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Green roofs could be a way to increase vegetation in the center of old Mediterranean cities. The need for conservation of local character and biodiversity requires the use of native plant species, whereas the deficiency of water, particularly in semiarid regions, requires the use of species with reduced irrigation needs. Moreover, the aged buildings lead to the use of lightweight green roof constructions. Therefore, research was undertaken to investigate the possibility of using three Mediterranean aromatic xerophytes, Artemisia absinthium L., Helichrysum italicum Roth., and H. orientale L., at an extensive green roof in Athens, Greece. Simultaneously, the possibility of using locally produced grape marc compost was investigated. Substrate type and depth and irrigation frequency effects on growth of these species were studied. Rooted cuttings were planted mid-May in plastic containers with a green roof infrastructure fitted (moisture retention and protection of the insulation mat, drainage layer, and filter sheet) and placed on a fully exposed third floor flat roof at the Agricultural University of Athens. Two types of substrates were used, grape marc compost:soil:perlite (2:3:5, v/v) and peat:soil:perlite (2:3:5, v/v, as a control), as well as two substrate depths, 7.5 (shallow) and 15 cm (deep), and two irrigation frequencies, sparse (5 or 7 days in shallow and deep substrate, respectively) and normal (3 or 5 days in shallow and deep substrate, respectively). Increased contents of macroelements, total phosphorus (P) and potassium (K) in particular, were recorded in the compost-amended substrate, whereas both substrates had similar physical properties. Plant growth was recorded from May to October. The deep compost-amended substrate, independent of irrigation frequency, resulted in taller plants with bigger diameter and aboveground dry weight in all species. However, a remarkable result was that shallow compost-amended substrate with sparse irrigation resulted in similar or even bigger plant growth of all plant species compared with deep peat-amended substrate with normal irrigation. Thus, all three species were found suitable for use in Mediterranean extensive or semi-intensive green roofs, whereas the use of grape marc compost in the substrate allowed for less water consumption and the reduction of substrate depth without restriction of plant growth at the establishment phase and the first period of drought. The intense urbanization of recent decades has led to the development of huge cities, even in the Mediterranean, where large smooth areas, usually offered for the development of big cities, are not commonly available as a result of the natural terrain and the presence of historical settlements. Thus, most Mediterranean cities are crammed around their old nucleus, which in many cases is characterized as a historical heritage. These cities lack areas that could be converted into conventional green spaces, and thus there is an increasing interest in green roof systems. However, green roofs are still relatively uncommon in Mediterranean countries, although these areas would significantly benefit from the ecological and technical functions of this technology (Benvenuti and Bacci, 2010; Fioretti et al., 2010; Savi et al., 2013). In the last decade mainly, a number of research works, reviews, and books (Dunnett and Kingsbury, 2008; Getter and Rowe, 2006; Oberndorfer et al., 2007) refer to ecosystem services provided by green roofs in urban areas such as improved stormwater management (Fioretti et al., 2010; Nagase and Dunnett, 2012), thermal insulation and energy conservation (Jaffal et al., 2012), reduction of the urban heat island effect (Bowler at al., 2010; Mackey et al., 2012), mitigation of air pollution (Currie and Bass, 2008), CO2 sequestration (Getter et al., 2009; Li et al., 2010), increased biodiversity and provision of habitats (Maclvor and Lundholmb, 2011) as well as increased aesthetic value of buildings and the city overall. Furthermore, green roofs could contribute to the socialization of the multistory building tenants in dense cities, replacing the traditional house courtyard or terrace as an assembly place. In addition, the visitation or even the volunteer work of some people, e.g., elderly or children, at a green roof could support their psychosomatic health and provide environmental education (Kweon et al., 1998). Major parameters that should be taken under consideration before applying green roof systems in Mediterranean regions are water availability, particularly in those areas with semiarid climate, biodiversity, and local character preservation. The use of native xerophytes in Mediterranean green roofs could fulfill all these conditions (Benvenuti and Bacci, 2010). A number of works refer to the ability of certain plant species to cope with drought in green roofs (Benvenuti and Bacci, 2010; Kotsiris et al., 2012; Nagase and Dunnett, 2010; Nektarios et al., 2011; Thuring et al., 2010). Even in regions characterized by cooler and wet climates, the growth environment of green roofs is considered severe as a result of high exposure to solar radiation, wind, wide temperature fluctuations, and limited water availability. Therefore, Sedum taxa have been widely investigated and are a popular choice for extensive green roofs, because these plants combine shallow root systems with high tolerance to drought resulting from crassulacean acid metabolism (Durhman et al., 2007; Rowe et al., 2012). Recognizing green roofs as tools for augmentation of urban biodiversity and habitats (Cook-Patton and Bauerle, 2012; Lundholm and Peck, 2008), a diversity of species capable to establish meadows on extensive green roofs have raised the attention of researchers (Dvorak and Volder, 2010; Maclvor and Lundholmb, 2011; Nagase and Dunnett, 2010, 2013). Apart from biodiversity reasons, mixing multiple species in a green roof was shown to enhance plant performance and ecological services through optimal water loss and roof surface cooling (Butler and Orians, 2011; Dvorak and Volder, 2010). Also plant species with taller height, larger diameter, and larger shoot and root biomass, as grasses and forbs, were more effective than sedum in reducing water runoff Received for publication 05 July 2013. Accepted for publication 25 Aug. 2013. To whom reprint requests should be addressed; e-mail mpapaf@aua.gr. HORTSCIENCE VOL. 48(10) OCTOBER 2013 1327 from green roofs (Nagase and Dunnett, 2012). The weight of a green roof is another issue that should be taken into consideration in constructions, particularly in the Mediterranean, where most cities are composed of aged buildings that possibly will not support a heavy green roof. Green roof weight depends mainly on the substrate type and depth. Therefore, it is desirable to identify plant species and substrate types to ensure satisfactory growth in shallow substrate. Interestingly, the Mediterranean xerophyte Dianthus fruticosus survived through summer in southern Greece with minimum irrigation (15% Epan), even at 7.5-cm substrate depth (Nektarios et al., 2011). The ideal substrate should be lightweight, well drained with adequate water and nutrientholding capacity and should not break down over time (Fassman et al., 2010; FLL, 2010). According to the FLL (2010) guidelines, organic content for an extensive green roof should be up to 20% by volume. Higher levels of organic matter are not recommended because it might cause shrinkage of the substrate over time and can result in increased nitrogen (N) and P runoff (Getter and Rowe, 2006). Furthermore, compost-rich substrates may result in plants with increased foliage, prone to increased transpiration and thus more susceptible to water stress conditions (Nagase and Dunnett, 2011). Often the organic matter of green roof substrates is composed of peat, bark compost, or green waste compost. Peat is used because of its high waterholding capacity and readily available water, high cation exchange capacity, and low bulk density (Ampim et al., 2010; Fassman et al., 2010). However, its use in horticulture has become an environmental issue, because it is considered a non-renewable resource that should be preserved (Alexander et al., 2008). A parallel environmental issue is the disposal of agroindustrial wastes; composting being part of the solution to this problem. Locally available recycled materials are recommended for use in green roof substrates (Getter and Rowe, 2006; Molineux et al., 2009) contributing to the reduction of construction cost and carbon footprint. Grape marc compost is found in large amounts in the traditionally wine-producing Mediterranean region and has been used efficiently for greenhouse horticulture (Papafotiou et al., 2011a, 2011b; Reis et al., 2001). In the present work, we investigated the possibility of using native aromatic xyrophytes in extensive green roofs in semiarid Mediterranean regions, simultaneously with the possibility of using locally produced grape marc compost as a substrate component, to support urban biodiversity and carbon footprint reduction, preserving local character and increasing enjoyment. Aromatic plants, apart from evoking memories by their aroma, particularly for elderly persons, can attract wild life such as butterflies and bees, having simultaneously repellent effects on mosquitoes (Fussell and Corbet, 1992; Lafuma et al., 2001). Three shrubby (40 to 50 cm height), aromatic, Mediterranean xerophytes, members of the Asteraceae family, Artemisia absinthium L., Helichrysum italicum Roth., and H. orientale L., were selected for evaluation as for their adaptation to extensive green roof conditions in Athens, Greece. These plant species grow naturally on rocky sunny slopes, have fibrous roots, gray leaves, and bear numerous yellow flower heads from April to June in H. orientale and June to September in H. italicum and A. absinthium (Polunin, 1981). No information was found on the type of pho

Growth of Native Aromatic Xerophytes in an Extensive Mediterranean Green Roof as Affected by Substrate Type and Depth and Irrigation Frequency