulches,whileothersconsidereditavaluablewaytoinvolvethecommunitywhichcouldbelow-costifvolunteerswereinvolved.Thisdisagreementmaybeduetopersonalvaluesandexperiences,whethertheprojectrequireshighefficiencyandweedcontrolsuccess,andtheamountofcommunityinvolvementalreadyinthearea.
4.1:PlantingPlanningandSitePreparationEstablishingearlycanopycovertoreduceweedswas,asintheliterature,important(CouncilEmployee,personalcommunication,September20,2020).Theeffectivenessofcanopyshadeisshowninfigure1.However,earlycanopycreationisatradeoffasplantingdenselyrequiresmoreplantsandearlymaintenance,increasingcosts(NurseryEmployee,personalcommunication,September18,2020).Plantingdesignandtimingcanhelpreduceexpenses.Forexample,whitecloverwasusedinKarameatocreategroundcover,reduceerosionandfixnitrogentoincreasethesuccessofsubsequentnativeplantings(EnvironmentalAgency,personalcommunication,September21,2020).Plantingworkhorsespeciesfirstandenrichmentspeciestwotothreeyearsafterwardcanincreaseshading,thereforereducingtheweedingeffortrequiredfromothertechniques(Landscape Figure1:Thecanopyshadeisveryeffectiveinreducingthegrassy
weedsatthisfieldsiteinTempleton.Source:Burgin,2020
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architect,personalcommunication,September17,2020).ExamplesofsuccessionalplantingspeciesisprovidedinAppendixB.
Clearingofimmediatevegetationisstandardpracticeforplantingnativeplants(Figure2)(Ecologist,personalcommunication,September24,2020;CouncilEmployee,personalcommunication,September20,2020;EnvironmentalAgency,personalcommunication,September21,2020).Theuseofchemicalsprayswasthemainmethodusedbyintervieweestoclearimmediatevegetationandiscoveredinthechemicalcontrolsection4.5.
Therelationshipbetweenplantingmethodandtheplant’sabilitytocombatweedsdidnotappearintheliteraturebutwasdiscussedasbeingsignificantintheinterviews.Plantingqualitydeterminesaplant’shealthandgrowthrate,andthereforeitsabilitytoout-growcompetitors.Directseedingwasusedforsteepslopes(EnvironmentalAgency,personalcommunication,September21,2020).Insomecases,directseedingoutpacednurserygrownandtransplantedplantsatthesamesite.Furtherinformationonthiswasnotabletobegatheredbutcouldbeapointforfutureresearch.
4.2:NaturalProcessesGorsewasidentifiedasasuitablenurseplantprovidedthatthesitehadadequaterainfall,anearbyseedsource,andthatfireandbrowsinganimals(includingseedpredators)werekeptoutofit(Ecologist,personalcommunication,September24,2020).Thesameintervieweediscussedusingmānukaandkānukaasnativenurseplantsbutsaidthesespeciesarenotverycompetitiveandbestestablishinbaresoilsorgrazedgrass.Importantly,theysaidmostnurseplantsareshadeintolerant.However,theEnvironmentalagency(personalcommunication,September21,2020)discussedplantingmānukaandkānukaintogorsewhichtheythenoutcompeted.Theecologistalsodiscussedsuccessfulbiologicalcontrol,suchasusingtheragwortfleabeetletocontrolragwort.
4.3:MechanicalControlOverall,theintervieweessaidmechanicalcontrolisnon-feasibleandlabourintensive,leadingtoitbeingaless-preferredcontroltechnique.However,mechanicalcontrolishighlydesiredinsomeareasdespiteitshighcost.EnvironmentalAgency(personalcommunication,September21)discussedhowmechanicalcontrolmethodssuchasbrushcuttersandhandpullingaremorefeasiblefornativeplantsonthesteepslopesoftheporthills,becauseofthedifficultytomanagethemanyotherway.
TheChristchurchCityCouncil(CCC)hasstartedtousesteamasacontroltechnique(personalcommunication,September20,2020).Theboilersystemisexpensiveandthesteamdoesnotbreakdownthewholeplant,allowingregrowthofweeds.Therefore,thistechniqueneedstobecarriedoutregularly.
AnEnvironmentalAgency(personalcommunication,September21,2020)controlsnoxiousweedssuchasperennialswithacutandpastemethod.Therootiscutatitsbaseandapasteisappliedontopto
Figure2:ImmediatevegetationhasbeenclearedaroundnewnativeplantsatthisfieldsiteontheHeathcoteRiver.Source:Hopper,2020
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stopregrowth.Thisisimportantforperennials,astheyarecapableofvegetativereproduction.Therefore,regularcuttingtechniqueswillnotinfluenceperennialsduetotheirabilitytoregrowtheirshoots.
Sitespecificationsdetermineweedcontroltechniquesused;sensitivesitessuchasatthePortHillsuseprimarilymechanicalcontrolduetotheenvironmentaleffectsofchemicalcontrol.Mechanicalcontroloptionsareveryintensive,buttheliteratureshowsthatitisstillpossibletocarryitouteffectively.Innovationiskeytotakethehardworkoutofmechanicalcontrol,suchasbycreatingautomatedrobots.Mechanicalcontrolisalsomoreviablearoundwaterwaystominimisetheuseofchemicalcontrol(Botanist,personalcommunication,September17,2020).
4.4:MulchesandMatting:TheCCCusessingleusematsquarestoprotectthebaseofplantsfromweeds(Figure3)(Councilemployee,personalcommunication,September17,2020).Coconutfibreisnowapopularchoiceforthis,butDOCismovingtowardswool.Mulchesaregoodformoistureandweedsuppression(Nurseryemployee,personalcommunication,September18,2020),whereasmatoptionscanbedetrimentaltoplantsbecauseofalackofmoistureifplacedondryground.Hempfibreisprovingtobeausefuloption,butduetoitscostitisnotagoodlarge-scaleoption(Nurseryemployee,personalcommunication,September18,2020).Theenvironmentalagencyisproducingcompostblanketsfromgreenwasteandmulchwhichsimulateforestfloorconditionsfortransplantednatives(Environmentalagency,personalcommunication,September21,2020).Theenvironmentalagencydiscussedwooloptionbutfoundappletraystobeusefulandeasytoapply,despiteitnotlastingaslong.Itis,however,crucialthatmattingisnotsothickastoimpedewaterpermeabilityandthatthemattingisappliedonmoistsoil(Environmentalagency,personalcommunication,September21,2020).Woodmulchisthemosteffectiveoptionforsteeperterrainasitbindstogether,butitisbesttousesemi-decomposedmulchsoasnottoremovenitrogenfromsoils(Landscapearchitect,personalcommunication,September17,2020).Theconsensusinalltheinterviewswasthatplasticmatoptionsshouldbeavoidedsincetheybreakdownrelativelyrapidly,leavingbehindplasticfibresinthesoil.Someoftheinterviewees’companieshadadaptableguidelines,butageneralstandardofthebestoptionsdidnotexist.
Therewerearangeofgoodoptionsdiscussedintheinterviews,butgenerallymulchoptionswerethoughttohitthebestbalanceofcostandlabourrequirements,withcoconut-fibreandwoolmattingoptionsbecomingincreasinglypopular.
Figure3:PlantingsalongtheHeathcoteRiverillustratingtheuseofmattingsquaresforweedcontrol.Source:Hopper,2020
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4.5:ChemicalControlFormostinterviewees,chemicalcontrolispreferredovermechanicalcontrolonlargesitesduetotimeandlabourcosts,butcarefulandtargetedapplicationiscrucialtoavoidkillingnativeplants(Nurseryemployee,personalcommunication,September18,2020).Anumberofmeasurescanbetakentoreducethisrisksuchasonlysprayingatlowwindvelocities,placingconesovertheplants,checkingforbranchesunderneathmatting,weedingaroundtheplantsfirstorcuttingandpastingchemicalsintotheweed’strunk(CouncilEmployee,personalcommunication,September20,2020;EnvironmentalAgency,personalcommunication,September21,2020;Ecologist,personalcommunication,September24,2020).Galantcanbeusedtocontrolexoticgrasseswhennativeplantsarepresent(Ecologist,personalcommunication,September24,2020).Organicherbicidesexist,butsometimesdonotkilltheweed’srootsandcanbedangeroustoworkwith(Councilemployee,personalcommunication,September20,2020).
4.6:FinancesandKeyFindingsItwasdifficulttoestablishcostsfortheweedcontroltechniquesaspricesvaryhugelybetweensitesandtheweedswhichneedcontrolling.Estimatedcostsrangedbetween$40,000and$70,000perhectareofnativeplantings(Nurseryemployee,personalcommunication,September18,2020;EnvironmentalAgency,personalcommunication,September21,2020).Thisisagreatdealmorethanthe$4,000perhectarethegovernmentsuppliesthroughOneBillionTrees.Giventhevariabilityinprices,mostofthissectionshalldiscussapproximaterelativecostsinsteadofhardfigures.
Designingyourplantingsequencetoincreaseshadingisacheapandeasywaytoreducetheamountofmoneyspentonotherweedcontroltechniques,althoughitrequiresahigherdensityofplants.Nurseplantsareacheapandlow-intensityformofweedcontrol(Burrowsetal.,2015).Biologicalcontrolischeapandlow-effortonceitisestablished(Paynteretal.,2012)butthereispotentialforittohaveunwantedeffects(Barrattetal.,2007).Biologicalcontrolisalsoslowtointroduce(Ogden,2020).
Chemicalcontrolisthecheapestweedcontroloptionbutshouldbeappliedcarefullytoavoidkillingnativeplants,eitherbyaccidentaldirectapplicationorviaspraydrift.Mulchesandmattingarethesecond-cheapestoption,butsleevesaroundplantsareexpensive.Itcosts$3perplanttoattachasleeveandaddafertilisertablet(Councilemployee,personalcommunication,September20,2020).However,otherintervieweesfeltmulchesandmattingwerethecheapestweedcontroloption.
Mechanicalcontrolislabourintensive,buthand-weedingmaybemoreappropriatethanchemicalcontrolinurbanareasandinstanceswherethecommunitywantstobeinvolved.Providedthecommunitycanaccessthesite,usingvolunteerweedingisalow-costwaytomanageweedsthatalsoengagesthecommunityinlocalrestorationandreducestheuseofchemicalcontrol.
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5:SummaryandConcludingCommentsPlantingdenselyinrippedsoilswherevegetationhasbeenclearedappearstobethebestmethodforsitepreparation.Plantinghealthyplantsincreasesgrowthrates,allowingforquickercanopycreationandshadingofweeds.Growthratescouldpotentiallybeimprovedbydirectseeding.Nurseplantsandclassicalbiologicalcontrolhavelimiteduseabilitybutcanbeeffectivewhenusedcorrectly.Plantingworkhorsespeciesfirstisafreewaytoreducethenumberofweedstocontrollater.
Chemicalcontrolisoftenthecheapest,fastestandmost-effectivecontroltechniquebutitcanhaveunwantedenvironmentaleffectsandkillnativeplants.Mattingandmulchesarethemostpopularalternativeandcanalsobecost-effective.
Therewasconsiderabledisagreementamongtheintervieweesontheroleofcommunityhand-weedingtosuppressweeds.Iftheprojectisaccessibletothepublic,involvingthecommunitycanfosterasenseofpublicengagementwithrestorationprojects.Itmayalsobebeneficialinsiteswherechemicalspraysmayhaveunwantedenvironmentalandhumanhealthimpacts.
ItishopedthatthesefindingscanhelpincreasethesuccessofweedcontrolinnativeplantingsinCanterbury.Theysuggestchemicalcontrolisbestforlarge-scaleprograms,butthatmatting,mulchesandmechanicalcontrolaremoreappropriateforsmall-scaleapproaches.Sitepreparationandmimickingsuccessionalplantingcanbeusedtoreducethelaterweedburden.Biologicalcontrolandnurseplantshavelimitedusefulnessbutcanbeeffectivewhenapplicable.
Therearesometechnicalknowledgegapsthatareimportanttofillwithmoreresearch.ThesehavebeenidentifiedinSection3.6.Additionally,ascientificfieldexperimentwouldprovidequantitativedatatostatisticallydetermineweedcontroleffectivenessandcomparetheircostsinacontrolledenvironment.Thisresearchhasimportantculturalimplicationsformanawhenuawhichwewereunabletoadequatelyaddressinthisreportandshouldbedevelopedfurther.
Finally,ourresearchemphasisestheimportanceofhavinglocallytailoredandeasilyaccessibleweedcontrolguidelines.DOCprovidesecosystem-specificguidance(DOC,n.d.)andwebsitessuchasWeedbustershaveawealthofinformationonindividualweeds.However,commentaryonhowtocontrolweedsusingtypicaltechniquesalongsidesitepreparationandnaturalprocesses,andtheirrelativecostsandtime-effectiveness,islacking.Aslocalsoilsandclimatesvary,whichmayalterweedcontroleffectiveness,itiscrucialtheseregion-specificguidelinesarecreated.Inturn,thisshouldincreasethesuccessofrevegetationattemptsinNewZealandtomitigatecarbonemissionsandimproveenvironmentalwellbeing.
AcknowledgementsWewouldliketothankBoydenEvans,ColinMeurk,FernFactor,RedTreeandoneanonymousintervieweeforparticipatinginourinterviewprocess,andAbbySuszkoforgivinghertimeandguidancethroughtheMāoriengagementprocess.WewouldliketogiveaspecialthankyoutoDiLucas,ourcommunitypartner,forherhelprefiningtheresearchaimandprovidinginterviewcontacts.WewouldliketogiveanotherspecialthankyoutoEdChallies,oursupervisor,forhisencouragementandadvicethroughouttheresearchprocess.
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AppendixA:InterviewQuestions
Thefollowingareguidingquestionsinterviewersusedtoundertakeaninterviewwith.Theyareopen-ended,allowingforintervieweestoexpandanddiscusstopicswemaynothavehadspecificquestionson.
Whatweedcontroloptionsdoyouhaveexperienceusing?(Andhowlongfor?)Howmuchplantinghaveyoudoneinexoticgrasses?-Bothlawnandpasture
Doyouhaveasetofguidelinesthatyoufollowfornativeplantings?Doyouknowofanyguidelinesthatpeopleuse?Whatisyourexperienceoftheeffectivenessofdifferentguidelines?
Whatwastheentireprocessofplanting?Howdidyoupreparethesite?Earlyweedcontrolmethods?Weedcontrolmethodsonceestablished?
Whatdiditcosttouseeachweedcontroloption?Andhoweasy/fastwasittoapplyeachweedcontroloption?Howlongdidittakefortheweedcontroltobeeffective?
Whataresomeweedsthatarecommonorhardtocontrol?(Oruseful!)Andwhat’sthebestwaytocontrolthem?
Arethereanyrestrictionswhenusinganycontrolmethods,likenotusingitatcertainlocations/undercertainconditionsorstaffprotectiveequipment/healthandsafety?
Didanyhaveunexpectedside-benefits(orproblems)?
Whatmechanicalweedcontrolmethodsdoyouuse,andhoweffectiveareeachofthecontrols?
Whichcommonweedsareusefulforprotectingandprovidingnutrientstoyoungnativeseedlingswhiletheyarebeingestablished?
Anytipsorrecommendationsforsomeonewho’snewtousingweedcontrol?
Whatmattingormulchoptionswouldyouconsidertobemosteffectiveingrasstypenativeplantings?
Whatisthecostofeachoftheseoptions?
Doyouhaveanyunusualmattingormulchoptionswhicharenotwellknownwhichcouldbeeffective(jute,wool,wood,plastic,steam)?
Whatdoyouconsidertobetherealisticcostofweedmaintenancefortwoyearsafterplantingperhectare?
Howcannativeplantingsbedesignedinsuchawaytomakeweedcontrolaseasyandviableaspossible?
Howcanweuse‘cuestocare’tomakenativeplantingsmoreappreciatedbythepublic?“Aesthetics”
Sitefactors(e.g.slope,aspect,soiltypeetc)thatmakeweedcontrolmoreorlesssuccessful?
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AppendixB:SpeciesListAnexampleofaspecieslistatasitewhichusedsuccessionalplantingtosuppressweeds(LandscapeArchitect,personalcommunication,September17,2020).Enrichmentspeciesareindicatedwitha‘y’inthefar-rightcolumn,whiletheprioritycolumnindicateshowsoonspeciesshouldbeplanted.