This study investigated the establishment of annual meadow including native and non-native species in an extensive green roof in Sheffield, UK. The study aimed to determine the feasibility of establishing annual plant species from a seed mixture and to determine the appropriate sowing rate as well as the necessity of watering during the first growing season from June to November 2006.
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Landscape and Urban Planning
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Research paper
Establishment of an annual meadow on extensive green roofs in the UK
Ayako Nagasea,!, Nigel Dunnettb,1
a Chiba University, Graduate School of Engineering, Division of Design Science, 1-33 Yayoicho, Inage-ku, Chiba-shi, Chiba 263-8522, Japanb University of Sheffield, Department of Landscape, Arts Tower, Western Bank, Sheffield S10 2TN, UK
h i g h l i g h t s
! Establishment of an annual meadow in an extensive green roof is investigated in Sheffield, UK.! It is possible to establish a long flowering annual meadow without irrigation at 7 cm depth of substrate in the extensive green roof.! A low sowing rate results in better conditions for individual plant growth when enough water is available.! A high sowing rate is necessary to ensure a sufficient plant number when water resources are not abundant.! Watering improves growth in most annual plant species.
a r t i c l e i n f o
Article history:Received 11 December 2011Received in revised form12 December 2012Accepted 16 December 2012
This study investigated the establishment of annual meadow including native and non-native speciesin an extensive green roof in Sheffield, UK. The study aimed to determine the feasibility of establishingannual plant species from a seed mixture and to determine the appropriate sowing rate as well as thenecessity of watering during the first growing season from June to November 2006. A 22-species seedmixture was sown on an experimental green roof with a substrate depth of 7 cm using two sowingrates (2 g/m2 and 4 g/m2) and two watering regimes (with and without watering). The watering regimeconsisted of application of water four times over the course of the experiment. Each combination ofsowing and watering regime yielded a successful aesthetic annual meadow green roof. Results showedthat a low sowing rate resulted in better conditions for individual plant growth when enough waterwas available. On the other hand, a high sowing rate was necessary to ensure a sufficient number ofplants when water resources were not abundant. The watering regime improved growth in most species;however, it was determined that an annual seed mixture could perform well without watering at thestudy site. The annual meadow possessed an abundance of flowering plants for an extended period oftime; plants started flowering one month after sowing and continued until the end of October. Successfulspecies during the first growing season included Alyssum maritimum, Echium plantagineum ‘Blue Bedder’,Gypsophila muralis, Iberis amara, Iberis umbellata ‘Fairy’, Linaria elegans and Linaria maroccana.
Although the installation of green roofs has taken place for manyyears (e.g. Derry and Toms, London, 1938), relevant peer reviewedresearch has only recently been published (Dvorak & Volder, 2010;Francis & Lorimer, 2011; Lundholm, 2006; Oberndorfer et al.,2007). Extensive green roofs are the most commonly deployedtype of green roof system and are characterized by the use of ashallow substrate (depth <20 cm), low costs and low maintenance(Dunnett & Kingsbury, 2008). Growth environments of green
roofs are considered severe due to limited water availability, widetemperature fluctuations, and high exposure to wind and solarradiation (Dunnett & Kingsbury, 2008; Oberndorfer et al., 2007).Selecting plants capable of surviving under these conditions is acritical step in successful green roof implementation. One of themost intensively investigated taxa is Sedum, since these plantshave shallow root systems and are able to efficiently utilize water,making them ideal candidates for tolerating the extreme condi-tions found on rooftops (Emilsson, 2008; Rowe, Getter, & Durhman,2012). Recently, alternative approaches that recognize green roofsas dynamic ecosystems and employ a diversity of species havegarnered the attention of researchers (Cook-Patton & Bauerle,2012). Some studies have examined the effects of plant mixturesas opposed to monocultures on green roof performance and havestressed that green roof performance could be improved throughgreater diversity of plant life forms in terms of climate change
Annual meadows that include both native and non-native plantspecies have great plant diversity and they may be able to toler-ate the harsh environments in extensive green roofs. Some annualplants have adapted to the brief growing seasons in steppe anddesert regions around the world and some grow at the first stagesof the vegetal succession in mild climate. Plant species in annualmeadows germinate, grow and flower under favourable conditionsand they lie dormant as seeds during periods of harsh conditions(Barber, 1954). There are a number of advantages to using annualplant meadows on extensive green roofs. First advantage is a reduc-tion of cost and maintenance. Establishment of an annual meadowcan lead to reductions in management cost due to lower levelsof imposed management practice (Bretzel, Pezzarossa, Benvenuti,Bravi, & Malorgio, 2009). Implementation of annual meadows canalso be achieved using direct sowing, a simple method capableof covering large areas at a low cost. For example, covering asquare metre of garden with herbaceous plants may cost around£20–25, while the cost of seeds needed in order to cover thesame area through direct sowing may be just 20–50p (Dunnett& Hitchmough, 2001). A seed-grown annual meadow is also sus-tainable, as appropriately selected species may be re-seeded fromyear to year (Kircher, 2004). Furthermore, very little fertilizer andherbicide are required in the cultivation of an annual meadow(Diboll, 2004). Second advantage is biodiversity benefit. Annualmeadows may play a crucial role in the maintenance of biodiversity.In previous fauna studies, biodiverse roofs, which recreate brown-fields on roofs in order to maximize biodiversity and comprisedof meadows that include annuals and perennials, tend to be used.Biodiverse roofs were able to support populations of spiders, bee-tles, wasps, ants and bees and they also contained rare invertebratetaxa from the above-mentioned groups, including some with veryspecialized niches within the UK (Francis & Lorimer, 2011; Grant,2006; Kadas, 2006). Kadas (2006) studied three diverse inverte-brate groups, Araneae, Coleoptera, and Hymenoptera, in biodiverseroofs in London and found that at least 10% of all species col-lected were designated nationally rare or scarce in accordancewith criteria established by the intergovernmental agency, NaturalEngland. Annual meadows can be particularly useful in suppor-ting both bees and butterflies as they contain many flowers withlong flowering periods. Positive correlations have been identifiedbetween bee abundance and floral abundance (Banaszak, 1996;Heithaus, 1974) as well as butterfly diversity and floral abundance(Steffan-Dewenter & Tscharntke, 1997). It has also been determinedthat bumblebees perform better within diverse plant communitiescontaining species that flower at different times, as these communi-ties provide more stable nectar and pollen resources (Cook-Patton& Bauerle, 2012; Menz et al., 2011). Non-native species in par-ticular extend the flowering season of annual meadows sinceplants native to UK meadows cease flowering by mid-summer(Hitchmough, de la Fleur, & Findlay, 2004). Third advantage isaesthetic. Annual meadows including various colour of flowersand many previous studies showed that people are attracted toflowers (e.g. Jorgensen, Hitchmough, & Calvert, 2002; Todorva,Asakawa, & Aikoh, 2004). Swiss study showed that annual mead-ows had the advantage of being aesthetically pleasing as manyannual plants display bright colours, a trait that has been shownto be preferentially selected for by humans (Lindemann-Matthiens& Bose, 2007). Another Swiss study showed that plant diversityin itself is attractive to humans because species-rich communi-ties are more likely to contain a species perceived as particularlybeautiful which might increase overall appreciation (Lindemann-Matthies, Junge, & Matthies, 2010). In Sheffield, UK, Özgüner andKendle (2006) studied public attitudes towards naturalistic versus
designed landscapes and determined that naturalistic landscapes,such as meadows, were associated with greater senses of natural-ness and feelings of freedom and were perceived as better placesto socialize.
Currently, studies to the performance of meadows on greenroofs have been limited. Two studies investigated changes inspecies composition in rooftop grass and wildflower meadowslocated in the U.S. (Dewey, Johnson, & Kjelgren, 2004) and Germany(Köhler, 2006) while another examined the effect of adding theseeds of annual species to a Sedum green roof in Bernburg, Germany(Kircher, 2004). Information currently available is not sufficient toencourage people to implement meadows in extensive green roofs.This study was carried out in order to investigate the performanceof annual meadows grown from a seed mixture on an exten-sive green roof. The first goal was to investigate the feasibility ofestablishing annual plant species from a seed mixture. Appropriateestablishment is important for successful use of annual meadowson extensive green roofs because seed germination performanceplays a major role in the persistence and dynamics of annual plants(Rivas-Arancibia, Montana, Velasco Hernández, & Zavala-Hurtado,2006). The second goal was to evaluate successfully establishedspecies in terms of emergence, growth and flowering and to deter-mine an appropriate sowing rate and watering regime for use inthe first growing season. Determination of an appropriate sowingrate for extensive green roofs is necessary since the environmenton the rooftop is harsher than it is on the ground. In particular, dif-ferences in water availability may affect plant emergence in greenroof annual meadows. It has been hypothesized that irrigation hasa significant effect on the performance of annual plant communi-ties located in extensive green roofs. It has also been hypothesizedthat it is possible to create an annual meadow without irrigationin the UK due to the presence of a generally mild climate, the UKtypically having relatively warm winters and a high likelihood ofregular rainfall throughout the summer (Dunnett, 2004).
2. Methods
This study was carried out on the roof of a four storey commer-cial building located in the city centre of Sheffield, UK. An extensivegreen roof was installed on the roof of the building (approximately150 m2), which was surrounded by a parapet 0.345 m in height.The green roof was flamed by timber (5.5 m " 6.0 m " 0.1 m) andhad a build-up that consisted of a root protection layer, geotex-tile made of polypropylene, with fleece backing for green roofdrainage (SSM 45), a drainage layer (Floradrain FD 25/25-E) and7 cm of commercial green roof substrate (Zinco semi-intensive:granules <0.063 mm in diameter #15%, salt content #2.5%, porosity64%, pH 7.8, dry weight 940 kg/m3, saturated weight 1360 kg/m3,maximum water capacity 42%, air content at maximum watercapacity 22%, water permeability $0.064 cm/s) (Alumasc ExteriorBuilding Products, 2006). All materials were obtained from Alumasc(Northamptonshire, UK). Sowing rates of 2 g/m2 and 4 g/m2 wereselected for use in this study because a sowing rate of 3–5 g/m2
had been used previously to establish annual meadows at groundlevel (Dunnett, 1999). This study employed a split-plot exper-imental design (Fig. 1). Three main plots with timber fames(2.2 m " 1.0 m) were established for both the watering regime andthe non-watering regime. Each of the six plots was divided into twosub-plots, which varied with respect to sowing rate, giving twelveplots in total (1.1 m " 1.0 m). Plots were located 1 m apart in orderto prevent water from travelling between plots.
The seed mixture employed in this study was comprised of equalproportions (by weight) of 22 annual species. The species used wereAdonis aestivalis, Anagallis arvensis, Alyssum maritimum, Centau-rea cyanus, Chrysanthemum segetum, Consolida regalis, Convolvulus
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Fig. 1. Layout of study plots and sample configuration of seedling emergence quadrats within plots.
Table 1Characteristics of species used in this study.
Species Family UK native Native Habitat Number ofseeds/g
Adonis aestivalis Ranunculaceae Non-native Europe from France and Spain eastwards to theCaucasus, Syria and Iran
Cornfields and waste grounds 85
Anagallis arvensis Primulaceae Native Europe and Asia eastwards to Iraq andAfghanistan
Waste places, cornfields, sand dunes,rive banks
2500
Alyssum maritimum Brassicaceae Non-native Europe – Mediterranean. Naturalized in Britain Dry sunny places in the Mediterranean 2500Centaurea cyanus Asteraceae Native Europe, Turkey Pine forest, rocky slopes, cornfield 220Chrysanthemum
segetumAsteraceae Native East Mediterranean, North Africa, Europe as far
north and west as Scotland and IrelandAcidic, sandy soil 600
Convolvulus tricolor Convolvulaceae Non-native Portugal and Mediterranean Dry, open, grassy places 100Consolida regalis Ranunculaceae Non-native Southeast Europe, eastwards to Iran and
TurkmeniaCornfields, steppe and west ground, atup to 1000 m
750
Coreopsis tinctoria Asteraceae Non-native North America, from Saskatchewan andMinnesota to Louisiana, Texas and Arizona
Moist low ground roadsides and wasteplaces
3000
Echium plantagineum‘Blue Bedder’
Boraginaceae Cultivar n/a n/a 250
Eschscholzia californica Papaveraceae Non-native Northwest America from Washington to SouthCalifornia
Grassland, chaparral and desert up to2000 m
200
Geranium molle Geraniaceae Native Britain, Europe to the Himalayas Dry grassland, dunes, waste places andcultivated ground
860
Gypsophila muralis Caryophyllaceae Non-native Much of central Europe (not theMediterranean), the Caucasus and Siberia, andnaturalized in eastern North America
Dry, sandy places 380
Iberis amara Brassicaceae Native West Europe from Southeast England andGermany and Italy
Chalky hills and cornfields 400
Iberis umbellata ‘Fairy’ Brassicaceae Cultivar n/a n/a 430Linaria elegans Scrophulariaceae Non-native North and central Spain and North Portugal Grassy roadsides and among bracken
in open pine forest15,000
Linaria maroccana Scrophulariaceae Non-native Morocco and North America Open ground and sandy fields 15,000Linum grandiflorum var.
rubrumLinaceae Non-native North Africa, California Fields and waste places 350
Linum usitatissimum Linaceae Non-native Not known as wild plant 300Papaver rhoeas Papaveraceae Native Europe and North Africa, east across Asia to
Northwest China, throughout temperate worldCornfields, disturbed ground 9000
Reseda odorata Resedaceae Non-native Egypt, naturalized in the Mediterranean andCalifornia
Open ground 750
Tripleurospermummaritimum
Asteraceae Native Eurasia, North Africa, North America Common on stony ground by the sea 2000
Viola tricolor Violaceae Native Most of Europe and Asia, southwards to centralTurkey and eastwards to Siberia and Himalayas
Grassy places and arable fields 1000
Adapted from Phillips and Rix (2002).
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tricolor, Coreopsis tinctoria, Echium plantagineum ‘Blue Bedder’,Eschscholzia californica, Geranium molle, Gypsophila muralis, Iberisamara, Iberis umbellata ‘Fairy’, Linaria elegans, Linaria maroccana,Linum grandiflorum var. rubrum, Linum usitatissimum, Papaverrhoeas, Reseda odorata, Tripleurospermum maritimum, and Viola tri-color. It was predicted that these plants would thrive in extensivegreen roof conditions since their habitats, which consist of openground, cornfields, grasslands and sandy fields, are seasonally arid.In general, selected species were also short, a trait selected forso that species could survive despite the strong winds presentin rooftop environments. It was also predicted that they pro-duce various colours, white, red, yellow, pink, purple and orange.Plant characteristics are summarized in Table 1. Seed mixture wasobtained from John Chambers Wildflower Seeds (Northampton-shire, UK) and seeds were sown on June 16, 2006. Seeds were mixedwith sand because they were too small to distribute over the sub-strate. 5000 mL of sand and 20 g of seeds were mixed thoroughly ina bucket prior to distribution. An amount of mixture composed ofapproximately 500 mL sand and 2 g seeds was distributed by handwithin low sowing rate (2 g/m2) plots. For high sowing rate plots, asimilar mixture was prepared using 5000 mL sand and 40 g seeds.An amount of mixture composed of approximately 500 mL sand and4 g seeds was distributed by hand within high sowing rate (4 g/m2)plots. After sowing, the substrate was gently raked in order to allowthe seeds to be incorporated. The surface was levelled using theedge of a rake. All plots were watered every other day from thedate of sowing until July 13, 2006 to ensure seedling emergence.
Irrigation treatment started in watering regime plots on July 20,2006. Once a week prior to watering, substrate moisture was mea-sured using a moisture sensor (SM200, Delta-T Devices, Cambridge,UK). Three measurement points were randomly chosen from eachplot and a total of eighteen points were measured for each wateringregime. When the mean substrate moisture of all eighteen pointswas less than 15%, water was applied as a fine spray to each plotusing a handheld hose. Application continued until water startedto drain off the plot. Between July 13 and August 11, 2006, plotswere watered a total of four times (July 20, July 27, August 4,August 11). Substrate moisture levels were less than 15% duringweeks in which no rainfall was observed. Non-watering regimeplots received water only from natural rainfall. Mean substratemoisture in both watering regime plots and non-watering regimeplots is shown in Fig. 2. Mean monthly temperature and rainfall forSheffield during 2006 are shown in Fig. 3. June and July 2006 weretypically dry, warm months. A relatively larger amount of rain wasobserved in August, however, recorded values of moisture contentremained low until the middle of the month.
Variables measured were as follows:
(1) Final number of plants and shoot dry weightHarvesting took place on October 30, 2006. Total emergence
of plants per plot for each species was determined at the timeof harvest. Three replicates existed for each combination ofsowing rate and watering regime (2 g/m2 watering, 2 g/m2 non-watering, 4 g/m2 watering, 4 g/m2 non-watering). Harvestedplants were dried in a drying oven at room temperature fora period of seven days, after which shoot dry weight was mea-sured. It was impossible to distinguish between L. elegans andL. maroccana subsequent to flowering; therefore, these twospecies were measured together during determination of finalnumber of plants and shoot dry weight. Shoot dry weight wasmeasured using a Precisa 2200c balance, which was accurateto 0.01 g. While only three replicates were employed in thesevariables due to budget and time constraints, a larger numberof replications is necessary in order to perform a more robuststatistical analysis.
(2) Seedling emergence
Fig. 2. Substrate moisture in each plot (n = 18). Error bars represent standard errorof mean values. Lower case letters show significance of t-test comparison within agiven day. Paired means displaying the same letter do not differ significantly fromeach other. Substrate moisture was measured prior to watering. Water was appliedonly when measured substrate moisture was below 15% and only within wateringregime plots (July 20, July 27, August 4, August 11).
Since it was impossible to determine total seedling emer-gence of all plant species in a given plot, three quadrats(30 cm " 30 cm) in each plot were determined randomly andset up using sticks and string (Fig. 1). A total of nine quadratsfor each combination of sowing rate and watering regime wereestablished in this manner. Seedling emergence of each speciesin a given quadrat was determined every three weeks (July28, August 18 and September 11, 2006). Seedling of L. elegansand L. maroccana as well as L. grandiflorum var. rubrum andL. usitatissimum were impossible to distinguish during earlydevelopmental stages and were measured together.
(3) Growth (height)Growth of 5 randomly selected representative plants from
each species was measured in each plot yielding a total fifteenreplicates for each of the 22 plant species examined. A totalof three hundred and thirty plants for each combination ofsowing rate and watering regime were measured in thismanner. Measurements of height were carried out on August5, August 30 and September 28, 2006. Plants were marked bysmall flags to ensure that the same plant was examined for
Fig. 3. Changes in mean monthly temperature and rainfall, Sheffield, UK, 2006.Source: Met Office (2007).
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Fig. 4. Mean total shoot dry weight of all plant species per plot (n = 3). Error barsrepresent standard error of mean values. n.s., not significant. Two-way ANOVAdetermined that watering, sowing rate and interaction between the two treatmentsdid not have a significant effect on shoot dry weight.
each measurement. Height was defined as the distance fromthe bottom of the plant to the highest leaf apex.
(4) Number of flowersThe number of flowering plants per plot was determined
weekly for each species. Flowering period was determinedby counting the number of weeks from the start of flowering(at least one plant of a given species was observed flowering)to the end of flowering (no plants of a given species wereobserved flowering).
A t-test was used in order to determine significance of differ-ences in substrate moisture between watering and non-wateringregime plots (Minitab Release 14) at a probability level of P < 0.05.
Significances of differences of other variables were determinedusing a two-way ANOVA (Minitab Release 14) at a probability levelof P < 0.05.
3. Results
It was shown that each combination of sowing and water-ing regime yielded a successful annual meadow green roof. Meantotal shoot dry weight of all plant species in a given plot for eachcombination of sowing rate and watering regime are shown inFig. 4. Although mean total shoot dry weight of all plant specieswas higher in watering regime plots than it was in non-wateringregime plots, no significant effects of watering regime, sowing rateor interaction between the two treatments could be determined.Across low sowing rate plots, total shoot dry weight was higher inwatering regime plots than it was in non-watering regime plots,while the same pattern was not observed across high sowing rateplots. Patterns in mean total number of plants were different fromthose observed in mean dry weight of plants (Fig. 5). Sowing rateappeared to have a significant effect on mean total number of plantsin a given plot, while no significant effect of watering or interac-tion between sowing and watering treatments could be found. Alarger total number of plants emerged in higher sowing rate plots.Across high sowing rate plots, total number of plants was larger innon-watering regime plots than it was in watering regime plots.The larger number of plants observed under non-watering regimeconditions was likely the result of reduced competition betweenplants due to smaller individual plant size. The watering regimeemployed in this study did not significantly affect final plant emer-gence or shoot dry weight. In addition, individual plant growth
Fig. 5. Mean total plant number per plot (n = 3). Error bars represent standard errorof mean values. P, probability. Sowing rate, sowing rate regime. Two-way ANOVAdetermined that sowing rate had a significant effect on total plant number.
tended to be improved in low sowing rate plots, as demonstratedby the higher dry weight of plants despite their relatively smallernumbers. The above results are illustrated in photographs of annualmeadow plots representing each combination of sowing rate andwatering regime (September 1) (Fig. 6).
Mean final plant number and shoot dry weight of each speciesper plot are shown in relation to sowing rate and watering regimein Tables 2 and 3, respectively. For the majority of species, watering,sowing rate and interaction between these treatments did not havea significant effect on either mean final plant number or mean shootdry weight. In C. cyanus, C. tinctoria, L. grandiflorum var. rubrum andT. maritimum, a significant difference existed in shoot dry weightbetween watering regimes. Sowing rate did not have a significanteffect on shoot dry weight in any of the plant species examined.In A. maritimum, E. plantagineum ‘Blue Bedder’, G. muralis, I. amaraand L. grandiflorum var. rubrum, sowing rate had a significant effecton mean total number of plants. In E. californica, watering regimehad a significant effect on total number of plants. Under water-ing regime conditions, many species displayed a higher shoot dryweight in low sowing rate plots; however, the opposite was truefor the non-watering regime, under which many species displayeda greater shoot weight in high sowing rate plots. In most species,a higher total number of plants emerged in high sowing rate plots.These results suggested that many of the examined species havegrown better with a reduced sowing rate when cultivated underthe watering regime whereas growth was reduced in plots of bothsowing rates under non-watering conditions. Greater shoot dryweights were observed in higher sowing rate plots under non-watering regime conditions due to a greater total number of plantsas opposed to increased individual size. There existed consider-able variation in shoot dry weight and number of plants betweenspecies. Only A. aestivalis did not germinate at all. A. maritimum, G.muralis, L. elegans and L. maroccana showed high shoot dry weightsas well as large numbers of plants. Some species, such as A. arven-sis, G. molle, T. maritimum and V. tricolor, produced large numbers ofplants; however, their shoot dry weights were smaller than thoseof other species.
The effects of sowing rate and watering regime on changes inmean seedling emergence of all plant species over time per quadratare shown in Fig. 7. Changes in mean seedling emergence of indi-vidual plant species over time per quadrat are shown in Appendix1. A large number of plant species displayed a tendency towardsdecreasing seedling emergence over time (A. maritimum, L. elegans,L. maroccana); however, in the majority of species, the number
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Fig. 6. Photographs of representative plots of annual meadow with low sowing rate (2 g/m2) and watering regime (top left), low sowing rate (2 g/m2) and non-wateringregime (top right), high sowing rate (4 g/m2) and watering regime (bottom left) and high sowing rate (4 g/m2) and non-watering regime (bottom right) on September 1.
of plants increased over time. In each of three measurementstaken, a significant effect of sowing rate on seedling emergencewas observed and, with the exception of measurements taken onSeptember 11, a significant effect of the interaction between sowingrate and watering regime was also observed. Plant emergence washigher in high sowing rate plots. In high sowing rate plots, no signif-icant difference existed with respect to watering and non-wateringregime; however, watering had a significant positive correlationwith emergence in low sowing rate plots.
The effects of sowing rate and watering on changes in meanheight of all plant species over time are shown in Fig. 8. Changesin mean height of individual plant species per plot over timeare shown in Appendix 2. The mean height of all plant specieswas significantly affected by watering in all three measurements.Therefore, it was determined that watering played an importantrole in encouraging plant growth, especially during the earlystages of annual meadow establishment. In both the wateringand non-watering regimes, many species showed better growth
Table 2Mean total shoot dry weight (g) of each species per plot (n = 3).
Two-way ANOVA was used to compare values within species. SE, standard error; n.s., not significant; P, probability; Sowing rate, sowing rate regime; Watering, wateringregime.
alongside a low sowing rate. A. aestivalis, C. tricolor, C. regalis, E.californica, G. molle, T. maritimum and V. tricolor were small in sizeand not prominent in study plots. At the beginning of August, theheight of most individuals of these species was less than 15 cm.As time passed, individuals from these species could be dividedinto three groups: short (under 10 cm, e.g. A. maritimum), medium(between 10 and 20 cm, e.g. E. plantagineum ‘Blue Bedder’, Iberisspp.) and tall (over 20 cm, e.g. C. tinctoria, G. muralis, Linaria spp.).When selecting annual plant species for use in green roofs it isimportant to consider variations in height in order to achieve amore interesting visual effect.
Changes in mean number of flowering plants per plot over timeare shown in Fig. 9 (high number of flowering plant species) andFig. 10 (low number of flowering plant species). Values of per plotnumber of emerging flowers shown are averages of all treatmentregimes. A. aestivalis, E. californica and G. molle had no flowers in anyof the treatment regimes. C. regalis produced only one flower, which
Fig. 7. Changes in mean seedling emergence of all plant species over time perquadrat (n = 9). Error bars represent standard error of mean values. P, probability;Sowing rate, sowing rate regime; Sowing rate " Watering, interaction between sow-ing rate regime and watering regime. Two-way ANOVA was used to compare valuesfrom each day. Sowing rate had a significant effect on emergence each day and inter-action between sowing rate and watering had a significant effect on emergence onSeptember 11.
had already disappeared by the time weekly measurements weretaken. Species for which the average number of emerging flowersper plot was less than 1.0 (C. tricolor, L. usitatissimum and V. tri-color) were not shown. A. maritimum and L. maroccana displayedparticularly large peak values of emerging flowers per plot in themiddle of August and beginning of September, respectively. E. plan-tagineum ‘Blue Bedder’, G. muralis, I. amara, I. umbellata ‘Fairy’ andL. elegans showed relatively constant numbers of emerging flowersper plot over the course of the study. These species also had longflowering periods. C. tinctoria, C. segetum and T. maritimum showedsmaller numbers of emerging flowers per plot; however, they flow-ered late and reached flowering peaks at the beginning of October.These species could be used to extend the term of flowering in anannual meadow green roof. Mean flowering period of each speciesis shown in Appendix 3. For the majority of species, no significant
Fig. 8. Changes in mean plant height of all plant species over time (n = 330). Errorbars represent standard error of mean values. P, probability; Watering, wateringregime; Sowing rate, sowing rate regime. Two-way ANOVA was used to comparevalues from each day. Watering regime had a significant effect on height each dayand sowing rate had a significant effect on height on August 30.
A. Nagase, N. Dunnett / Landscape and Urban Planning 112 (2013) 50– 62 57
Fig. 9. Changes in mean number of flowering plants per plot (high number of flow-ering plant species). Values shown are means taken from all treatment regimes(n = 12).
Fig. 10. Changes in mean number of flowering plants per plot (low number of flow-ering plant species). Values shown are averages taken from all treatment regimes(n = 12).
effect was exerted on flowering period by either watering, sowingrate or the interaction between the two treatments.
4. Discussion
A limited selection of plant species (such as Sedum spp.)has been used for extensive green roofs. This has, in part, beenthe result of previous studies that have concluded that regularirrigation or deeper substrates (depths greater than 15 cm) arenecessary for successful growth of herbaceous perennials inextensive green roofs, and that only Sedum spp. are able to survivein a thin substrate without irrigation (Monterusso, Rowe, & Rough,2005; Wolf & Lundholm, 2008). This study demonstrated thatit is possible to create colourful and long flowering green roofsusing annual meadow with a substrate depth of only 7 cm andwithout irrigation in Sheffield, UK. Embracing these results wouldexpand the range of options in planting design of extensive greenroofs for landscape designers and urban planners. Although it ispossible to install annual meadows without irrigation systemsunder average weather conditions in central UK, supplementalirrigation may be beneficial during dry and hot summers, sincethe high stress imposed on plants under these conditions has the
potential to reduce plant dry weight, number of flowers and seedsand the ability of those maintaining green roofs to re-seed, even inplants from arid regions (Goldberg, Turkington, Olsvig-Whittaker,& Dyer, 2001; Mott & McComb, 1975; Rivas-Arancibia et al., 2006;Rutledge & Holloway, 1994). In this study, plant growth wasencouraged using supplemental watering and it was determinedthat initial watering was particularly important to improve annualplant growth. Previous studies have also addressed the impor-tance of irrigation during the establishment stage in green roofs(Monterusso et al., 2005).
Final total number of plants was significantly higher in high sow-ing rate plots than it was in low sowing rate plots. These resultswere consistent with those of previous studies (Hitchmough et al.,2004; Shaw & Antonovics, 1986). Across high sowing rate plots, alarger number of seedlings emerged in non-watering regime plotsthan watering regime plots. These patterns could have been theresult of competition between plants. Shoot dry weight in water-ing regime plots was greater than it was in non-watering regimeplots. Larger numbers of seeds were able to germinate under non-watering regime conditions since most plants were relatively smalland exerted less of a competitive effect on one another. The oppo-site effect was observed in low sowing rate plots, as plant growthwas improved and exploitation competition appeared to be the pri-mary mechanism of interaction influencing growth in the annualcommunity (Goldberg et al., 2001; Inouye, Byers, & Brown, 1980).Therefore, it was determined that watering was an important factorinfluencing sufficient seedling emergence. Based on these results,it was concluded that a low sowing rate (2 g/m2) might be betterthan a high sowing rate (4 g/m2) when rainfall was sufficient. On thecontrary, a high sowing rate was determined to be more effectivein a dry environment as it would ensure a sufficient total numberof plants. Under non-watering regime conditions, seedling emer-gence rate was low in low sowing rate plots and individual growthwas not encouraged due to a lack of water. Under these conditions,a high sowing rate would likely improve the visual quality of theannual meadow due to the resulting increases in both floweringplants and total number of plants. As highlighted in the introduc-tion, visual quality of annual meadows depends on the flowers itcontains.
In order to determine the potential of individual annual speciesfor using extensive green roofs in the seed mixture used in thisstudy, emergence, height, number of flowering plants and lengthof flowering periods were summarized (Table 4). To develop a suc-cessful annual seed mixture, it is necessary to choose plant speciesthat possess a high emergence rate, good growth, high numbers offlowering and long flowering periods. In this study, A. maritimum, E.plantagineum ‘Blue Bedder’, G. muralis, I. amara, I. umbellata ‘Fairy’,L. elegans and L. maroccana fulfilled these requirements during thefirst growing season. These species are recommended for inclusionin seed mixtures intended for use in central UK in extensive greenroofs with substrate depths similar to those used in this study. A.maritimum and L. maroccana in particular showed a large numberof flowering and they formed the backbone of the annual plantmixture. The seed mixture used in this study contained amountsof seeds from each of the 22 species included that were equal withrespect to weight. Linaria spp. seeds are smaller than the seeds ofthe other species included (Table 1) and were therefore more preva-lent in the seed mixture. A. maritimum is a low-growing, creepingplant that is useful for filling gaps in a meadow. This species wasalso self-seeding and seedling emergence was observed just afterthe completion of autumn flowering. Linaria spp. are slender plants,and L. maroccana produced a mixture of white, yellow, pink andorange flowers that provided bright colours to the annual meadowextensive green roof. G. muralis plants are also slender; however,this species possesses a large flower, and stems were often brokenby strong winds. C. tinctoria flowered at a later time and could be
58 A. Nagase, N. Dunnett / Landscape and Urban Planning 112 (2013) 50– 62
Table 4Performance summary of each species.
Emergence Height Number offlowering plants
Length of floweringperiod
Potential for annualplant seed mixture
Adonis aestivalis No No No NoAnagallis arvensis High Short Small ShortAlyssum maritimum High Short Large Long HighCentaurea cyanus Low Medium Small MediumChrysanthemum segetum Low Medium Small MediumConvolvulus tricolor Low Low Small ShortConsolida regalis Low Low No NoCoreopsis tinctoria High Tall Small ShortEchium plantagineum ‘Blue Bedder’ Low Medium Medium Long HighEschscholzia californica Low Short No NoGeranium molle High Short No NoGypsophila muralis High Tall Medium Long HighIberis amara Medium Medium Large Long HighIberis umbellata ‘Fairy’ Medium Medium Medium Long HighLinaria elegans High Tall Large Long HighLinaria maroccana High Tall Large Long HighLinum grandiflorum var. rubrum Medium Tall Small ShortLinum usitatissimum Low Medium Small ShortPapaver rhoeas Low Medium Small ShortReseda odorata Medium Short Small MediumTripleurospermum maritimum High Short Small ShortViola tricolor High Short Small Short
Emergence: Mean plant number per plot at time of harvest. No emergence = 0, 0 < low # 20, 20 < medium # 50, 50 < high.Height: Mean plant height at the final measurement. No height = No emergence, 0 < short # 10 cm, 10 cm < medium # 20 cm, 20 cm < tall.Number of flowering plants: Mean number of flowering plants per plot at the peak of flowering in each plant species. No flowering plants = 0, 0 < small # 5, 5 < medium # 20,20 < large.Length of flowering periods: Length of flowering period per plot. No flowering plants = 0, 0 weeks < short # 5 weeks, 5 weeks < medium # 9 weeks, 9 weeks < long.
used to extend the flowering season of a seed mix. E. plantagineum‘Blue Bedder’, I. amara and I. umbellata ‘Fairy’ showed long floweringperiods and possessed outstanding flowering attributes. Gypsophilaspp. and Iberis spp. were successfully implemented in a previousstudy on annual seed mixtures for use in extensive green roofs(Kircher, 2004). Some species, such as C. cyanus and C. segetum,have provided an impressive floral display over a period of severalmonths when grown on the ground (Dunnett, 2004); however, theirflowering period was reduced in the rooftop environment. Of the22 species used, only A. aestivalis did not germinate at all. This mayhave been the result of not only dormancy and low seedling emer-gence rate, but of low initial numbers of seeds as well, which likelyresulted from the relatively large size of seeds in this species andthe way in which initial seed mixture composition was determined.Growth of A. arvensis, C. regalis, E. californica, G. molle, P. rhoeasand V. tricolor was restricted and inconspicuous, and some of theseplants did not produce flowers, while others had only a very smallnumber of flowers for a short period of time. The failure of thesespecies to establish themselves in the study plots was probably dueto competition with other species or their inability to adapt to thegreen roof environment. Furthermore, petals of E. californica and P.rhoeas were easily broken off by strong winds. Selection of plantspecies that can tolerate strong wind is an important considerationfor extensive green roofs.
In this study, performance of non-native and cultivar specieswas better than that of native species. Of the successful plantspecies mentioned previously, I. amara was the only native species.As Hitchmough and Woudstra (1999) pointed out, the inclusion ofexotic species into purposely sown native meadows allows for theaddition of dramatic colours effect as well as extension of the flow-ering season. Despite these observations, the use of native plantson green roofs has attracted considerable attention in recent yearsfor the following reasons: native plants provide a sense of placeand blend into the natural landscape; native plants are adaptedto local environments; native plants function as habitat for nativefauna and serve to increase biodiversity; and native plants are
less likely to become invasive than non-native species (Butler,Butler, & Orians, 2012). Research on performance of native plantspecies in green roofs is limited and it is necessary for futureresearch to examine the performance of seed mixtures contain-ing only native plant species. Moreover, the role of chorology andoptimal habitats for annual meadow green roofs was not clearin this study. Therefore, it is also interesting to study native andexotic annual plant community from various habitats to under-stand them.
This type of annual meadow cost 60 p/m2 (2 g/m2) to installand spreading the seeds took less than one hour. Furthermore,since fully grown plants do not have to be transported to theroofs, money that would otherwise have been spent on trans-portation and planting can be saved. These factors make annualmeadow particularly useful in the establishment of large area greenroofs. Annual meadows on extensive green roofs also require littlemaintenance. During the present study, no weeds colonized theextensive green roof examined as the substrate was thin and rela-tively dry. Another advantage of annual meadow green roofs is thereduction of competition with spontaneous plant species result-ing from initial installation of clean substrate. Seed banks of weedyplant species tend to be a serious problem when establishing annualmeadows on the ground (Prentis & Norton, 1992). The primarymaintenance act required for annual plant meadows in exten-sive green roofs is an annual mowing. According to Hitchmough(2004), annual plants are transient and even with proper manage-ment encouraging regeneration from self-sowing seeds, sowing isoften required on an annual basis. Previous studies have shown thatmanagement of meadow structure and composition and the abun-dance and availability of plant (mainly as seeds) and invertebratefood resources for birds are inextricably linked and it is impor-tant to leave seed heads without cutting over winter (Atkinson,Buckingham, & Morris 2004; McCracken & Tallowin, 2004; Vickeryet al., 2001).
Francis and Lorimer (2011) demonstrated that green roofs canbe used for urban reconciliation ecology, which is defined as the
A. Nagase, N. Dunnett / Landscape and Urban Planning 112 (2013) 50– 62 59
modification and diversification of anthropogenic habitats in orderto support a greater range of species without compromising landuse. They stressed that, in order for green roofs to be success-fully utilized in urban reconciliation ecology, the participation ofurban citizens is crucial. Use of annual meadow plants help toensure the participation of urban citizens. According to a publicopinion survey on wildflower mixtures, a long bloom season andmulti-coloured flowers are generally seen as favourable qualities(Rutledge & Holloway, 1994). It was also determined that the com-munities identified as the most aesthetically pleasing were thosethat possessed the greatest species richness and structural diver-sity (Lindemann-Matthiens & Bose, 2007; Lindemann-Matthies,Junge, & Matthies, 2010). Recently, many biodiverse roofs havebeen installed due to the associated biodiversity benefits; how-ever, they lack the lush green appearance of green roofs but theyhave a similar appearance to brownfields (Dunnett & Kingsbury,2008). In this study, biodiversity values of the annual meadowexamined were not measured; however, many bees, particularlybumblebees, were attracted to the study site due to the abun-dance of flowers. Especially, it was observed that bees visited E.plantagineum ‘Blue Bedder’ and Linaria spp. frequently and gener-ally, pollen dispersal success in entomophilous plants is influencedby visitation frequency of a pollinator (Galen & Stanton, 1989).Therefore, annual meadows could be one potential alternative tobiodiverse roofs in areas where aesthetics are important. In futureresearch, assessment of aesthetic value in annual plant meadowroofs and biodiverse roofs is required. It is important to note thatthe appearance of annual meadow green roofs in winter shouldbe considered, as some individuals might find them untidy if leftunmanaged during that time period (Rohde & Kendle, 1997). More-over, bare ground may become apparent during the wintertime(Dunnett, 2004). Combination of annual plants and bulbs or peren-nials during winter would serve an aesthetic purpose and mightalso help to minimize soil erosion on green roofs. Moreover, it is
required to use big aggregate of substrate to reduce soil erosion andto keep seeds into the substrate when annual meadow is used forextensive green roofs.
5. Conclusion
The annual plant seed mixture used in this study was easy toinstall, inexpensive and quick to establish, and the majority ofplant species used were drought tolerant and long flowering. Fur-thermore, annual meadows established using this seed mixturerequired little in the way of maintenance and irrigation. A low sow-ing rate is recommended in order to facilitate plant growth whenwater resources are abundant, while a high sowing rate would beideal under drier environmental conditions in order to ensure a suf-ficient total number of plants. Further study of this annual mixtureover a longer period is necessary to confirm the sustainability, bio-diversity value and detail flower colours (e.g. chromatic spectrum)of the annual meadows produced. In addition, the assessment ofaesthetic value of annual meadow is also required. Performance ofresulting annual meadows must also be assessed over a range ofgreen roof environments. This annual seed mixture might only beeffective in areas with climates similar to that of the UK; therefore,research on different seed mixtures for other regional extensivegreen roofs is essential.
Acknowledgements
We express our appreciation to Almasc for providing the exper-imental materials, to career-support program for woman scientistsin Chiba University for founding to proof our English, to Dr. NoelKingsbury in University of Sheffield for his valuable advice, to Mr.Min-Sung Choi in University of Sheffield for helping to set up theexperiment, to Dr. Shinichi Kurihara in Chiba University and Dr.Ann-Mari Fransson in Swedish University of Agricultural Sciencefor their advice of statistical analysis.
Appendix 1. Mean emergence of each species per quadrat (n = 9)
July 28 August 18 September 11
Probability Probability 2 g/m2 4 g/m2 SE Probability
Watering Non-watering
Watering Non-watering
Adonis aestivalis n/a n/a 0 0 0 0 n/a n/aAnagallis arvensis Watering P < 0.05 Watering P < 0.05 2.44 0.70 2.56 1.67 0.53 Watering P < 0.05Alyssum maritimum n.s. Sowing rate P < 0.01 12.22 12.22 18.78 21.67 1.80 Sowing rate P < 0.01Centaurea cyanus n.s. Sowing rate P < 0.05 0.78 0.89 1.00 1.00 0.35 n.s.Chrysanthemum segetum Sowing rate P < 0.05 n.s. 1.00 0.56 1.56 1.67 0.39 Sowing rate P < 0.05Convolvulus tricolor Watering P < 0.01 n.s. 0.44 0.56 0.67 0.33 0.29 n.s.Consolida regalis n.s. n.s. 1.22 0.22 0.78 1.22 0.32 Sowing rate * Watering
P < 0.05Coreopsis tinctoria Watering P < 0.01 n.s. 4.00 2.22 3.11 3.78 0.65 n.s.Echium plantagineum ‘Blue
Two-way ANOVA was used to compare values within species. Results of the first two measurements show only the probability derived from statistical analysis. SE, standarderror; n.s., not significant; P, probability; Watering, watering regime; Sowing rate, sowing rate regime; Sowing rate * Watering, interaction between sowing rate regime andwatering regime.
Appendix 2. Height of each species (cm) (n = 15)
August 5 August 30 September 28
Probability Probability 2 g/m2 4 g/m2 SE Probability
Watering Non-watering
Watering Non-watering
Adonis aestivalis n/a n/a 0 0 0 0 n/a n/aAnagallis arvensis Watering P < 0.01 Watering P < 0.01 4.15 1.48 3.11 1.84 0.44 Watering P < 0.01Alyssum maritimum Watering P < 0.01
Sowing rate P < 0.05Watering P < 0.01 6.79 6.09 6.27 6.02 0.51 n.s.
Centaurea cyanus Watering P < 0.01 Watering P < 0.01 13.19 14.17 18.72 13.37 1.73 n.s.Chrysanthemum
segetumWatering P < 0.05Sowing rate P <0.05Sowing rate * Watering P<0.01
Sowing rate P < 0.05 25.39 13.29 13.89 14.37 2.38 Watering P < 0.05Sowing rate P < 0.05Sowing rate * WateringP < 0.05
Convolvulus tricolor Watering < 0.05 Watering P < 0.05 8.11 6.77 7.20 6.65 1.12 n.s.Consolida regalis n.s. n.s. 1.53 1.56 0.96 0.71 0.38 n.s.Coreopsis tinctoria Watering P < 0.01
Sowing rate P < 0.05Sowing rate * WateringP < 0.05
Watering P < 0.05 34.36 25.52 30.72 24.57 2.06 Watering P < 0.01
Echium plantagineum‘Blue Bedder’
Watering P < 0.01 Watering P < 0.01 17.32 10.98 14.79 12.32 0.97 Watering P < 0.01
Eschscholzia californica Watering P < 0.01Sowing rate P < 0.01Sowing rate * WateringP < 0.01
Watering P < 0.01Sowing rate P < 0.01Sowing rate * WateringP < 0.01
6.43 3.46 3.97 3.01 0.62 Watering P < 0.01Sowing rate P < 0.05
Geranium molle Watering P < 0.01 Watering P < 0.01 4.26 2.59 3.37 2.37 0.33 Watering P < 0.01Gypsophila muralis n.s. Watering P < 0.01 28.73 21.15 25.77 20.79 1.78 Watering P < 0.01Iberis amara Watering P < 0.01 Watering P < 0.01
Sowing rate * WateringP < 0.01
18.73 16.22 16.11 15.87 0.69 Sowing rate P < 0.05
Iberis umbellata ‘Fairy’ Watering P < 0.01 Watering P < 0.01 14.99 9.63 11.71 10.93 0.76 Watering P < 0.01Sowing rate * WateringP < 0.01
Linaria elegans Watering P < 0.01Sowing rate P < 0.05
n.s. 40.30 37.59 34.37 37.20 2.65 n.s.
Linaria maroccana Watering P < 0.01 Watering P < 0.05 27.12 27.05 26.93 28.31 2.05 n.s.Linum grandiflorum var.
rubrumWatering P < 0.01Sowing rate P < 0.01
Watering P < 0.01Sowing rate P < 0.05
26.81 18.69 26.21 16.50 1.623 Watering P < 0.01
Linum usitatissimum Watering P < 0.01 Watering P < 0.01 22.15 14.97 21.53 15.65 0.82 Watering P < 0.01Papaver rhoeas Watering P < 0.01 Watering P < 0.01 15.52 12.57 16.43 6.49 1.91 Watering P < 0.01Reseda odorata Watering P < 0.01 Watering P < 0.05 10.53 8.72 8.96 7.03 1.42 n.s.Tripleurospermum
maritimumWatering P < 0.01 Watering P < 0.01 11.68 3.88 7.10 3.54 1.16 Watering P < 0.01
Sowing rate P < 0.05Viola tricolor Watering P < 0.05 n.s. 1.63 1.65 1.79 0.99 0.40 n.s.
Two-way ANOVA was used to compare values within species. Results of the first two measurements show only the probability derived from statistical analysis. SE, standarderror; P, probability; n.s., not significant; Watering, watering regime; Sowing rate, sowing rate regime; Sowing rate * Watering, interaction between sowing rate regime andwatering regime.
A. Nagase, N. Dunnett / Landscape and Urban Planning 112 (2013) 50– 62 61
Appendix 3. Mean flowering period of each species per plot (week) (n = 3)
Two-way ANOVA was used to compare values within species. SE, standard error; P, probability; n.s., not significant; Watering, watering regime; Sowing rate * Watering,interaction between sowing rate regime and watering regime.
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