HORTSCIENCE 42(3):588–595. 2007.

Effect of Substrate Depth on InitialGrowth, Coverage, and Survival of25 Succulent Green Roof Plant TaxaAngela K. Durhman and D. Bradley Rowe1

Department of Horticulture, Michigan State University, A212 Plant & SoilSciences Building, East Lansing, MI 48824

Clayton L. RughDepartment of Crop and Soil Sciences, Michigan State University, EastLansing, MI 48824

Additional index words. vegetative roof, eco-roof, living roof, plant evaluations, Graptope-talum, Phedimus, Rhodiola, Sedum, Crassulaceae

Abstract. Because of greater interest in green roofs in the United States, it is critical toincrease the number and geographic range of proven plant resources for long-termsurvival on rooftops. Successful plant taxa for extensive green roofs must establishthemselves quickly, provide high groundcover density, and tolerate extreme environ-mental conditions. Furthermore, dead load weight restrictions on many buildings maylimit the substrate depth that can be applied. The objective of this study was to evaluatethe effect of substrate depth on initial establishment and survival of 25 succulent planttaxa for green roof applications in the midwestern United States. Survival, initial growth,and rate of coverage were compared for plants grown in three substrate depths (2.5, 5.0,and 7.5 cm) on 24 roof platforms. Plant coverage was determined from image analysis ofweekly digital photographs. Results indicate deeper substrates promote greater survivaland growth; however, in the shallowest depth of 2.5 cm, several species continued topersist. Of the 25 species initially planted, only 47% survived in the deepest substrateof 7.5 cm. Recommended species at the depths tested for climates similar to southernMichigan include Phedimus spurious Raf. ‘Leningrad White’, Sedum acre L., S. album L.‘Bella d’Inverno’, S. middendorffianum L., S. reflexum L., S. sediforme J., and S. spuriumBieb. ‘Summer Glory’. Subsidiary species that are present at specific substrate depths butmay not exhibit an ability to cover large areas include S. dasyphyllum L. ‘Burnatii’, S.dasyphyllum L. ‘Lilac Mound’, S. diffusum W., S. hispanicum L., and S. kamtschaticumFisch. The primary deterrent for these subsidiary species was little to no survival at 2.5cm. Deeper substrates promoted greater survival and growth for nearly all species tested.

Vegetated green roofs provide numerousbenefits to the built environment such as areduction in stormwater runoff, buildinginsulation, and mitigation of the urban heatisland effect (DeNardo et al., 2005; Getterand Rowe, 2006; Liu, 2004; VanWoert et al.,2005a). Many extensive (shallow) greenroofs consist primarily of low-maintenancesucculent perennial species such as Sedum L.,Delosperma N.E.Br., and Sempervivum L.;grasses like Festuca L.; and herbaceousplants such as Allium L. and Dianthus L.(Dunnett and Kingsbury, 2004; Dunnett and

Nolan, 2004; Köehler, 2003; Snodgrass andSnodgrass, 2006). As more green roofsbecome established in the United States, itis critical to increase the number and geo-graphic range of proven plant resources forlong-term survival on rooftops.

Likely candidates can be found in extremeenvironments such as rock outcroppings orunder alpine conditions. Species classified aschamaephytes grow in alpine regions wheresnow cover protects the shoots and budsagainst water loss. Chamaephytes are sub-shrubs and herbs with vegetative shoots thatlie along the ground and remain intact at thebeginning of an unfavorable season (Raun-kiaer, 1934). Sedum are classified as passivechamaephytes because response during unfa-vorable conditions results in shorter inter-nodal length and reduced shoot lengths. Interms of growth habit, their low-growing,spreading groundcover characteristics makethem ideal for covering extensive greenroofs. Plants that cover the substrate in ashort period of time reduce potential erosionproblems and inhibit weeds. Species that arelong-lived, reseed themselves, or spread veg-etatively should continue to provide 100%coverage as long as environmental conditions

are favorable. German guidelines require atleast 60% vegetative coverage to be approvedas a green roof (FLL, 1995).

In addition to morphologic and growthhabit characteristics, many succulents areideal for extensive green roofs because theyare physiologically adapted to withstandharsh environmental conditions (Gebauer,1988). Some have been documented to ex-hibit Crassulacean acid metabolism (CAM),a metabolic pathway that enables them toadapt to water-stressed environmental con-ditions (Gebauer, 1988; Sayed, 2001; Ting,1985). CAM plants usually have fewer sto-mata than C3 and C4 plants, and these stomatacan open at night for the uptake of CO2, thusreducing daytime water loss. In a controlledgreenhouse study, Durhman et al. (2006)found that several species of Sedum survivedand maintained active photosynthetic metab-olism even after 4 months without water.Another drought-resistant mechanism ofCAM plants is to store water in the succulentleaves (Sayed, 2001).

In northern climates, winter cold hardi-ness is a major factor in overwinteringsurvival. In Madrid, Spain, researchers foundthat when subjected to 10 �C for 11 h,S. forsterianum Sm. and various strains ofS. album L. suffered, but S. micranthumBast., S. rupestre L., and S. ochroleucumChaix all survived at a substrate depth of3.5 cm (Gómez-Campo, 1996). In NorthAmerica, Boivin et al. (2001) reported that,for the six species tested in Quebec, greaterfreezing injury occurred at shallow substratedepths of 5 cm compared with 9 cm or 11.5cm. Monterusso et al. (2005) compared 18native forbs and grasses with nine species ofSedum over three seasons (2001–2004) on aroof platform in Michigan. All nine of theSedum thrived, but only four of the 18 nativetaxa were found to be acceptable in 10 cm ofsubstrate. It is important to note that althoughQuebec experiences colder winters thanMichigan, the Quebec study was conductedon a heated building, whereas the Monterussostudy and this study took place on unheatedroof platforms.

The U.S. Dept. of Agriculture (USDA)has developed a plant hardiness zone map toquantify average annual minimum tempera-ture data for the purpose of predicting plantsurvival. Many ornamental plant specieshave been assigned to a range of hardinesszones where they are most likely to survive.However, some of the species and cultivarsexamined in this study have not been pre-viously reported for use on Michigan greenroofs and do not have published USDAhardiness zones. Thus, this study offers newplant recommendations for locations withsimilar climates.

Successful candidates for extensive greenroofs exhibit characteristics such as rapidestablishment, high groundcover density, tol-erance to extreme environmental conditions,and successful winter recovery (ASTM,2006; Getter and Rowe, 2006). Substratedepth can influence all of these factors. Long-term persistence is also important because

Received for publication 10 Dec. 2006. Acceptedfor publication 23 Feb. 2007.Funding for this study was provided by Ford MotorCompany, Dearborn, Mich.; ChristenDETROITRoofing Contractors, Detroit, Mich.; WolfgangBehrens Systementwicklung, GmbH, Groß Ipp-ener, Germany; International Sedum Society,Northumberland, U.K.; Perennial Plant Associa-tion; Michigan Department of Agriculture; andthe Michigan Agricultural Experiment Station.This paper is a portion of an MS thesis submittedby Angela K. Durhman.1To whom reprint requests should be addressed;e-mail rowed@msu.edu

588 HORTSCIENCE VOL. 42(3) JUNE 2007

green roofs are dynamic systems. However,this article concentrates on initial establish-ment. Therefore, the objective of this studywas to evaluate 25 succulent plant species forgreen roof applications in the midwesternUnited States by measuring the effect ofsubstrate depth on initial growth rates, cov-erage, and survival.

Materials and Methods

An initial growth and coverage study wasconducted on raised roof platforms at theHorticulture Teaching and Research Centerat Michigan State University (MSU), EastLansing, Mich. The study was a split-plotcompletely random design with substratedepth as the main plot factor and species asthe subplot factor. Each species was repli-cated eight times within each substrate depthfor a total of 600 plants.

Platforms. Twenty-four 123 cm · 123-cmraised-roof platforms were constructed. Eachpressure-treated wood platform was built perthe same ASTM International standards thatwould be required for a commercial buildingand equipped with layers of insulation, water-proofing, a green roof drainage system, rootbarrier, substrate, and a 2% slope for drain-age. In each plot, excess water drainedthrough three drilled holes at the base of theslope �3 cm in diameter and covered by amesh filter screen. The tops of each individ-ual wood frame plot were bordered withflexible meter tape for rescaling and orien-tating the images.

Platforms included a green roof drainagelayer (XF108) and vegetation carrier (XF301;Wolfgang Behrens Systementwicklung, GmbH,Groß Ipener, Germany). The drainage layerconsisted of a geotextile fabric with attachednylon coil. The nylon coils faced down wheninstalled and the total thickness of this layerwas �1.5 cm. A water retention fabric layer,�0.75 cm thick, was added with the capacityto hold up to 800 g�m–2 of water. The waterretention fabric layer was composed of arecycled synthetic fiber mixture of polyester,polyamide, polypropylene, and acrylic fibers.The vegetation carrier consisted of a geo-textile fabric with nylon coils attached andfilled with substrate.

Substrate. Substrate depths of 2.5 cm,5.0 cm, and 7.5 cm were randomly assignedto the 24 platforms. Substrate consisted of40% heat-expanded slate (gradation of 3 to5 mm) (PermaTill; Carolina Stalite Com-pany, Salisbury, N.C.), 40% U.S. Golf Asso-ciation grade sand (Osburn Industries,Taylor, Mich.), 10% Michigan Peat (OsburnIndustries, Taylor, Mich.), 5% Dolomite(Osburn Industries), 3.33% composted yardwaste (Renewed Earth, Kalamazoo, Mich.),and 1.67% composted turkey litter (Her-bruck’s, Saranac, Mich.). Substrate propor-tions are based on volume. At the time ofplanting, electrical conductivity and pH ofthe media were 3.29 mmho�cm–1 and 7.9,respectively. All treatments had 100 g�m–2 ofNutricote type 100, 18N–6P–9K controlled-release fertilizer (Agrivert, Webster, Texas)

hand-applied 47 d after planting on 28 July2003 and the following summer on 29 July2004 at the same rate.

Plant species. Stem and leaf cuttings of25 Crassulaceaen plant species were excisedfrom stock plants growing in the MSU PlantScience Greenhouses on 11 June 2003.Length of the unrooted cuttings ranged from2 to 4 cm but were uniform in size withinspecies. Cuttings were stored overnight at5 �C and propagated the next day on theoutdoor platforms (day 1). Cuttings wereplaced on 20-cm centers with 25 individualspecies per plot. The location of individualcuttings within each plot was randomlyassigned. Species included Graptopetalumparaguayense subsp. Rose, Phedimus spuriusRaf. ‘Leningrad White’, Rhodiola pachycladaL., R. trollii L., Sedum acre L., S. albumL. ‘Bella d’Inverno’, S. clavatum C., S. con-fusum Hemsley, S. dasyphyllum L. ‘Burnati’,S. dasyphyllum L. ‘Lilac Mound’, S. diffusumW., S. hispanicum L., S. kamtschaticum Fisch.,S. mexicanum Britt., S. middendorffianum L.,S. moranense Kunth, S. pachyphyllum Clau-sen, S. reflexum L., S. sediforme J., S. ‘RockeryChallenger’ H., S. ‘Spiral Staircase’ H.,S. spurium Bieb. ‘Summer Glory’, S. surculo-sum var. luteum Cos., S. · luteoviride C., andS. · rubrontinctum C.

During the first 22 d of the study, theplatforms were covered with a shadecloth.To help acclimate the plants, the shadeclothwas removed, except on bright sunny days,up until day 31, at which time it was removedpermanently.

Irrigation. During the establishmentperiod, plots were overhead-irrigated withRain Bird (Azusa, Calif.) Xerigation XS-180spray heads fixed to 30.5-cm Polyflex risers.The risers were placed at increments measur-ing 120 cm. For the first 20 d, the plots wereirrigated for 5-min cycles at 0700, 1100, 1400,1700, and 2000 HR. Each 5-min cycle appliedenough water to saturate each plot, misting�4.0 mm (30 ml) per plot. Irrigation durationwas reduced to 2-min cycles from days 21 to41. After day 41, automated irrigation endedbut occurred periodically to maintain planthealth the first year. In the second growingseason, supplemental irrigation was not used.

Weed species. During the establishmentperiod, numerous weed seedlings emergedand included Cirsium arvense L. Eleusineindica L., Eragrostis cilianensis All., Mol-lugo verticillata L., Panicum capillare L.,Populus deltoides Marshall, Salix nigraMarsh., and Senecio vulgaris L. Emergingweeds were hand pulled up to day 33. Theywere then allowed to grow until day 86, atwhich time all weeds were removed. There-after, plots were managed to remain weed-free for ease of data collection to maintain theoriginal goals of measuring growth rates fordesired planted species.

Data collection. Measurements of two-dimensional plant coverage were recorded bytaking weekly digital images (32 MB, 1800 ·1200 pixel, fine quality). A portable camerastand was constructed to raise a camera�163 cm above the platforms. A digital

camera (FUJIFILM MX-2900 zoom, 2.3mega pixels; Fuji Photo Film Co., Ltd.,Tokyo, Japan) equipped with an F3.3/F7.6wide conversion lens was suspended on thecamera stand. The focal distance was set at 22mm and the focal range set at 0.9 m.Although planted on 12 June 2003, imageswere first taken on 8 July (day 27). Weeklyanalysis occurred during the initial growingseason defined as the time up until the plantsentered dormancy in late fall and a hard frostoccurred on 28 Oct. 2003 (day 139). Datacollection resumed the next spring on 24 Mar.2004 (day 287). This method was used until19 May 2004 (day 343) when it became toodifficult to distinguish individual speciesbecause plant canopies began to overlap.

Survival rates were recorded during estab-lishment, after the first growing season, thenext spring, and at the end of the secondgrowing season (Table 1). The establishmentperiod was defined as the period up to 7 dafter supplemented irrigation ended when90% of the individuals had rooted. Persis-tence for the first year was scored on 28 Oct.2003 (day 139) after a hard frost. To consideroverwintering success, presence of individu-als at day 139 were compared with theirpresence on 12 May 2004 (day 336). A finalassessment of persistence during the secondgrowing season was made on day 482 after ahard frost on 5 Oct. 2004.

Image analysis. Plant growth rates andhorizontal vegetative coverage were deter-mined in a nondestructive method by usingSigmaScan Pro 5.0 image analysis software(SPSS Science, Chicago). Vertical heightwas not measured. Coverage (plant commu-nity development) in each plot was measuredto compare growth relative to substrate depth(Fig. 1). Digital images were analyzed todetermine the percentage of the total hori-zontal vegetative canopy attributed to eachindividual. Image area was delineated for thequadrat area using the two-point rescalingfunction, and then individual plants wereanalyzed using the manual trace mode (Olm-stead et al., 2004). Manual trace mode wasnecessary because the software programcould not automatically distinguish color,intensity, and hue differences between plantmaterials and substrate. A preliminary testestablished the accuracy of the method oftaking weekly images, analyzing them in animage analysis program, and converting toactual centimeters squared. By measuringpaper images of a known area (10 cm2), itwas determined that the measurements were94% accurate relative to actual size.

Vegetative growth was recorded byweekly image analysis beginning on day 27(8 July 2003), after the 26 d establishmentperiod when individual cuttings rooted.Because of snow cover, analysis of weeklyimages resumed the next spring on day 287(24 Mar. 2004). Because it became toodifficult to distinguish individual speciesboundaries, image analysis collection endedon day 342 (18 May 2004).

As a result of size variability amongpropagules after the 26 d establishment

HORTSCIENCE VOL. 42(3) JUNE 2007 589

period and at the beginning of the secondseason, the increase in area was calculated toshow vegetative groundcover relative tostarting size. During 2003, this increase wasdefined as final area of coverage on day 139(28 Oct.) minus the initial coverage whenimage analysis was first used on day 27 (8July). Likewise, this value was calculated thesecond season (2004) as the final area on day342 (18 May) minus the initial on day 287 (23Mar.) that spring. Measurements for eachspecies were averaged across the eight repli-cations at each species depth. Growth ratewas defined using the area of coverage graphsto measure the slope of area divided by time.

Statistical analysis. Data were analyzedseparately for 2003 and 2004 years. Signifi-cant differences between species growth anddepth on specific weeks were determinedusing multiple comparisons (least significantdifferences) with Tukey-Kramer adjustments(PROC MIXED, SAS version 8.02; SASInstitute, Cary, N.C.). Survival percentageswere compared using a mixed model inwhich time and depth were factors andspecies was nested in depth. Survival datadid not require a log transformation becauseit was observed to be normally distributed.Overall coverage of vegetation analyzed atthe end of the study was tested using leastsignificant differences (PROC GLM, SASversion 8.02; SAS Institute).

Results and Discussion

Survival. In general, plants grown in thedeeper substrate depths of 5.0 and 7.5 cmexhibited higher survival rates than thosegrown at the 2.5-cm depth (Table 1). Allindividual cuttings did not survive the prop-agation period; however, no single speciesexperienced complete mortality for alleight replications at any depth. Rhodiolapachyclada, S. clavatum, S. kamtschaticum,S. ‘Rockery Challenger’, and S. surculosumvar. luteum exhibited less than 100% propa-gation survival on some substrate depths aftersupplemental irrigation was ended. In addi-tion, S. clavatum and S. surculosum var.luteum displayed less than 100% propagationsurvival at all three depths.

At the end of the first season (day 139),plant mortality was most prominent at the2.5-cm depth where survival percentagedeclined for R. pachyclada, R. trollii,S. dasyphyllum ‘Lilac Mound’, and S. surcu-losum var. luteum. Individuals grown in the7.5-cm depth were least affected by the endof the first season because only S. surculosumvar. luteum experienced a significant declinein survival. Regardless of depth, S. clavatumdid not survive the first growing season andS. surculosum var. luteum only survived atthe 7.5-cm depth.

In addition to initial establishment andgrowth, plant hardiness of the shoots and rootsystems are critical for longevity, stability,and appearance of extensive green roofs(Boivin et al., 2001; Durhman et al., 2004;Getter and Rowe, 2006). Substrate depthappeared to influence plant cold hardinessT

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590 HORTSCIENCE VOL. 42(3) JUNE 2007

with deeper substrate depths of 5.0 and 7.5cm supporting greater overwintering survivalthan those grown at the 2.5-cm depth. At theshallow substrate depth of 2.5 cm, only nineof the 25 species overwintered comparedwith 12 and 14 species for the 5.0- and7.5-cm depths, respectively. Deeper sub-strates likely provided greater moisture reten-tion and root protection from temperaturefluctuations and allowed for more verticalspace for plant roots to grow before reachingthe root barrier. A more stable environmentallows plants to grow stronger and healthier,which affects their ability to survive harshclimatic conditions of drought and temper-atures. However, even with deeper sub-strates, mortality during winter could be theresult of death of the root systems, which aregenerally not as cold-tolerant as the tops ofplants (Wu and Cosgrove, 2000).

In this study, one must remember that theplants were growing on roof platforms so theambient air temperature was the same aboveand below the green roof. This would makethe root systems more susceptible to freezing.The winter of 2003–2004 was typical for EastLansing with a minimum temperature of–24.3 �C recorded at the research site and141 d with a minimum temperature below0 �C (Fig. 2). On the roof of a heated building,the rooting substrate would be warmed some-what from heat transfer from the buildingbelow. If freezing of root systems was thecause of death, then one would expect that if aparticular species survived on a roof plat-form, then it would also survive on the roofsof unheated and heated buildings.

Rhodiola pachyclada, R. trollii, S. dasy-phyllum ‘Burnatii,’ S. dasyphyllum ‘LilacMound,’ S. diffusum, S. hispanicum, S. kamt-schaticum, S. sediforme, and S. spurium‘Summer Glory’ all increased their survivalrates when grown in deeper substrates. Noplants of Sedum dasyphyllum ‘Burnatii’ sur-vived at 2.5 cm, but survival increased to50% and 100% at 5.0 and 7.5 cm, respec-tively. Similarly, S. hispanicum and S. kamt-schaticum exhibited a dramatic increase insurvival rates when the depth was increasedfrom 2.5 to 5.0 cm. At the end of the secondseason, all plants of five species were stillalive regardless of substrate depth. Knowl-edge of how a species will perform at varioussubstrate depths is important when choosingplant species for a green roof where substratedepth must be kept to a minimum because ofbuilding weight restrictions.

Results for S. acre, S. album, S. kamt-schaticum, S. middendorffianum, S. reflexum,and S. spurium support previous research thatthese species can survive on extensive greenroofs in the midwestern United States(Monterusso et al., 2005; Rowe et al.,2006a, 2006b). In addition, all P. spurius‘Leningrad White’ and S. sediforme survivedregardless of substrate depth. Species such asS. mexicanum, which exhibited high cover-age values and a fast rate of establishmentduring the first growing season but no wintersurvival, may be more suited for green roofsin warmer climates.

Growth rate. Substrate depth affectedgrowth rate, although not immediately (Fig.1). This is probably because the developingroot systems were not yet large enough toexploit the entire depth of the substrate.Growth after establishment varied acrossspecies. Depending on substrate depth, sev-eral species established and grew quicklyearly in the season. Differences in initialgrowth rates could be attributed to individu-als’ propagation potential, aggressiveness toestablish in an open area, and resourceallocation.

When image analysis first began on day27 (8 July 2003) until day 55 (5 Aug. 2003),S. album ‘Bella d’Inverno’ (1.6 cm2�d–1) was

the only species that exhibited a growth rategreater than 1.5 cm2 in coverage per day at adepth of 2.5 cm. This value improved to threespecies at a 5.0 cm with S. album ‘Bellad’Inverno’ (1.9), S. mexicanum (1.7), andS. spurium ‘Summer Glory’ (1.8) all exhibit-ing growth greater than 1.5 cm2�day–1. At7.5 cm, eight species were above this value:P. spurius ‘Leningrad White’ (2.5), S. acre(2.0), S. album ‘Bella d’Inverno’ (3.5),S. diffusum (1.7), S. hispanicum (1.5),S. mexicanum (3.2), S. middendorffianum(1.6), and S. spurium ‘Summer Glory’ (2.2).

Between days 55 and 125 (14 Oct. 2003),three, eight, and 14 species exhibited agrowth rate greater than 1.5 cm2�d–1 at the

Fig. 1. Growth of 25 taxa (Graptopetalum, Phedimus, Rhodiola, and Sedum) cultivated on green roofplatforms at three depths (2.5, 5.0, and 7.5 cm). Growth was calculated from digital image analysis.Data collected once a week for 20 weeks, resumed the next spring on week 41, and ended on week 49when plants were too dense to discriminate. Error bars represent standard error. Break in x-axis denoteswinter.

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2.5-, 5.0-, and 7.5-cm depths, respectively. At2.5 cm, species included S. album ‘Bellad’Inverno’ (2.2), S. diffusum (1.7), andS. mexicanum (1.7). At 5.0 cm, S. acre (3.3),S. album ‘Bella d’Inverno’ (3.7), S. diffusum(4.7), S. hispanicum (2.8), S. mexicanum (4.0),S. middendorffianum (1.7), S. reflexum (1.5),and S. sediforme (1.9) fit into this category. At7.5 cm, the list included P. spurius ‘LeningradWhite’ (2.9), S. acre (5.6), S. album ‘Bellad’Inverno’ (7.5), S. dasyphyllum ‘Burnati’(2.7), S. diffusum (4.3), S. hispanicum (4.0),S. kamtschaticum (1.6), S. mexicanum (7.2),S. middendorffianum (2.8), S. moranense(1.6), S. reflexum (2.7), S. sediforme (2.9),S. spurium ‘Summer Glory’ (2.1), and S. ·rubronticum (1.6). From day 125 to the firsthard frost on day 139 (28 Oct. 2003), S. acre(1.8) was the only species at any depth that

had a growth rate greater than 1.5 cm2�d–1.This occurred at a depth of 7.5 cm.

After winter, growth resumed for mostspecies the second season. There was little orno observable vegetation present on day 287for deciduous species such as S. kamtschati-cum and R. pachyclada. However, becauseregeneration occurred later in spring, growthrates improved. Some species had vegetativedieback in the plant’s center (semidecidu-ous), although surrounding tissues wereactively recovering from winter injury orgrowing. For this observation, plant materialthat looked healthy (turgid or leaf colorsimilar to the previous year’s growth) wasrecorded. Species that exhibited diebackincluded S. dasyphyllum ‘Burnatii’, S. dasy-phyllum ‘Lilac Mound’, S. hispanicum, and S.album ‘Bella d’Inverno’. However, by May,

winter injury was no longer observed. Oneinteresting observation was S. middendorffia-num, S. spurium ‘Summer Glory’, andS. kamtschaticum had much faster growthrates in the second year compared with theirperformance the prior year. Sedum acre andS. album ‘Bella d’Inverno’ had consistentlyincreasing growth rates.

During this second season, there was zeroto minimal growth at 2.5 cm between days286 (23 Mar. 2004) and 314 (20 Apr. 2004)with S. middendorffianum displaying thehighest growth rate of 0.6 cm2�d–1. At5.0 cm, S. middendorffianum (4.7) was stillthe only species with a growth rate above1.5 cm2�d–1. At a depth of 7.5 cm, P. spurius‘Leningrad White’ (2.5), S. acre (3.3),S. album ‘Bella d’Inverno’ (2.3), S. kamt-schaticum (3.3), S. middendorffianum (14.7),S. reflexum (3.7), and S. spurium ‘SummerGlory’ (2.2) all exhibited rapid early growth.Other species such as S. mexicanum,S. moranense, and S. · rubronticum thatdisplayed rapid growth the previous yearwere absent at this stage because they didnot survive the winter.

During the next 28 d up until day 342(18 May 2004), rapid growth occurred forP. spurius ‘Leningrad White’ (2.2), S. acre(2.6), S. album ‘Bella d’Inverno’ (4.3),S. middendorffianum (3.6), S. reflexum(1.9), and S. spurium ‘Summer Glory’ (2.5)at 2.5 cm and P. spurius ‘Leningrad White’(4.2), S. acre (8.9), S. album ‘Bella d’Inverno’(9.0), S. kamtschaticum (6.9), S. middendorf-fianum (11.0), S. reflexum (5.4), and S. spu-rium ‘Summer Glory’ (7.8) at 5.0 cm. At 7.5cm, these values were even higher: P. spurius‘Leningrad White’ (8.9), S. acre (14.6),S. album ‘Bella d’Inverno’ (15.8), S. hispani-cum (9.6), S. kamtschaticum (10.6), S. mid-dendorffianum (15.8), S. reflexum (9.1), andS. spurium ‘Summer Glory’ (6.6). After day342, it became too difficult to distinguishspecies by image analysis from digital photo-graphs because some species were beginningto grow over the top of others to form multiplecanopy layers. However, visual observationsconfirmed that plants continued to spreadthroughout the second growing season(Durhman, 2005). Sedum acre, S. album‘Bella d’Inverno’, and S. middendorffianumdisplayed the most growth and exhibited thehighest percentage of cover by day 482 (5Oct. 2004).

Across all species, plant vigor (i.e., thefastest growth rates) was greatest at thedeepest substrate depth of 7.5 cm. Althoughthe 2.5-cm depth did not promote growth tothe same extent as the deeper substrates,plants remained alive. This agrees with thework of VanWoert et al. (2005b) whoreported that watering was necessary every14 d to support growth of a mixture of sedumin a green roof substrate with a 2-cm mediadepth but only once every 28 d when thesubstrate depth was increased to 6 cm.Although growth was diminished, theseplants survived 88 d without water (Van-Woert et al., 2005b). Over time, growth rateswithin depths varied across plant species,

Fig. 1. Continued

592 HORTSCIENCE VOL. 42(3) JUNE 2007

especially for some species including S. acre,S. album ‘Bella d’Inverno’, S. diffusum,S. hispanicum, S. mexicanum, and S. mid-dendorffianum (Fig. 1). This is attributable inpart to favorable growing conditions such asamount and duration of rainfall and temper-atures (Fig. 2).

Coverage. As expected, those species thatexhibited the fastest growth rate covered thegreatest area when image analysis ended onday 343 (Fig. 1). Species with the greatestamount of coverage included P. spurius‘Leningrad White’, S. acre, S. album ‘Bellad’Inverno’, S. hispanicum, S. middendorffia-num, and S. reflexum. An exception wasS. mexicanum, which exhibited high cover-age values and a fast rate of establishmentacross all depths during the first growingseason. However, it was not able to survivewinter and completely disappeared by thesecond season. In fact, at the end of the firstseason, S. mexicanum covered a greater areathan all other species except for S. album‘Bella d’Inverno’.

For most species, coverage was signifi-cantly different at each depth with the great-est area of coverage occurring for plantsgrowing in 7.5 cm of substrate (Fig. 1). Byday 343, plants growing in 2.5, 5.0, and 7.5cm of substrate had reached 47%, 74%, and96% coverage, respectively (P # 0.05). The47% coverage on the 2.5-cm depth had stillnot reached the minimum 60% coverage to beapproved as a green roof according to GermanFLL standards (FLL, 1995). The most vigor-ous spreader, S. middendorffianum, covered1242 cm2 at a depth of 7.5 cm but only

670 cm2 and 220 cm2 at 5.0 and 2.5 cm,respectively (Fig. 1). However, not all spe-cies exhibited a significant difference incoverage between depths of 5.0 and 7.5 cm.Sedum spurium ‘Summer Glory’ covered 377and 452 cm2 at 5.0 and 7.5 cm, respectively,but only 108 cm2 at a depth of 2.5 cm.

Increases in coverage within each seasonalso mirrored overall coverage and wasdependent on substrate depth (Table 2).Similar to total coverage, during the imageanalysis phase of the second season, theincrease in horizontal growth for Sedumspurium ‘Summer Glory’ at depths of 5.0(229 cm2) and 7.5 cm (245 cm2) was notdifferent but was much greater than horizon-tal growth at 2.5 cm (65 cm2). There werealso differences among seasons. Some spe-cies such as S. dasyphyllum ‘Burnatii’,S. dasyphyllum ‘Lilac Mound’, and S. sed-iforme did not expand further or decreased incoverage regardless of substrate depth (Table2) during the second growing season. Thismay be the result of increased competitionduring the second season from plants thatwere more vigorous. Fast establishment andsubstrate coverage are desirable character-istics for green roof plant taxa. Fast initialgrowth is important because the faster theplants cover the substrate surface, the fewerthe number of plants required and the lessexpensive they will be to purchase andinstall.

Life form characteristics influenced spe-cies survival, growth, and coverage. Raun-kiaer (1934) classified the genus Sedum aspassive chamaephytes, meaning evergreen

or deciduous vegetative shoots lay along theground and remain intact at the beginningof the unfavorable season. Evergreen speciessuch as S. acre and S. album retain theirvegetation over the Michigan winter. Addi-tionally, their vegetative shoots quickly rootand grow in different areas of the plots earlyin the growing season. In the spring, theyhave an obvious spatial advantage by pre-dominating a particular area within the plot.In contrast, deciduous plants like S. kamt-schaticum are not frost-tolerant and above-ground shoot tissues die in late fall withadverse weather conditions, although newvegetative growth occurs from regenerativebuds in the spring. However, this influencesthe coverage present early in the season.They are at a disadvantage because they mustcompete spatially against evergreen species;however, their growth rates are comparablelater in the growing season (Fig. 1).

Although not apparent in this study,improved coverage and presence at the endof the second growing season by S. hispani-cum was attributable mainly to its prolificreseeding ability in late summer, especiallycompared with other species tested. In thesecond year, S. hispanicum flowered through-out June and July with seedlings emergingby the beginning of August. Other speciesthat reseeded in the second season includeS. acre and S. album ‘Bella d’Inverno’.Overall, plants selected for this trial generallyreproduce easily by asexual means of stemor leaf cuttings without the use of com-mercial rooting compounds (Stephenson,2002). Over time, original plants couldhave easily reestablished themselves in theplots by vegetative means, thereby increasingtheir coverage and presence at the end ofthe study.

Conclusion

Most of the species examined within thisstudy have not been previously reported foruse on green roofs in the Michigan climate.Furthermore, some of these species andcultivars do not have published USDA har-diness zones. Therefore, this study offersnew plant recommendations for use on greenroofs. Of the 25 species initially planted, only47% survived in the deepest substrate of 7.5cm. Recommended species at the depthstested for climates similar to southern Mich-igan include P. spurius ‘Leningrad White’, S.acre, S. album ‘Bella d’Inverno’, S. midden-dorffianum, S. reflexum, S. sediforme, and S.spurium ‘Summer Glory’. Subsidiary speciesthat are present at specific substrate depthsbut may not exhibit an ability to initiallycover large areas include S. dasyphyllum‘Burnatii’, S. dasyphyllum ‘Lilac Mound’,S. diffusum, S. hispanicum, and S. kamtscha-ticum. The primary deterrent for these sub-sidiary species was little to no survival at 2.5cm. Deeper substrates promoted greater sur-vival and growth for nearly all species tested;however, in the shallowest depth of 2.5 cm,several species were observed to form stable

Fig. 1. Continued

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communities. In choosing a green roof sys-tem, it is important to consider both substratedepth and plant species growth factors forsustained growth.

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Fig. 2. Daily maximum and minimum temperatures (�C) and precipitation (millimeters) during theexperimental study (12 June 2003 through 06 Oct. 2004). Weather data were taken using the MichiganAutomated Weather Network’s East Lansing weather station (located adjacent to the research site).

Table 2. Increase in coverage (cm2) as calculated from image analysis from days 27 to 139 (8 July 2003to 28 Oct. 2003) and 287 to 343 (24 Mar. 2004 to 19 May 2004).

Increase in area of coverage (cm2) within each season

2003 Depth (cm) 2004 Depth (cm)

Taxa 2.5 5 7.5 2.5 5 7.5

G. paraguayense 26 a 46 aP. spurius Leningrad White 47 b 106 b 281 a 61 b 127 ab 320 aR. pachyclada –2 a 6 a 23 a 19R. trollii –3 a –1 a 3.6 a 12S. acre 128 c 288 b 472 a 67 c 265 b 501 aS. album Bella d’Inverno 210 c 330 b 641 a 118 c 260 b 507 aS. clavatumS. confusum 7 a 31 a 83 aS. dasyphyllum Burnatii 9 c 106 b 205 a 7 a 8 aS. dasyphyllum Lilac Mound –9 a 21 a 42 a –14 a 17 aS. diffusum 149 b 384 a 355 a 32 a 66 aS. hispanicum 98 c 224 b 342 a 6 b 3 b 289 aS. kamtschaticum 12 b 73 ab 121 a –1 c 222 b 387 aS. mexicanum 150 c 338 b 599 aS. middendorffianum 58 c 144 b 237 a 118 c 441 b 855 aS. moranense 38 b 57 b 128 aS. pachyphyllum 26 b 77 ab 109 aS. reflexum 50 b 133 ab 235 a 46 c 175 b 359 aS. Rockery Challenger 30 a 42 a 42 aS. sediforme 76 b 155 ab 234 a –20 a –13 a –10 aS. Spiral Staircase 29 a 66 a 85 aS. spurium Summer Glory 35 b 124 ab 197 a 65 b 229 a 245 aS. surculosum var. luteum 17S. · luteoviride 12 a 48 a 84 aS. · rubrotinctum 48 b 100 ab 124 aMean separation in rows among depths within each growing season for each taxa were tested using leastsignificant difference with Tukey-Kramer adjustments (P # 0.05) (n = 8). Tests were separated by yearbefore analysis. Blanks denote no surviving plants for specific species.

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