Hereamidthesteelandconcretecanyons,greengrassgrows.Ahawthorntree(山楂树)standsinnewsoil,andfreshlydugplantsbendinthewind.ButChicagoCityHallhereseemsanunlikelyspotforagardenofanyvariety—especially20,000squarefeetofgardens—onitsroof.Asoneofahandfulofsimilarprojectsaroundthecountry,thegardenispartofa$1.5milliondemonstrationprojectedbythecitytoreduceits"urbanheatislands",saidWilliamAbolt,thecommissioneroftheDepartmentofEnvironment.Heatislands-darksurfacesinthecity,likerooftops-soakupheat.Theretentioncanbakeabuilding,makingitstubborntocooling.TheroofofCityHall,a90-year-oldgraystonelandmarkonLaSalleStreetintheheartofdowntown,hasbeenknowntoreachtemperaturesubstantiallyhotterthantheactualtemperatureonthestreetbelow.Thegardenwillprovidegreeneryandshade."Andthat,"saidthecityofficials,"willsavethecitydollarsonthoseblisteringsummerdays."Theprojectsavingsfromcoolingisabout$4,000ayearonanewroofwhoselifespanisabout50percentlongerthanthatofatraditionalroof.Thesprawlingopen-airrooftopgardenisbeingcarefullybuiltonamulti-tieredbedofspecialsoil,polystyrene,egg-carton-shapedconesand"waterproofmembrane"malltokeeptherooffromleaking,orcavingunderthenormalcombinedweightofsoil,rainandplantlife.Thedesigncallsforsoildepthsof4inchesto18inches.Whenthelastplantsandseedlingsareburiedandthelastbitofcompostislaid,thegardenwillhavecircularbrickstepping-stoneswindinguptohills."TheprimaryfocusofwhatwewanttodowastoestablishthislaboratoryonthetopofCityHalltogetpeopleinvolvedandunderstandingtheirimpactontheenvironmentandhowthelittlethingscanmakeanimpactonthequalityoflife",Mr.Aboltsaid,addingthattheplantsalsohelptocleartheair.Rooftopgardens,inplaceswhereconcretejungleshaveerasedplantsandtrees,arenotnew,noteveninChicago.Armsofgreenerydanglingoverterracesorsproutingfromrooftops,commoninEurope,arebecomingmoresointheUnitedStatesaspeoplebecomeincreasinglyconsciousabouttheenvironment.RichardM.Daley,whourgedtheenvironmentaldepartmenttolookintotheprojectafternoticingrooftopgardensinHamburg,Germanyafewyearsago,haspraisedthegardenasthefirstofitskindonapublicbuildinginthecountry.Itwillholdthousandsofplantsinmorethan150species-wildonionandbutterflyweed,sky-blueasterandbuffalograss-toprovidedataonwhatspeciesadaptbest.Smallplantsrequiringshallowsoildepthswerechieflyselected.
