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.

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