water

Article Performance of Three Different Native Mixtures for Extensive Green Roofs in a Humid Subtropical Climate Context

Giampaolo Zanin 1,* and Lucia Bortolini 2

1 Department of Agronomy, Food, Natural Resources, Animals and Environment (DAFNAE), University of Padova, Viale dell’Università, 16, 35020 Legnaro, Italy 2 Department of Land, Environment, Agriculture and Forestry, University of Padova, Agripolis, Viale dell’Università 16, 35020 Legnaro, Italy; [email protected] * Correspondence: [email protected]; Tel.: +39-049-827-2902

 Received: 28 September 2020; Accepted: 9 December 2020; Published: 11 December 2020 

Abstract: Most of the services and benefits of green roofs are related to the substrate as well as the vegetation layer. Although plant selection should be made on the basis of green roof typology, morphology, and climate conditions, very often, species only are used worldwide. However, they do not always guarantee the best performances; hence, it is important to investigate different plant species and their performance in different climate contexts. Herein, an experiment was conducted using three plant mixes (i.e., a Sedum mix, a perennial herbaceous mix, and a suffruticose mix), grown in boxes containing two substrates (a volcanic substrate or a recycled crushed brick substrate) and two drainage/storage layers (a preformed layer or a mineral layer), in factorial combination. The Sedum mix showed a high canopy cover, comparable to or even higher than that of the other mixes, particularly when supplemental irrigation was stopped. However, the actual crop coefficient (Kc act) of the herbaceous and suffruticose mixes was often higher than that of the Sedum mix. The results also showed that both the substrate and the drainage/storage layer may improve Kc act values as a consequence of their capacity for stormwater retention.

Keywords: Sedum; herbaceous perennials; suffruticose; canopy cover; actual crop coefficient

1. Introduction The presence and coverage of have great influence on the services and benefits of green roofs [1], among which the most appreciated are those related to stormwater management, such as the delay in runoff peak and the direct retention of a portion of the rainfall, even if most of this retention is a function of the growth substrate [2–5]. Additionally, the vegetation and substrate of green roofs can also provide a wildlife habitat for many species and can improve biodiversity [1]. Urban surfaces and roof coatings contribute to the urban heat island (UHI) effect. Green roofs can mitigate the UHI effect and can improve building energy performance thanks to the characteristics of their layers and the presence of vegetation. In particular, the lower roof temperatures in summer result in energy savings from lower air conditioning costs. Green roof vegetation can mitigate air temperature via evapotranspiration phenomena, but albedo (reflectivity) is also a key determinant of roof temperature, with more reflective vegetation correlated with lower surface temperatures [6–10]. Green roofs can enhance the ornamental and landscape design both inside and outside urban areas, increasing the esthetic value of property [11,12]. Furthermore, despite vegetation suffering from pollution, at the same time, it is the main means through which pollutants can be reinserted into natural environments thanks to interception and biological re-elaboration [13], also achieved under

Water 2020, 12, 3484; doi:10.3390/w12123484 www.mdpi.com/journal/water Water 2020, 12, 3484 2 of 15 green roof conditions [14]. In particular, the main reduced pollutants are nitrogen oxides (removed by absorption and sometimes used by the plant as a source of nitrogen), sulfur dioxide (absorbed and reduced, but toxic in large quantities), carbon monoxide (removed in small quantities), chlorine compounds (effectively removed), and ozone (removed very effectively, but rapidly toxic for many species) [15]. Recently, green roofs have gained interest in Italy, and are among the preferred natural water retention measures (NWRMs) with vegetation [16,17], which also include rain gardens and bioretention basins. However, their spreading is generally based on the application of ready-to-use packages, which are poorly tested under specific environmental conditions. As mentioned above, among green roof components, the growing substrate type and thickness play the most important role in runoff reduction, but the vegetation layer can also affect runoff by intercepting, retaining, and transpiring rainwater [18–20]. The stormwater retention efficiency of a green roof is, in fact, a consequence of its evapotranspiration feature and, focusing on the contribution of vegetation, of its water use efficiency (i.e., crop coefficient (Kc)). Despite Sedum species being the most used plant material worldwide [21,22], they do not always assure the most benefits in terms of performance, for example, stormwater retention, cooling effect, and natural biodiversity preservation [23–27], and sometimes do not even guarantee plant survival [28]. Hence, there is great interest in exploring different plant species, with major emphasis on native species adapted to water shortage [23,29]. Herein, a study was carried out to test the green roof solution that is most suitable in the humid subtropical climate context of the Veneto Plain and that is able to better manage rainfall water for reducing outflow volumes from building roofs into the urban drainage systems. Twelve green roof microcosms derived from the factorial combination of three mixtures of native plants, two substrates, and two drainage/storage layers were studied. Specifically, in this paper, the plant performances in terms of plant cover and actual crop coefficient are reported.

2. Materials and Methods The experiment was conducted in the Agripolis Campus of the University of Padova, located in Legnaro (45◦20026” N; 11◦5800” E) in the Veneto region (northeastern Italy) (Figure1). The plain of the Veneto region is characterized by a humid subtropical climate, with a mean annual temperature of 13–14 ◦C and annual precipitation ranging from 700 to 1000 mm [30]. Precipitation is concentrated in spring and autumn, with quite frequent high-intensity events, especially during summer, and convective storms. During the test, weather data (precipitation, air temperature, and air humidity) were recorded by the weather station located on campus. The trial involved the setting of different solutions of green roofs (microcosms) using plastic boxes (polypropylene (PP)) with a height of 22 cm and approximately 0.44 m2 of surface. A drain hole in the center of the bottom of the box was created to convey percolation water (outflow) into tanks, where it was collected and measured for the evaluation of the crop coefficient. A tubular steel structure kept the boxes raised approximately 1 m off of the ground (Figure2). Each green roof microcosm consisted of a combination of different layers, namely, a drainage/storage layer, a filter sheet, and a substrate with vegetation. The drainage/storage layers were as follows: A preformed layer (PL) in recycled high-density polyethylene (HDPE) (Bauder DSE 40, • Bauder GmbH, Stuttgart, Germany); A mineral layer (ML) composed of expanded perlite (Igroperlite Type 3, Perlite Italiana S.r.l., • Corsico, Italy). The substrates were as follows: A volcanic substrate (VS) (Vulcaflor Extensive, Europomice S.r.l., Milan, Italy); • A recycled crushed brick substrate (RS) (“Rockery Type Plants”, Zinco GmbH, Nuertingen, Germany). • Water 2020, 12, 3484 3 of 15 Water 2020, 12, x FOR PEER REVIEW 3 of 16 Water 2020, 12, x FOR PEER REVIEW 3 of 16

Figure 1. Location of of the the Agripolis Agripolis Campus Campus in in the the portio portionn of of the the Venetian Venetian Plain Plain of of the the Veneto Veneto region (northeasternFigure 1. Location Italy). of The The the Venetian VenetianAgripolis Plain Campus is the in refere referential the portiontial region n of the for Venetian the climatic Plain conditions of the Veneto considered region in(northeastern this study. Italy). The Venetian Plain is the referential region for the climatic conditions considered in this study.

Figure 2. The microcosms of green roofs at the experimental site of Agripolis Campus. Figure 2. TheThe microcosms microcosms of green roofs at the experimental site of Agripolis Campus. Each green roof microcosm consisted of a combination of different layers, namely, a drainage/storageEachThe water green retention layer,roof amicrocosm capacityfilter shee of t, the consistedand substrates a substrate of wasa withcombination 36 mmvegetation. and 48of mm different for VS andlayers, RS, respectively,namely, a whiledrainage/storageThe that drainage/storage of the drainage layer, a /filterstorage layers shee layerweret, and wasas follows:a 16.5substrate mm and with 18 vegetation. mm for PL and ML, respectively. The other •hydrological AThe preformed drainage/storage characteristics layer (PL) layers of in the recycled were different as high-density follows: layers were polyethylene reported by Bettella(HDPE) et(Bauder al. [30]. DSE 40, Bauder • GmbH,A preformed Germany); layer (PL) in recycled high-density polyethylene (HDPE) (Bauder DSE 40, Bauder GmbH, Germany);

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• A mineral layer (ML) composed of expanded perlite (Igroperlite Type 3, Perlite Italiana S.r.l., Italy). The substrates were as follows: • A volcanic substrate (VS) (Vulcaflor Extensive, Europomice S.r.l., Italy); • A recycled crushed brick substrate (RS) (“Rockery Type Plants”, Zinco GmbH, Germany). The water retention capacity of the substrates was 36 mm and 48 mm for VS and RS, respectively, Waterwhile2020 that, 12 of, 3484 the drainage/storage layer was 16.5 mm and 18 mm for PL and ML, respectively.4 ofThe 15 other hydrological characteristics of the different layers were reported by Bettella et al. [30]. On these different combinations of layers, three different mixtures of plant species were On these different combinations of layers, three different mixtures of plant species were transplanted: transplanted: • A mixture of Sedum species (SE): Sedum album L., Sedum acre L., Sedum reflexum L., and A mixture of Sedum species (SE): Sedum album L., Sedum acre L., Sedum reflexum L., • Sedum sexangulare L.; and Sedum sexangulare L.; • A mixture of herbaceous (HE) perennials: Melica ciliata L., Artemisia alba Turra, Bromus erectus A mixture of herbaceous (HE) perennials: Melica ciliata L., Artemisia alba Turra, Bromus erectus • Huds, and Dianthus carthusianorum L.; Huds, and Dianthus carthusianorum L.; • A mixture of suffruticose (SF): Potentilla pusilla Host, Clinopodium nepeta L., Thymus A mixture of suffruticose (SF): Potentilla pusilla Host, Clinopodium nepeta L., Thymus serpillum L., • serpillum L., Euphorbia cyparissas L., Anthemis tinctoria L., Campanula spicata L., and Euphorbia cyparissas L., Anthemis tinctoria L., Campanula spicata L., and Dianthus hyssopifolius L. Dianthus hyssopifolius L. The species were selected from among native plan plantsts found in dry natural habitats because they were considered to have higher adaptability to the specific specific environmental context and to green roof conditions; plants were collected in dry grasslands or sandy and rocky soils close to the experimental site, or were provided by Veneto Veneto Agricoltura’s Cent Centrere for Biodiversity and and Ou Outside-Foresttside-Forest Activities, Activities, Veneto’s regionalregional agencyagency forfor innovationinnovation inin thethe primaryprimary sector.sector. All 12 combinations (3 plant mixes 2 substrates 2 storage/drainage layers) were replicated All 12 combinations (3 plant mixes ×× 2 substrates × 2 storage/drainage layers) were replicated three times, for a total of 3636 microcosms.microcosms. Three Three bo boxesxes were filled filled with gravel and used as controls (traditional roofroof withwith drainage drainage element), element), whose whose significance significance will will be illustrated be illustrated in a forthcomingin a forthcoming paper. Thepaper. experimental The experimental design design was a randomized was a randomized complete complete block (Figure block 3(Figure). 3).

SE HE SF HE SF SF SE HE RF SE HE SE EAST Gravel VS VS VS RS VS RS RS RS RS VS VS RS Block 1 ML ML PL PL ML ML PL ML PL PL PL ML

RF HE SE SF HE SE HE SF SE HE SE SF Grave RS VS RS VS RS RS VS RS VS RS VS VS Block 2 l PL PL ML PL PL PL ML ML ML ML PL ML

SE HE SE SF HE SF SE RF HE SF HE SE Grave VS VS RS VS RS VS VS RS VS RS RS RS WEST Block 3 l PL PL ML ML PL PL ML PL ML ML ML PL

RF Vegetation layer SE = Sedum mix HE = Herbaceous perennial SF = Suffruticose RS Substrate layer VS = Volcanic RS = Reclycled crushed bricks PL Drainage/Storage layer ML = Mineral PL = Preformed plastic

Figure 3. RandomizedRandomized complete complete block block layout layout of of the the expe experimentalrimental site site with with the indication of the conditions of the substrate, drainagedrainage/storage,/storage, and vegetation layerslayers ofof eacheach greengreen roofroof microcosm.microcosm.

The experiment experiment began began in in May May 2014 2014 when when planti plantingng material, material, grown grown in 10 in cm 10 containers, cm containers, was wastransplanted. transplanted. A total of A 15 total plants of 15per plantsmicrocosm per microcosmwere used. During were used. the initial During phase, the daily initial irrigations phase, ofdaily 5 L irrigations(approximately of 5 L11 (approximately mm) were given 11 to mm) each were microcosm given toto eachensure microcosm normal plant to ensure growth. normal After plant growth. After June 2014, supplemental irrigations were performed only when strong symptoms of wilting occurred, and this management lasted throughout the experiment until May 2016, when no more supplemental watering was given. During the two years, 12 irrigations were performed. The soil moisture data of the growing media were recorded using 5TM probes positioned at a depth of 6 cm and an EM50 data logger (Decagon Devices Inc., Washington, DC, USA), set to record data every 10 min (Figure4). Unfortunately, we soon realized that neither type of substrate allowed a correct and continuous measurement of the humidity value over time, making it impossible to compare the growth of the plants with the humidity of the substrates. Water 2020, 12, x FOR PEER REVIEW 5 of 16

June 2014, supplemental irrigations were performed only when strong symptoms of wilting occurred, and this management lasted throughout the experiment until May 2016, when no more supplemental watering was given. During the two years, 12 irrigations were performed. The soil moisture data of the growing media were recorded using 5TM probes positioned at a depth of 6 cm and an EM50 data logger (Decagon Devices Inc., Washington, DC, USA), set to record data every 10 min (Figure 4). Unfortunately, we soon realized that neither type of substrate allowed Watera correct2020, 12and, 3484 continuous measurement of the humidity value over time, making it impossible5 of to 15 compare the growth of the plants with the humidity of the substrates.

Figure 4. ProbesProbes and and data data logger logger for the data record recordinging of the humidity of the substrates.

2.1. Surveys Surveys on on the the Development of Plant Cover On the 19th of every month at approximately 2:00 p.m., starting from June 2014, a photographic survey ofof eacheach microcosm microcosm was was carried carried out out in orderin order to evaluateto evaluate the trendthe trend of the of vegetation the vegetation canopy canopy cover. Acover. photo A ofphoto each microcosmof each microcosm was taken was with taken a remotely with a controlledremotely controlled GoPro Hero4 GoPro camera Hero4 (Woodman camera (WoodmanLabs, Inc., SanLabs, Mateo, Inc., San CA, Mateo, USA), placedCA, USA), on aplaced mobile on stand a mobile passing stand through passing the through boxes (Figurethe boxes5), (Figurein order to5), always in order obtain to stablealways photos. obtain The stable photos photos. were processedThe photos using were Adobe processed Photoshop using Lightroom Adobe (AdobePhotoshop Inc., Lightroom San Jose, CA, (Adobe USA) Inc., to correct San Jose, any lensCA, distortions,USA) to correct shadows, any lens or highlights distortions, to obtain shadows, cleaner or highlightscolors and to cropobtain the cleaner photo socolors that and only to the crop area the inside photo the so box that was only framed. the area ImageJ inside [31 ]the was box used was to framed.remove allImageJ that was[31] notwas vegetation used to remove from the all image,that was and not MATLAB vegetation® software from the (MathWorks image, and Inc., MATLAB Matick,® softwareMA, USA) (MathWorks was used for Inc., the Matick, calculation MA, ofUSA) the was plant used cover for [ 32the] incalculat orderion to obtain of the theplant percentages cover [32] in of ordercanopy to cover.obtain Anthe examplepercentages of an of original canopy picturecover. An and example its elaboration of an original are given picture in Figure and its6. elaboration Water 2020, 12, x FOR PEER REVIEW 6 of 16 are given in Figure 6.

Figure 5. TheThe mobile mobile stand stand with with a a camera camera to take photos of the plant cover.

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Figure 6. Example of an original photo and its sequence of elaboration. Figure 6. Example of an original photo and its sequence of elaboration. 2.2. Evaluation of the Crop Coefficient 2.2. Evaluation of the Crop Coefficient On a monthly basis, all microcosm boxes were weighed by means of an electrical scale associated withOn a pallet a monthly jack. For basis, each all microcosm,microcosm boxes the monthly were weighed actual evapotranspiration by means of an electrical (ET) wasscale empirically associated determinedwith a pallet as jack. follows: For each microcosm, the monthly actual evapotranspiration (ET) was empirically determined as follows: ET = P + I D ∆W − − where P is the precipitation, I is the irrigationET = water, P + I −D D is− Δ theW drainage (outflow), and ∆W is the diwherefference P is in the weight precipitation, of the boxes I is between the irrigation two successive water, D weighings, is the drainage and represents (outflow), the and di ffΔerenceW is the in waterdifference content in weight of boxes of the between boxes twobetween dates. two All succe the datassive areweighings, expressed and in represents millimeters. the Indifference particular, in water content of boxes between two dates. All the data are expressed in millimeters. In particular, ΔW data were converted to millimeters by considering the area of each box. Hence, the actual crop coefficient (Kc act) was calculated according to [33]:

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Water 2020, 12, x FOR PEER REVIEW 8 of 16 ∆W data were converted to millimeters by considering the area of each box. Hence, the actual crop coefficient (Kc act) was calculated according to [33]: Kc act = ET/ET0 K = ET/ET where ET0 is the monthly reference evapotranspiratic act on0 calculated using the Hargreaves–Samani formula modified by Berti et al. [34]. where ET0 is the monthly reference evapotranspiration calculated using the Hargreaves–Samani formula2.3. Statistical modified Analysis by Berti of the et Data al. [34 Co].llected on the Microcosms of Green Roofs 2.3. StatisticalThe study Analysis was arranged of the Data as Collecteda 3 × 2 × on 2 thefactorial Microcosms experiment of Green (3 Roofs plant mixes × 2 substrates × 2 drainage/storage layers) in a completely randomized block design with three replications. Data were The study was arranged as a 3 2 2 factorial experiment (3 plant mixes 2 substrates 2 analyzed using a three-way analysis ×of variance× (ANOVA), and when the ANOVA× was significant× drainage/storage layers) in a completely randomized block design with three replications. Data were (p < 0.05), the means were differentiated by Tukey’s honestly significant difference (HSD) test. Values analyzed using a three-way analysis of variance (ANOVA), and when the ANOVA was significant expressed as percentages were angular-transformed prior to the ANOVA. (p < 0.05), the means were differentiated by Tukey’s honestly significant difference (HSD) test. Values expressed as percentages were angular-transformed prior to the ANOVA. 3. Results 3. Results 3.1. Weather Conditions 3.1. Weather Conditions The meteorological data recorded during the experiment period are reported in Figure 7. TemperaturesThe meteorological were particularly data recorded high during in summer the experiment 2015 when period the average are reported maximum in Figure temperature7. Temperatures was werehigher particularly than 30 °C high for in two summer months. 2015 The when year the 2014 average was maximum characterized temperature by a rainy was summer higher than and 30 fall,◦C forwhereas two months. 2015 and The 2016 year had 2014 dry was summers. characterized During by a the rainy first summer year of and data fall, collection whereas 2015(September and 2016 2014– had dryAugust summers. 2015), Duringprecipitation the first was year 839 of mm, data whereas collection in(September the second 2014–August year (September 2015), 2015–August precipitation 2016), was 839it was mm, 856 whereas mm. in the second year (September 2015–August 2016), it was 856 mm.

300 40 ETo P T Tmin Tmax

225 30

150 20 and Precipitation (mm) Temperature (°C) Temperature

0 75 10 ET

0 0 J JASONDJ FMAMJ JASONDJ FMAMJ JA 2014 2015 2016

Figure 7. Monthly reference evapotranspiration (ET0), precipitation (P) and average (T), maximum Figure 7. Monthly reference evapotranspiration (ET0), precipitation (P) and average (T), maximum (Tmax),(Tmax), andand minimumminimum (Tmin)(Tmin) temperaturetemperature duringduring thethe experimentalexperimental period.period.

3.2. Plant Cover Evolution The evolution of plant cover after the transplant reflectedreflected the initial plant growth (higher for the herbaceous perennials and susuffruticoseffruticose thanthan forfor thethe SedumSedum species)species) andand diddid notnot changechange substantially over thethe summer.summer. StartingStarting fromfrom April April 2015, 2015, the the plants plants began began to to grow, grow, reaching reaching the the highest highest coverage coverage in October–Novemberin October–November 2015 2015 (Figure (Figure8); the 8); greatestthe grea growthtest growth rate wasrate manifestedwas manifested by the bySedum the Sedumspecies species that, fromthat, from the lower the lower coverage coverage at the at beginning the beginning of 2015, of 2015, achieved achieved significantly significantly higher higher values values compared compared to theto the other other plant plant mixes, mixes, starting starting from from August. August. During During the the second second winter, winter, all all species species showedshowed aa generalgeneral coveragecoverage reduction,reduction, but but it wasit was lower lower for theforSedum the Sedumspecies. species. During During summer summer 2016, a consistent2016, a consistent decrease decrease in plant coverage was noted in all plant mixes due to the drought and the absence of supplemental irrigation, which caused the death of some plants, but it was definitely higher in the

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inWater plant 2020 coverage, 12, x FOR PEER was notedREVIEW in all plant mixes due to the drought and the absence of supplemental9 of 16 irrigation, which caused the death of some plants, but it was definitely higher in the herbaceous perennialsherbaceous and perennials suffruticose and than suffruticose in the Sedum thanplants. in the At Sedum the end plants. of the At experimental the end of period,the experimental the Sedum mixperiod, had the twice Sedum the coveragemix had twice of the the other coverage mixes (Figureof the other8A). mixes (Figure 8A).

100 A SE HP SF 80 a a a a b a a a a ab a a c b a 60 a b a a b b c b b b a a a a b b b a b b a b b b b b b b a a b a 40 a ab a b b b b b a b b b b b b b a 20 c b b b Canopy cover (%) coverCanopy b 0 100 B VS RS 80 *** ** *** * 60 *** ns *** *** *** *** *** *** 40 *** *** ***

ns ns 20 *** *** * *** *** Canopy cover(%) ***

0 100 C PL ML 80 ns ns * ns ns 60 *** ns ns ns ns ns *** ns 40 *** ns

ns ns ns ns 20 ns ** ns

Canopy cover(%) ns

0

FigureFigure 8. VariationVariation of of canopy canopy cover cover during during the the ex experimentperiment as asaffected affected by byplant plant mix mix (A), ( Asubstrate), substrate (B), (andB), anddrainage/storage drainage/storage layer layer (C). (SEC). = SE Sedum= Sedum mix,mix, HP = HP herbaceous= herbaceous perenni perennials,als, SF = SFsuffruticose,= suffruticose, VS = VSvolcanic= volcanic substrate, substrate RS ,= RSrecycled= recycled crushed crushed brick brick substrate, substrate, PL PL = =preformedpreformed plastic plastic drainage/storage drainage/storage layer,layer, MLML == mineral drainage/storage drainage/storage layer. On each date, values with different different letters are significantly significantly different according to Tukey’s HSD test p 0.05). *, **, and *** = significant for p-values of <0.05, <0.01, different according to Tukey’s HSD test p≤ ≤ 0.05). *, **, and *** = significant for p-values of <0.05, <0.01, andand <<0.0010.001 according to the analysis of variancevariance (ANOVA). nsns == not significant.significant.

The ANOVA highlighted that the canopy cover was also affected by the substrate (Figure8B). The ANOVA highlighted that the canopy cover was also affected by the substrate (Figure 8B). The highest values of canopy cover were shown in the recycled crushed brick substrate (RS) with The highest values of canopy cover were shown in the recycled crushed brick substrate (RS) with differences that were 30% higher than those observed in the volcanic substrate (VS) at most of the differences that were ≥≥30% higher than those observed in the volcanic substrate (VS) at most of the sampling points (Figure8B). The e ffect of the drainage/storage layer was found to be significant only sampling points (Figure 8B). The effect of the drainage/storage layer was found to be significant only in five months, with differences that were not higher than 12% in summer 2015 (Figure8C). in five months, with differences that were not higher than 12% in summer 2015 (Figure 8C). Interaction effects were observed only at three consecutive sampling points from October to Interaction effects were observed only at three consecutive sampling points from October to December 2015 and at four from May to August 2016. In the first case, the significance concerned the December 2015 and at four from May to August 2016. In the first case, the significance concerned the plant mix × substrate interaction; the values for November 2015 (when differences were significant for p ≤ 0.001) are reported in Figure 9 as representative of this interaction. The different substrates affected only the canopy cover of the herbaceous perennial mix, which was higher with RS than with VS. In the second case, the plant mix × drainage/storage layer interaction, only the canopy cover of

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plant mix substrate interaction; the values for November 2015 (when differences were significant × for p 0.001) are reported in Figure9 as representative of this interaction. The di fferent substrates ≤ Wateraffected 2020, only 12, x FOR the canopyPEER REVIEW cover of the herbaceous perennial mix, which was higher with RS than10 withof 16 VS. In the second case, the plant mix drainage/storage layer interaction, only the canopy cover of the × thesuff suffruticoseruticose plant plant mix wasmix awasffected affected by the by drainage the dr/ainage/storagestorage layer, with layer, ML with achieving ML achieving better coverage better coveragethan PL, asthan shown PL, as in shown Figure 9inB, Figure which 9B reports, which the reports values the of Mayvalues 2016 of May ( p < 20160.001). (p < 0.001).

VS RS A PL ML B 100 a 100 a a ab bc bc cd 80 d 80 b bc c c 60 60

40 40 Canopy cover (%) cover Canopy

20 (%) cover Canopy 20

0 0 SE HP SF SE HP SF Plant mix Plant mix

Figure 9. Interaction effects of plant mix substrate (A) (November 2015) and plant mix Figure 9. Interaction effects of plant mix ×× substrate (A) (November 2015) and plant mix × drainage/storagedrainage/storage layer layer ( (BB)) (May (May 2016). 2016). Histogram Histogram values values with with different different letters letters indicate indicate significant significant differencesdifferences according according to to Tukey’s Tukey’s HSD HSD test test (p (p < <0.05).0.05). SE SE = Sedum= Sedum mix,mix, HP HP = herbaceo= herbaceousus perennials, perennials, SF =SF suffruticose,= suffruticose, VS = VSvolcanic= volcanic substrate, substrate, RS = recycl RS = edrecycled crushed crushed brick substrate, brick substrate, PL = preformed PL = preformed plastic drainage/storageplastic drainage/ storagelayer, ML layer, = mi MLneral= mineral drainage/storage drainage/ storagelayer. layer. 3.3. Actual Crop Coefficient 3.3. Actual Crop Coefficient The actual crop coefficient (Kc act) varied over time, ranging, on average, from the minimum The actual crop coefficient (Kc act) varied over time, ranging, on average, from the minimum value value in July 2016 (0.19 mm) to the maximum in October 2015 and January 2016 (1.06 and 1.04 mm, in July 2016 (0.19 mm) to the maximum in October 2015 and January 2016 (1.06 and 1.04 mm, respectively) (Figure 10). respectively) (Figure 10). The ANOVA often revealed a significant effect of the three main factors. As for the plant mix The ANOVA often revealed a significant effect of the three main factors. As for the plant mix (Figure 10A), with the exception of July 2015, when significant differences were found, the lowest Kc act (Figure 10A), with the exception of July 2015, when significant differences were found, the lowest Kc values were observed in the Sedum mix. The values of herbaceous perennial and suffruticose mixes act values were observed in the Sedum mix. The values of herbaceous perennial and suffruticose mixes were higher than those of the Sedum mix, and were similar for the first two-thirds of the monitored were higher than those of the Sedum mix, and were similar for the first two-thirds of the monitored period; following this, the Kc act values of the suffruticose mix were sometimes higher (Figure 10A). period; following this, the Kc act values of the suffruticose mix were sometimes higher (Figure 10A). The substrates also influenced the Kc act values and, in seven out of the nine months in which the The substrates also influenced the Kc act values and, in seven out of the nine months in which the differences were significant, RS increased its values compared to VS (Figure 10B). The drainage/storage differences were significant, RS increased its values compared to VS (Figure 10B). The layer affected the Kc act values much less than the other main factors did. Significant differences were drainage/storage layer affected the Kc act values much less than the other main factors did. Significant found in the last period of the trial, where, in comparison to PL, ML was found to improve the Kc differences were found in the last period of the trial, where, in comparison to PL, ML was found to values (Figure 10C). improve the Kc values (Figure 10C).

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2.0 A SE HP SF a a c act b 1.5 a

K a a c a a a b a a a b b b a b b 1.0 c b ns ns c c a ns b ns ns 0.5 b

0.0 2.0 B VS RS

c act 1.5 K *** **

1.0 * ** * ns ns ns *** *** * *** n 0.5 s ***

0.0 2.0 C PL ML

c act 1.5 K ns ns

1.0 ns * ns *** ns ns *** *** ns *** 0.5 ns **

0.0

Figure 10. Variation of the actual crop coefficient (K ) during the experiment as affected by plant Figure 10. Variation of the actual crop coefficient (Kcc actact) during the experiment as affected by plant mix (A), substrate (B), and drainagedrainage/storage/storage layer (C).). SESE == Sedum mix, HP = herbaceous perennials, SF = suffruticose,suffruticose, VS = volcanic substrate, RS = recycled crushed brick substrate, PL == preformed plastic drainagedrainage/storage/storage layer, layer, ML ML= mineral = mineral drainage drainage/storage/storage layer. layer. On each On date, each values date, withvalues diff erentwith differentletters are letters significantly are significantly different according different toaccording Tukey’s HSDto Tukey’s test (p

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1.0 PL ML A1.0 PL ML B 1.0 PL ML C1.0 PL ML D a 0.8 a0.8 0.8 0.8 a b a b b a b ab c b ab 0.6 0.6 b 0.6 0.6 d d d c b c c act c act K 0.4 0.4 K 0.4 0.4

0.2 0.2 0.2 0.2 Canopy cover (%) cover Canopy Canopy cover (%) cover Canopy

0.0 0.0 0.0 0.0 SE HP SF VS RS SE HP SF VS RS Plant mix Substrate Plant mix Substrate

Figure 11. Interaction effect of plant mix drainage/storage layer and substrate drainage/storage Figure 11. Interaction effect of plant mix × drainage/storage layer and substrate × drainage/storage layerlayer in in April April 2016 2016 ( (AA,,BB,, respectively) respectively) and and June June 2016 2016 ( (CC,,DD,, respectively) respectively) on on the the actual actual crop crop coefficient coefficient

(K(Kcc act act).). Histogram Histogram values values with with different different letters letters indicate indicate significant significant differences differences according according to to Tukey’s Tukey’s HSDHSD test test ( (pp << 0.05).0.05). 4. Discussion 4. Discussion In this experimental setting, the Sedum plants had lower coverage than the other plant mixes In this experimental setting, the Sedum plants had lower coverage than the other plant mixes and and in the first summer, the growth for all species was poor; however, starting from April 2015, plant in the first summer, the growth for all species was poor; however, starting from April 2015, plant growth increased noticeably, also assisted by supplemental irrigations, so the coverage reached 90% growth increased noticeably, also assisted by supplemental irrigations, so the coverage reached 90% for Sedum and approximately 80% for the other plant mixes. Winter senescence reduced the canopy for Sedum and approximately 80% for the other plant mixes. Winter senescence reduced the canopy cover of all mixes, but from late winter to April 2016, the coverage increased again. Then, the HP and cover of all mixes, but from late winter to April 2016, the coverage increased again. Then, the HP and SF coverage strongly decreased as a consequence of drought, which resulted in the death of several SF coverage strongly decreased as a consequence of drought, which resulted in the death of several plants. In contrast, the Sedum canopy cover remained high and stable (Figure8A). The high capacity of plants. In contrast, the Sedum canopy cover remained high and stable (Figure 8A). The high capacity Sedum species to survive and grow under harsh conditions (i.e., drought, high temperature and solar of Sedum species to survive and grow under harsh conditions (i.e., drought, high temperature and radiation, and wind) is well known, also in comparison to other herbaceous species [35–37]. Because of solar radiation, and wind) is well known, also in comparison to other herbaceous species [35–37]. the crassulacean acid metabolism (CAM) of Sedum plants, their stomata can open at night to let carbon Because of the crassulacean acid metabolism (CAM) of Sedum plants, their stomata can open at night dioxide enter, so that daytime stomata closure reduces transpiration [35,38]. Sedum species also have to let carbon dioxide enter, so that daytime stomata closure reduces transpiration [35,38]. Sedum great coverage as a result of their prostrate growth habit that, by shading, reduces soil evaporation [39], species also have great coverage as a result of their prostrate growth habit that, by shading, reduces further increasing water use efficiency. In contrast, herbaceous forb and grass species are, in general, soil evaporation [39], further increasing water use efficiency. In contrast, herbaceous forb and grass much more sensitive to drought because of their higher evapotranspiration requirements, and they species are, in general, much more sensitive to drought because of their higher evapotranspiration need a deeper substrate in order to increase water retention and/or require irrigation [37,40]. requirements, and they need a deeper substrate in order to increase water retention and/or require As seen in Figure8B,C, the plant coverage was also a ffected by the substrate and the irrigation [37,40]. drainage/storage layer, particularly the former. The higher water retention capacity of RS vs. VS and As seen in Figure 8B,C, the plant coverage was also affected by the substrate and the of ML vs. PL is likely the major contributor to this response, highlighting the importance of these drainage/storage layer, particularly the former. The higher water retention capacity of RS vs. VS and layers in supporting plant growth. However, other aspects may also be involved, such as the different of ML vs. PL is likely the major contributor to this response, highlighting the importance of these thermal capacities of the substrates, which can influence plant growth by affecting evaporation and, layers in supporting plant growth. However, other aspects may also be involved, such as the different hence, water availability to plants [41]. thermal capacities of the substrates, which can influence plant growth by affecting evaporation and, As for the empirically determined K , the values gradually decreased to very low values hence, water availability to plants [41]. c act from October to December 2015. This is related to the low transpiration due to plant rest, but also As for the empirically determined Kc act, the values gradually decreased to very low values from to the drought period that occurred during November and December 2015. Over the next months, October to December 2015. This is related to the low transpiration due to plant rest, but also to the we observed a new increase linked to the natural vegetative restart, increasing actual evapotranspiration. drought period that occurred during November and December 2015. Over the next months, we During summer 2016, K did not increase like it did the previous summer; this may be because the observed a new increase clinked act to the natural vegetative restart, increasing actual evapotranspiration. water deficit was not fulfilled by supplemental irrigation, causing some plants to die. It is known that During summer 2016, Kc act did not increase like it did the previous summer; this may be because the irrigation can prevent plant death and can support evapotranspiration, which can increase from 3 to water deficit was not fulfilled by supplemental irrigation, causing some plants to die. It is known that 5 L m 2 on a daily basis in summer [42]. As CAM metabolism allows a reduction in water consumption irrigation− can prevent plant death and can support evapotranspiration, which can increase from 3 to 5 L m−2 on a daily basis in summer [42]. As CAM metabolism allows a reduction in water consumption during a drought period [38], the Sedum species had a lower Kc act than the herbaceous perennial and suffruticose species did.

Water 2020, 12, 3484 12 of 15

during a drought period [38], the Sedum species had a lower Kc act than the herbaceous perennial and suffruticose species did. Sometimes, to estimate the performance of green roofs, the Kc of low-water-use plants (i.e., 0.3) is used [43,44]. This value is similar to those estimated by Starry et al. [45] for three Sedum species, ranging from 0.21 to 0.50 for S. album, from 0.22 to 0.55 for S. sexangulare, and from 0.25 to 0.71 for (formerly classified as S. kamtschaticum) according to the season, and to the Kc of 0.35–0.52 calculated by Lazzarin et al. [46]. In the present experiment, the average Kc act of 0.57 reveals that the Kc used to model the performance of the green roofs may have underestimated the evapotranspiration features of the Sedum species, particularly if we consider that we calculated Kc act and not Kc, which is related to a condition with no water stress [33]. In contrast, it has to be considered that some Sedum species are facultative CAM plants, and in moist conditions, they switch from CAM to C3 metabolism, in which plants’ stomata are open during the daytime, increasing the evapotranspiration of water in comparison to strict CAM succulents [47]. Furthermore, the herbaceous perennial and suffruticose species had higher Kc act values (0.68 and 0.65, respectively) than the Sedum species, but they did not reach the value used by Vanuytrecht et al. [48] (i.e., 1.0) for grasses and herbs or those found by Miller [49] for common forbs. It must be remembered that we did not calculate ET in well-watered conditions. Our values are more in line with the Kc used for high-water-use plants (i.e., 0.6) [43,44]. The lower water use, or lower evapotranspiration, of Sedum compared to other species is important to guarantee plant survival in low-maintenance green roofs. Indeed, our results proved once again that Sedum species can survive without supplemental irrigation during summer, while irrigation is mandatory for other species. However, the high-water-use efficiency of Sedum may turn out to be a drawback, as it limits the utilization of excess water [19,50] and reduces potential summer thermal benefits associated with evapotranspirative cooling [51]. Hence, in accordance with what has been reported by other authors, when selecting plants for green roofs, consideration should be given to which are the more important benefits to pursue [51,52]. The results also showed that the recycled substrate (RS) and the mineral drainage/storage layer (ML) generally improved the Kc act values as a consequence of their better water retention capacity [20], which promoted plant growth.

5. Conclusions Along with the capacity to reduce stormwater runoff, evapotranspiration is a key factor for green roofs. Succulent species, particularly Sedum, are often used to vegetate green roofs, and our results confirmed that in a humid subtropical climate context, their drought tolerance guarantees their long-term survival, as well as rapid and full surface coverage, even with no supplemental irrigation. Our results demonstrate that the components of green roofs (i.e., the substrate and storage/drainage layer) are also important for overall plant performance, as long as they contribute as a water reservoir to support plant growth. Considering the actual crop coefficient of the tested species, even though the Sedum mix exhibited higher values than expected, those of the herbaceous perennial and suffruticose species were often superior, implying higher maintenance input to ensure plant survival but, potentially, higher benefits.

Author Contributions: Conceptualization, G.Z. and L.B.; data curation, G.Z. and L.B.; investigation, G.Z. and L.B.; methodology, G.Z. and L.B.; validation, G.Z. and L.B.; formal analysis, G.Z.; writing—original draft, G.Z. and L.B.; writing—review and editing, G.Z. and L.B.; supervision, L.B. All authors have read and agreed to the published version of the manuscript. Funding: This work was financed by the Progetto di Ateneo 2012 N. CPDA122584 titled, “In situ sustainable management of stormwater runoff by mean of green roofs: evaluation of systems suitable for Venetian Plain” (University of Padova). Conflicts of Interest: The authors declare no conflict of interest. Water 2020, 12, 3484 13 of 15

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