CatChing Rain: Rain garden Construction Checklist

Look for the other helpful fact sheets in this series: 1. Low Impact Development2. LID Stormwater Regulations3. LID Development Process4. Pavement Maintenance5. Rain Garden Maintenance6. Rain Garden Construction

Checklist7. Rain Garden Construction

Sequencing

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6Effective rain gardens start with successful installations! Here are tips from experienced rain garden builders.

1. Educate Yourself & All Field Personnel• Ifyou’renewtoraingardens(orbioretentionfacilities),learnmore!Herearesomeresources:

Rain Garden Handbook for Western Washington;avideo:BuildingaRainGarden:KeepingourPacificNorthwestWatersclean(http://raingarden.wsu.edu/);andtheLIDTechnicalGuidanceManualforPugetSound,2012.

• TakeaworkshopofferedthroughWSUExtensionoryourlocalconservationdistrict.• Besureallyourfieldstaffunderstandtheprojectfully.

2. PlanningReview the design and site plans well in advance to ensure that:

• apercolationtestdugtoatleast2feethasalreadybeendonetoensureproperdrainageonsite• enoughdetailisprovidedforyoutofullyunderstandtheplan• theplantsareappropriateandavailablefromregionalsuppliers• siteaccessandstagingareasaredefinedandworkable• therightschedulehasbeenset iftheraingardenispartofa largerconstructionproject

(seeourcompanionfactsheet:“ConstructionSequencingforRainGardens”).

3. Procurement• Beginprocurementofplantsandmaterialsearly.Someraingardenplantsmayhavelimited

availabilityorbeavailableonlyseasonally,especiallybare-rootrushesandsedges.• Ifusingarborists’woodchips formulch, seeka sourceearlyanddesignateanaccessible

deliverysite.Useareputablesourceformulch,freeofdebrisorweedseeds.

4. Materials Specifications• Ifusingnativesoils,doublecheckthattheydrainappropriately.• Check with suppliers to be sure they will meet the plan’s requirements for specified

bioretentionsoilmixorcompost.Raingardenshaveparticularrequirementsthatmustbemet,andnotallsuppliershaveappropriatematerials.

• Order“washed”rocktoarmorinflowsandoverflowsandinspecttherockondeliverytomakesureitwasreallywashed.

5. Construction ConcernsExcavation:

• Ensure the rain garden area is excluded from other construction operations to preventcompaction—heavyvehiclescancompactsoilsupto3feetbelowinasinglepass!

• Usethesmallestpossibleequipmentforthejobtopreventcompactingsoils.• Knowthelocationofallutilitiesbeforedigging—call811forfreelocatorservice.• Rememberthatthecapacityoftheraingardenwillbedeterminedbythefootprintofthe

excavatedbottom—besureyoufollowtheplanswhensizingthegarden.• Checkallexcavationcalculationsbeforeyoudig:howdeeptodig,wheretosettheinflow

level,wheretosetthepondingdepth,wheretoplacetheoverflow.(Seehttp://raingarden.wsu.edu/foraraingardencalculationsworksheet.)

• Neverplaceexcavationequipmentintheexcavatedraingarden—backawayasyoudig.• Thebottomoftheexcavatedraingardenshouldbeatthesameelevationthroughoutfor

evendistributionofthestormwater.• Scarifyor“rip”thebottomtoadepthofatleast2inches,thenrakesmoothbeforeapplying

thebioretentionsoilmixspecified.Berm & Slopes:

• Priortosettingthegradeofsideslopesinsidetheraingarden,breakupcompactednativesoilstoallowbettermixingofexistingsoilsandnewbioretentionsoilsorcompostplacedontop.

• Extrasoilsfromexcavationcanbeusedtocreateaberm.• Makesurethebermiswellcompactedandaminimumof2feetwideatthebase(usuallywider).

Continued >

A pit test to calculate soil drainage rates is a critical first step. Photo: Erica Guttman

Measuring elevations before and during construction will ensure your rain garden manages water properly. Photo: Erica Guttman

CatChing Rain: Rain garden Construction Checklist 6

Developed with funding provided by the Washington State Department of Ecology and The Russell Family Foundation. Author: Erica Guttman, WSU Extension. WSU programs and employment are available to all without discrimination. Evidence of noncompliance may be reported through your local Extension office.

Continued: Rain Garden Construction Checklist, Construction Concerns, Berm & Slopes

• Steepsideslopesarehardtoplantandmaintain,andcanbeasafetyhazard.Ifnototherwisespecified,makesideslopes3:1oreven4:1,rememberingthattheshallowertheslope,thelargertheraingarden’soverallfootprintwillbetostillachievetherequiredspaceatthebottom.

• Generally,bermsshouldbelandscapedwithshrubs,groundcovers,andperennials—turfishardtomaintainonaslopeandcanposeamaintenanceproblembyinvadingtheraingarden.

• Establishaborderaroundtheperimetertodeterthespreadofgrassesfromadjacentlawnareas.Inflow & Overflow:

• Stormwatercanbeconveyedtotheraingardenthrougharock-linedswale,orthroughanundergroundpipe.

• Inflowgradeshouldflowataminimumof1/4-inchdropperfootofrun;iftherunissteep,buildsmallrockcheckdamsacrosstheswaleevery5to10feettoslowthewater.

• Swalesneedtobewellarmoredwithdrainrockapplied2-3inchesatminimumtopreventbringingsiltintotheraingarden.

• Undergroundpipespecificationsshouldconsiderthefutureexpectedabove-groundload,forinstancefromvehicles.

• Inflowpipesshouldbeinstalledtomaximizeeaseofmaintenanceandpreventpotentialclogging.• Theopeningofinflowpipesmustbefullycleartoallowwatertoenterfreely.• Several inchesofdrain rock shouldbeplacedunderneath thepipeandextend into the

gardenafewfeettopreventerosionasstormwaterflowsin.• Raingardenplans should always include at least one clearlydefinedoverflow, either as

araisedgratewithinthegardenorasalowpointintheberm.Theseareasmustalsobearmoredwithcleandrainrocktopreventerosion,andshouldextendabout3to4feetoutsidethegardentoslowwater.

• Theoverflowwatershouldbedirectedtoastormdrain,vegetatedlandscape,oranotherraingarden—neverontoaneighboringpropertyortowardsastructureofanykind.

• The placement of the rain garden’s inflow and overflow are dependent on the existingtopographyandthepondingdepth(usuallyatleast6to12inches)calledforintheplan.

• Theoverflowmustbeplacedlowerthanthedownspoutconnectionpointofthebuilding,butatleast6inchesabovethepondingdepth(seeRainGardenHandbookforWesternWashington).

• Alwaysuseareliableelevationmeasuringdevice,suchasaproperlycalibratedlaserlevel.Plants & Early Care:

• Raingardenemergents,suchasrushes,sedges,andbulrushes,canbepurchasedbare-rootorinplugs.

• Whenplanting,besurethatadeepnarrowholeisdugsothattherootsdonotbendbackupward(“J-roots”)andinsteadgostraightdown.Aplantingbarandweedingknifehelpwiththis.

• Useaplantingbarorweedingknifetobringthesoilbackovertheemergent’sroots.• Ifusingbare-rootplantsofanykind,keeptheirrootsmoistandcooluntilthey’replanted.• Confirmany recommendedfield adjustmentswith thedesigner, such asmovingplants

thatwillultimatelybetootalltomaintainsightdistancesorimproperlysitedplantsforsunconditions.Checkwiththedesignerbeforemakinganyfieldchangestothespecifiedplantlayoutforanyreason.

• Waterwellimmediatelyafterplantingandbeforemulching.Mulch & Armoring:

• Afterplanting,first applymulchas specified in theplan.Mulches areusually coarse intexture,suchasarborists’woodchipsorcoarsecompost,andtheplanmaycallforapplyingdifferentkindsofmulchtothebottomthantothesidesandtop.

• Mulchshouldbeappliedthicklybetweenplants(2to3inchesunlessotherwisespecified),moreheavilyatthetopedgesifnoplantsarecalledfor(4-6inches),andverylightlyrightaroundthebaseofplants,beingcarefulnottoplacemulchagainstwoodystems.

• Aftermulch,thelastfinishtoapplyiscleandrainrockorcobblestoarmorinflowsandoverflows.Applyaccordingtospecifications(4to8inchesunlessotherwisespecified).

• Bothmulchandrockshouldbeappliedbyhandtopreventdamagetotheplantsorfrommixingmaterials.

Applying clean drain rock to inflows and overflows prevents erosion and siltation of the rain garden. Photo: Erica Guttman

Be sure to follow proper planting guidelines so the rain garden gets off to a good start. Photo: Erica Guttman

Proper rain garden construction is the key to long-term success. Photo: Erica Guttman