Plant Species for Green Roofs in Mediterranean Area

Plant Species for Green Roofs in Mediterranean Area: History

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Subjects: Plant Sciences

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Owing to intense urbanization and global change with the consequent extreme climate effects, interest in green roofs, even extensive ones, in the Mediterranean environment has increased. To this end, the choice of plant species is crucial because, owing to the identification of the most suitable plants, it will be possible to expand this type of green infrastructure and increase its ecosystem services in the urban environment.

ecosystem services
green roofs
Mediterranean areas
plant species

1. Introduction

The growth of the global population (in 2022, 8 billion people on Earth were exceeded) and the rate of urbanization (the UN Urbanization Statistics in 2018 [1] estimate that over 60% of the world population lived in urban areas and that by 2030, the number of megacities with more than 10 million inhabitants will be equal to 43) indicate that the problems linked to cities have become central to the quality of human life. In fact, overpopulation determines that cities have become heterotrophic organisms with their own metabolism [2], which bases their growth and expansion on the indiscriminate use of resources (energy and raw materials, often non-renewable), favored by the proliferation of means of transport and supported by industrial development and today’s technologies. Urban agglomerations return heat and pollution to the environment, alter bio-geo-chemical cycles, and cause irreversible loss and fragmentation of natural habitats [3]. Furthermore, cities not only consume the resources immediately available within their physical boundaries, but also have a pervasive effect on large areas, linked to the production of commercial goods and services necessary for their sustenance and development. To mitigate the failures of intense urbanization, a fundamental role for the environmental quality of the city is assured by green infrastructures and ecosystem services [4]. Green roofs play an important role among the green infrastructures. Since building roof surfaces cover 20–25% of urban areas [5], green roofs can effectively contribute to increasing ecosystem services (reduction in air temperature, interception of rainfall, reduction in pollution, etc.). This contribution varies among the different types of green roofs, which, based on the height of the substrate and the level of maintenance requirements, can be divided into extensive (height of the substrate often less than 10 cm, cheaper, weight approximately 60–150 kg m⁻²), semi-intensive (15–30 cm, expensive, weight 25% above or below 150 kg m⁻²), and intensive (>30 cm, very expensive, weight 200–500 kg m⁻²) [5].

Vegetation can ensure the long-term effect of a green roof, also through its evolution, as a result of interactions with the environment and between the different species used. Therefore, the identification of the most suitable plant species is one of the most important aspects of green roofs, which involves green roof designers, who must evaluate numerous parameters and individuate the advantages and disadvantages of possible plant species choices. In recent years, ecological aspects have become more prevalent than aesthetic aspects in the selection of plant species. The positive environmental effects of these types of green infrastructure in urban areas have been highlighted above all [6], which strongly influence the same criteria for choosing plants. Green roofs are considered indispensable ecosystem structures for cities, and are capable of ensuring heat island mitigation, temperature balancing of buildings, better management of water runoff, and greater urban biodiversity [7,8]. Based on the function that is considered prevalent, the selection criteria change, sometimes significantly; therefore, it is necessary to be well aware of the objectives that underlie the identification of different species [6].

2. Plant Species for Green Roofs in the Mediterranean Area

In many regions with hot and dry climates, including the Mediterranean, green roof technology is not widespread [35], mainly due to the difficult climate (summer drought and high temperatures) and the limited availability of water. These characteristics impose severe restrictions on the growth and survival of green roofs [36,102]. Plants are assumed to not survive in semi-arid climates on unirrigated green roofs with substrate depths of less than 5 cm, especially during the summer drought or establishment phases [35,118]. Furthermore, summer water scarcity is a recurring problem in the Mediterranean and climate change will lead to even more severe water scarcity because summer precipitation is expected to decrease by 5% per decade [119]. This can lead to irrigation becoming an unsustainable, regulated, and limited option. Therefore, it is necessary to select plant species that are capable of adapting to the absence of irrigation [89]. Mediterranean areas contain habitats rich in native plant species that have the potential to be used on extensive green roofs [89] because they are believed to be better adapted to local climatic conditions and require little maintenance [34]. From a biodiversity perspective, one could also assume that green roofs constitute a new habitat for some Mediterranean plants whose natural environments are in danger. The choice can rely on the fact that many native plant species of the Mediterranean (in particular, the xerophytes) express morpho-functional and physiological adaptations that make them particularly suitable for green roofs; in fact, they present changes in the structures of the leaves (imbricated or often linear, with thick and waxy cuticle, sunken stomata, pubescent surface) and roots (deep roots, large root hair, rapid development in young plants), reduction in photosynthesis, and leaf drop phenomena under conditions of drought, high solar radiation, and high temperatures in summer [42].

Although research in the Mediterranean environment is less extensive than that carried out in a continental climate, numerous native species have been considered. Azenas et al. [120], for example, analyzed the response of five Mediterranean species—Brachypodium phoenicoides (L.) Roem. & Schult., Crithmum maritimum L., Limonium virgatum (Willd.) Fourr., Sedum sediforme (Jacq.) Pau, and Sporobolus pungens Kunth—grown in a non-limiting water regime or restoring 50% of evapotranspiration. The plant species were selected because they grow in natural habitats characterized by shallow soils with low organic substance content, high solar radiation, and extreme temperatures, namely, under conditions similar to those found on a green roof. Furthermore, some of these (S. sediforme) were CAM or CAM-facultative succulent species. The behavior of these plant species was monitored for two years. All plant species survived and exhibited a suitable aesthetic performance and vegetation coverage. S. sediforme recorded the least changes in appearance, the highest biomass production, and the lowest water consumption. However, B. phoenicoides appears to be an interesting alternative due to its valuable aesthetic characteristics and water consumption during the rainy season, suggesting a potential role of this species in the regulation of rainwater related to runoff reduction. S. pungens performed well in summer but presented poor aesthetic value during winter. L. virgatum, a plant that grows on rocky coasts, has shown good aesthetic value, both for its flowering and compact shape, and a high carbon sequestration capacity. In contrast, the use of C4 species, such as S. pungens, in urban green roofs in the Mediterranean climate is limited by the difficulty of this species to survive in winter and regrow in early spring [120].

To broaden the diffusion of green roofs in the Mediterranean environment, the contribution of four native plant species in Portugal—Antirrhinum linkianum Boiss. & Reut., Asphodelus fistulosus L., Centranthus ruber (L.) DC. and Sedum sediforme (Jacq.) Pau—resilient and drought tolerant, was analyzed. Growth, and aesthetic value were evaluated under two irrigation regimes (return of 100 and 60% evapotranspiration). A. linkianum had the highest number of flowers, longest seed production duration, and the highest area coverage, demonstrating its suitability for use. The level of irrigation did not significantly affect flowering and green coverage for any of the plant species and irrigation costs could be reduced by adopting deficit irrigation [121].

Attention has also been paid to therophyte species; annual plants contribute significantly to the vegetation of the Mediterranean basin, but their presence on green roofs has been limited to date [89], which is due to the brevity of their cycle, the difficulty of regeneration, and the lack of competitiveness compared to perennial plants. The absence of these plants during the summer months results in a modest cooling effect during the hot season. Van Mechelen et al. [89] analyzed the plants present in natural habitats in southern France, that presented characteristics similar to those of green roofs, and identified 372 potentially usable species on the basis of some functional parameters; of these 35% are therophytes, which indicates that many annual species can be taken into consideration. Therophytes have interesting properties such as a short flowering period and the production of many seeds. Their conservation value may also be important, as many annual plants are threatened in Mediterranean areas [122]. Other traits such as CAM metabolism, stress tolerance, and succulence have been shown to be important for successful green roofs [25].

The screening tool provides a potential list; however, definitive proof is obtained through experimental tests. Despite these limitations, the plant characteristics approach offers interesting possibilities for Mediterranean regions and can also help adapt green roof designs to future climate change [89].

Sage species native to Greece—i.e., Salvia fruticosa Mill., S. officinalis L., S. pomifera ssp. pomifera, S. ringens Sm., S. tomentosa Mill., and interspecific hybrids—were evaluated for their inclusion in an extensive green roof in a Mediterranean climate in the summer period with regular or reduced irrigation (every 2–3 days with substrate humidity of 16–22% v/v and 4–5 days with substrate humidity of 7–11% v/v). Regardless of the irrigation frequency, S. pomifera ssp. pomifera x S. ringens and S. officinalis x S. pomifera ssp. pomifera showed the highest survival rate among all hybrids and species, as well as satisfactory growth, while S. fruticosa recorded the lowest survival, demonstrating that numerous Salvia species can be used in extensive green roofing in arid regions [123].

The possible use of two Mediterranean shrubs, Arbutus unedo L. and Salvia officinalis L., on green roofs was analyzed. The first species presented a substantial isohydric response (owing to the reduction in the stomatal opening at the first signs of stress, it was able to contain an excessive lowering of the water potential) and the second anisohydric (the plant appeared capable of withstanding strong variations in water potential while only partially limiting stomatal closure). Both species can be used on the Mediterranean green roof, even if the anisohydric species appear to be more sensitive to the characteristics of the substrate [63].

Reducing soil temperature while maintaining a relatively high air temperature has been shown to improve the growth and functional status of both roots and shoots and enhance plant survival [124]. This contrasts with the need to reduce the depth of the substrate to limit weight and installation costs [125]. However, the depth of the substrate is not always a limiting factor in the adoption of shrubs. Savi et al. [126] investigated the behavior of two drought-adapted shrubs of two-years-old (Cotinus coggygria Scop. and Prunus mahaleb L.) grown in experimental modules with a 10 or 13 cm deep substrate. The results highlighted how the reduced depth of the substrate translated into less severe water stress than hypothesized and that the shallower substrate indirectly stimulates lower water consumption as a consequence of the reduced plant biomass; therefore, it is possible to hypothesize a green roof with the use of stress-resistant shrubs in sub-Mediterranean areas, even in the presence of a substrate only 10 cm deep.

The performance of native Mediterranean plants on green roofs could be improved by adopting a plant community instead of a monoculture. Varela-Stasinopoulou et al. [127] analyzed the growth, flowering, and self-reproduction rate of three plant communities, artificially created and made up of native Mediterranean plants, placed in substrates of different depths (8 and 15 cm) and with two irrigation regimes (high, 20% ETo and low, 10% ETo). The plant communities simulated those on the islands of Crete and Greece. Each of the three artificial plant communities comprised nine species and subspecies. Deeper substrates significantly improve the growth, flowering, and survival of most taxa. The irrigation regime was not significant for any species except for one, indicating that minimal amounts of water may be sufficient for irrigation. Four species failed to flower, whereas 15 species managed to self-reproduce.

Information on the plant species proposed for extensive green roofs in the Mediterranean region is presented in Table 1. The selected papers were experimental trials conducted in the Mediterranean environment. No information has been reported on plant species performance because the operative and stressful conditions are quite different. The list is full of over 180 species and/or cultivars belonging to over 40 families, most of them of Mediterranean origin, attesting to a large number of plant species that can be counted even with substrate depths of less than 20 cm. Among biological forms, chamaephytes (~40% of the total) and hemicryptophytes (~30%) stand out.

Table 1. Plant species studied for extensive green roofs ¹ in the Mediterranean region.

Plant Species ²	Botanical Family ³	Plant Life Form ⁴	Chorotypes ⁵	Substrate Depth (cm)	References
Achillea millefolium L.	Asteraceae	H	Eurosib.	4, 7, 10, 19	[77,128]
Aeonium arboreum Webb & Berthel.	Crassulaceae	NP	Macarones.	6	[129]
Allium carinatum L.	Amaryllidaceae	G	Stenomedit.	20	[130]
Allium roseum L.	Amaryllidaceae	G	Stenomedit.	10	[131]
Allium sphaerocephalon L.	Amaryllidaceae	G	Paleotemp.	5, 10	[37]
Alyssum alyssoides L.	Brassicaceae	T	Eurimedit.	5, 10	[37,131]
Alyssum saxatile L.	Brassicaceae	C	Medit.	14	[132]
Anemone hortensis L.	Ranunculaceae	G	N-Eurimedit.	20	[130]
Anthemis arvensis L.	Asteraceae	T	Stenomedit.	20	[130]
Anthemis maritima L.	Asteraceae	H	W-Medit.	15, 20	[36]
Anthyllis vulneraria L.	Fabaceae	H	Eurimedit.	10	[131]
Antirrhinum majus L.	Plantaginaceae	C	W-Medit.	20	[130]
Antirrhinum linkianum Boiss. & Reut	Plantaginaceae	C	Endem. Portugal	11	[121]
Aptenia cordifolia (L.f.) Schwantes	Aizoaceae	C	Africa	5, 6	[129,133,134]
Arbutus unedo L.	Ericaceae	P	Stenomedit.	18	[63]
Armeria maritima (Mill.) Willd.	Plumbaginaceae	H	Subcosmopol.	4, 7, 10, 11 ± 1	[128,135]
Armeria maritima (Miller) Willdenow subsp. maritima	Plumbaginaceae	H	Subcosmopol.	19	[77]
Armeria maritima ‘Rosea’	Plumbaginaceae	H	Subcosmopol.	14	[132]
Armeria pungens Hoffmanns. & Link	Plumbaginaceae	C	W-Europ.	15, 20	[36]
Artemisia absinthium L.	Asteraceae	C	E-Medit.	7.5, 10, 15	[19,86]
Arthrocnemum macrostachyum (Moric.) K.Koch	Amaranthaceae	C	Medit.	10	[136,137]
Asphodelus fistulosus L.	Asphodelaceae	H	Subtrop.	11	[121]
Atriplex halimus L.	Amaranthaceae	P	Stenomedit.	7.5, 10, 15	[19,138]
Atriplex portulacoides L.	Amaranthaceae	C	Circumbor.	10	[139]
Ballota acetabulosa Benth.	Lamiaceae	C	W-Asia	8	[140]
Blackstonia perfoliata (L.) Huds.	Gentianaceae	T	Eurimedit.	10	[131]
Brachypodium phoenicoides (L.) Roem. & Schult.	Poaceae	H	W.Stenomedit.	15	[42,65,120,141]
Brachyscome multifida DC.	Asteraceae	H	Australia	19	[77]
Calamintha nepeta Savi	Lamiaceae	C	Medit.-Mont.	15, 20	[36,130]
Calendula arvensis L.	Asteraceae	H	Eurimedit.	10	[131]
Carpobrotus edulis (L.) N.E.Br.	Aizoaceae	C	S-Africa	5, 6, 9, 12, 15	[133,142]
Carpobrotus rossii (Haw.) Schwantes	Aizoaceae	C	S-Africa	10	[143]
Carthamus carduncellus L.	Asteraceae	H	NW-Medit.	5, 10	[37]
Centaurea cyanus L.	Asteraceae	T	Stenomedit.	20	[130]
Centranthus macrosiphon Boiss.	Caprifoliaceae	H	W.Stenomedit.	10	[131]
Centranthus ruber (L.) DC.	Caprifoliaceae	C	Stenomedit.	11, 11 ± 1, 15, 20	[36,121,135]
Cerastium tomentosum L.	Caryophyllaceae	C	Endem. Italy	6, 9, 12, 14, 15	[132,142]
Chrysanthemum myconis L.	Asteraceae	T	Stenomedit.	20	[130]
Chrysocephalum apiculatum (Labill.) Steetz	Asteraceae	H	Endem. Italy	19	[77]
Cistus salviifolius L.	Cistaceae	NP	Stenomedit.	10, 13	[87]
Clinopodium acinos Kuntze	Lamiaceae	T	Eurimedit.	5, 10	[37]
Colchicum autumnale L.	Colchicaceae	G	C-Europ.	20	[130]
Consolida regalis Gray	Ranunculaceae	T	Eurimedit.	20	[130]
Convolvulus cneorum L.	Convolvulaceae	C	N-Medit.	7.5, 10, 15	[19,138,144]
Convolvulus sabatius Viv.	Convolvulaceae	G	W.Stenomedit.	19	[77]
Cotinus coggygria Scop.	Anacardiaceae	NP	Medit.-Turan.	10, 13	[87,126]
Crithmum maritimum L.	Apiaceae	C	Eurimedit.	7.5, 15, 20	[36,120,141,145,146]
Crocus vernus (L.) Hill	Iridaceae	G	Eurimedit.	20	[130]
Delosperma cooperi (Hook.f.) L.Bolus	Aizoaceae	C	S-Africa	14	[132]
Delosperma N.E.Br. ‘Kelaidis’	Aizoaceae	C	S-Africa	14	[132]
Delosperma N.E.Br. sp.	Aizoaceae	C	S-Africa	14	[132]
Dianella caerulea Sims ‘Breeze’	Asphodelaceae	H	Australia	10	[143]
Dianthus carthusianorum L.	Caryophyllaceae	H	C-Europ.	15, 20	[36,130]
Dianthus deltoides L.	Caryophyllaceae	H	Euroasiat.	10	[131]
Dianthus deltoides L. ‘Leuchtfunk’	Caryophyllaceae	H	Euroasiat.	14	[132]
Dianthus fruticosus subsp. fruticosus	Caryophyllaceae	H	Endem. Greece	7.5, 15	[147]
Dianthus gratianopolitanus Vill.	Caryophyllaceae	H	C-Europ.	6, 9, 12, 15	[142]
Dianthus superbus L.	Caryophyllaceae	H	Euroasiat.	5, 10	[37]
Dorycnium hirsutum (L.) Ser.	Fabaceae	C	Eurimedit.	14	[132]
Drosanthemum floribundum (Haw.) Schwantes	Aizoaceae	C	S-Africa	5, 6, 8, 10, 11 ± 1, 12	[133,135,148]
Dymondia margaretae Compton	Asteraceae	H	S-Africa	11 ± 1	[135,149]
Echium plantagineum L.	Boraginaceae	H	Eurimedit.	8	[150]
Echium vulgare L.	Boraginaceae	H	Europ.	8	[150]
Emerus major Mill.	Fabaceae	NP	C-Europ.	10, 13	[87]
Erodium cicutarium (L.) L’Hér.	Geraniaceae	T	Subcosmop.	10	[131]
Erophila verna (L.) DC.	Brassicaceae	T	Circumbor.	5, 10	[37]
Euphorbia characias L.	Euphorbiaceae	NP	Stenomedit.	15, 20	[36]
Euphorbia cyparissias L.	Euphorbiaceae	H	C-Europ.	5, 10	[37]
Euphorbia pithyusa L.	Euphorbiaceae	C	W-Medit.	15, 20	[36]
Festuca arundinacea Schreb.	Poaceae	H	Paleotemp.	8, 16	[54]
Frankenia laevis L.	Frankeniaceae	C	Stenomedit.	11 ± 1	[135,149]
Geranium molle L.	Geraniaceae	H	Eurasiat.	10	[131]
Glaucium flavum Crantz	Papaveraceae	H	Eurimedit.	15, 20	[36]
Halimione portulacoides (L.) Aellen	Amaranthaceae	C	Circumbor.	5, 10	[134,136]
Hardenbergia violacea (Schneev.) Stearn	Fabaceae	P	Australia	19	[77]
Helianthemum Gray ‘Fire Dragon’	Cistaceae	C	-	14	[132]
Helianthemum nummularium Mill.	Cistaceae	C	Europ.-Caucas.	5, 10	[37]
Helichrysum italicum (Roth) G.Don	Asteraceae	C	S-Europ.	7.5, 10, 15, 20	[19,36,86,151]
Helichrysum italicum subsp. microphyllum (Willd.) Nyman	Asteraceae	C	Eurimedit.	15, 20	[36]
Helichrysum orientale (L.) Gaertn.	Asteraceae	C	Endem. Medit.	7.5, 8, 10, 15	[19,86,140]
Helichrysum stoechas (L.) Moench	Asteraceae	C	Stenomedit.	11 ± 1, 15, 20	[36,135]
Hemerocallis L. ‘Stella de Oro’	Asphodelaceae	G	-	14	[132]
Heuchera L. ‘Electra’	Saxifragaceae	H	-	10	[152]
Heuchera L. ‘Obsidian’	Saxifragaceae	H	-	10	[152]
Hypericum calycinum L.	Hypericaceae	C	Medit.-Mont.	14	[132]
Hypochaeris radicata L.	Asteraceae	H	Europ.-Caucas.	10	[131]
Hyssopus officinalis L.	Lamiaceae	C	Orof. Eurasiat.	19	[77]
Hyssopus officinalis subsp. aristatus (Godr.) Nyman	Lamiaceae	C	Medit.	14	[132]
Indigofera australis Willd.	Fabaceae	P	Australia	19	[77]
Iris chamaeiris Bertol.	Iridaceae	G	NW-Stenomedit.	20	[130]
Iris lutescens Lam.	Iridaceae	G	NW-Stenomedit.	5, 10, 11 ± 1	[37,135,149]
Jacobaea maritima (L.) Pelser & Meijden	Asteraceae	C	Stenomedit.	19	[77]
Lagurus ovatus L.	Poaceae	T	Eurimedit.	5, 10	[37,153]
Lampranthus spectabilis (Haw.) N.E.Br.	Aizoaceae	C	S-Africa	6, 8, 10, 12	[148]
Lavandula angustifolia Mill.	Lamiaceae	NP	Stenomedit.	6, 8, 10, 12	[148]
Lavandula dentata L.	Lamiaceae	NP	Paleosubtrop.	15	[152]
Lavandula stoechas L.	Lamiaceae	NP	Stenomedit.	15, 20	[36]
Lavandula stoechas subsp. luisieri (Rozeira) Rozeira	Lamiaceae	NP	Endem. Spain	15	[42]
Leontodon tuberosus L.	Asteraceae	H	Stenomedit.	15, 20	[36,130]
Ligustrum vulgare L.	Oleaceae	NP	Eurasiat.	10 or 13	[87]
Limonium virgatum (Willd.) Fourr.	Plumbaginaceae	C	Eurimedit.	11 ± 1, 15	[120,135,141]
Linum bienne Mill.	Linaceae	H	Eurimedit.	5, 10	[37]
Lobularia maritima (L.) Desv.	Fabaceae	C	Stenomedit.	5, 10	[37,131]
Lomandra longifolia Labill. ‘Tanika’	Asparagaceae	H	Australia	10	[143]
Lomelosia cretica (L.) Greuter & Burdet	Caprifoliaceae	C	Stenomedit.	7.5, 10, 15	[19,138]
Lotus creticus L.	Fabaceae	C	Stenomedit.	10, 11 ± 1	[135,153]
Medicago arborea L.	Fabaceae	P	NE-Medit.	6, 8, 10, 12	[148]
Melissa officinalis L.	Lamiaceae	H	Eurimedit.	8	[140]
Muscari comosum (L.) Mill.	Asparagaceae	G	Eurimedit.	10	[131]
Myoporum parvifolium R.Br.	Scrophulariaceae	C	Australia	10	[143]
Narcissus tazetta L.	Amaryllidaceae	G	Stenomedit.	20	[130]
Nepeta cataria L.	Lamiaceae	H	E-Medit.	19	[77]
Nigella damascena L.	Ranunculaceae	T	Eurimedit.	20	[130]
Olearia axillaris (DC.) Benth.	Asteraceae	T	Australia	19	[77]
Origanum dictamnus L.	Lamiaceae	H	Endem. Crete	7.5, 10, 15	[19,138]
Origanum majorana L.	Lamiaceae	H	Saharo-Sind.	7.5, 10 or 15	[19]
Origanum onites L.	Lamiaceae	C	E-Medit.	8 or 16	[54]
Origanum vulgare L.	Lamiaceae	H	Eurasiat.	19	[77]
Ornithogalum umbellatum L.	Liliaceae	G	Eurimedit.	10, 20	[130,131]
Otanthus maritimus (L.) Hoffmanns. & Link	Asteraceae	C	Medit.-Atlant.	10, 20	[36]
Paliurus spina-christi Mill.	Rhamnaceae	P	SE-Europ.	10, 13	[87]
Pallenis maritima (L.) Greuter	Asteraceae	H	W-Medit.	7.5, 10, 11 ± 1, 15	[19,135,141,154,155]
Pennisetum clandestinum Hochst. ex Chiov.	Poaceae	H	E-Africa	5	[134]
Petrorhagia prolifera (L.) P.W.Ball & Heywood	Caryophyllaceae	T	Eurimedit.	5 and 10	[37]
Petrorhagia saxifraga Link	Caryophyllaceae	H	Eurimedit.	10	[131]
Phillyrea angustifolia L.	Oleaceae	P	Stenomedit.	10, 13	[87]
Phlox douglasii Hook. ‘McDaniels Cushion’	Polemoniaceae	H	N-America	14	[132]
Pistacia lentiscus L.	Anacardiaceae	P	S-Medit.	10, 13	[87]
Plantago afra L.	Plantaginaceae	T	Stenomedit.	5, 10	[37]
Prunus mahaleb L.	Rosaceae	P	S-Europ.	10, 13	[87,126]
Pyrus pyraster (L.) Burgsd.	Rosaceae	P	Eurasiat.	10, 13	[87]
Rosmarinus officinalis L.	Lamiaceae	NP	Stenomedit.	8, 15, 19	[42,77,140]
Salvia fruticosa Mill.	Lamiaceae	C	E-Medit.	8, 10	[123,140,156]
Salvia L. hybr	Lamiaceae	C	-	10	[123,156]
Salvia officinalis L.	Lamiaceae	C	Stenomedit.	10, 13, 18	[63,87,123,156]
Salvia officinalis L. ‘Berggarten’	Lamiaceae	C	Stenomedit.	10	[152]
Salvia pomifera subsp. pomifera	Lamiaceae	C	Endem. Crete	10	[123]
Salvia ringens Sm.	Lamiaceae	C	Endem. Greece	10	[123,156]
Salvia tomentosa Mill.	Lamiaceae	C	NE-Medit.	10	[123]
Santolina chamaecyparissus L.	Asteraceae	NP	Medit.	7.5, 10, 15	[19]
Santolina rosmarinifolia L.	Asteraceae	NP	Medit.	11 ± 1	[135]
Saponaria ocymoides L.	Caryophyllaceae	H	Orof. S-Europ.	14	[132]
Satureja illyrica Host	Lamiaceae	C	S-Europ.	14	[132]
Satureja montana L.	Lamiaceae	C	W-Medit.	15, 20	[36,151]
Scabiosa columbaria L.	Dipsacaceae	H	Eurasiat.	15, 20	[36,130]
Scilla autumnalis L.	Asparagaceae	G	Eurimedit.	20	[130]
Scrophularia canina L.	Scrophulariaceae	H	Eurimedit.	15, 20	[36]
Scrophularia peregrina L.	Scrophulariaceae	T	Stenomedit.	10	[131]
Sedum acre L.	Crassulaceae	C	Europ.	4, 5, 6, 7, 10, 12	[37,128,131,157]
Sedum album L.	Crassulaceae	C	Eurimedit.	4, 5, 6, 7, 10, 12, 14	[37,128,131,132,133,157,158]
Sedum floriferum Praeger	Crassulaceae	C	Siberia	14	[132]
Sedum hispanicum L.	Crassulaceae	C	SE-Europ.	5, 14	[132,133]
Sedum L. spp.	Crassulaceae	C	-	6, 10	[129,152]
Sedum ochroleucum Chaix	Crassulaceae	C	Medit.-Mont.	5	[133]
Sedum reflexum L.	Crassulaceae	C	C-Europ.	6, 12, 14	[132,157]
Sedum rupestre L.	Crassulaceae	C	C-Europ.	15, 20	[36]
Sedum sediforme (Jacq.) Pau	Crassulaceae	C	Stenomedit.	5, 6, 7.5, 8, 10, 11, 12, 13, 15, 16, 19	[54,65,120,121,133,141,148,158,159,160]
Sedum sexangulare L.	Crassulaceae	C	C-Europ.	6, 10, 12, 14	[132,157,158]
Sedum spurium M.Bieb.	Crassulaceae	C	Europ.-Caucas.	14	[132]
Sedum spurium M.Bieb. cf. ⁶ ‘Coccineum’	Crassulaceae	C	Europ.-Caucas.	10	[158]
Sedum spurium M.Bieb. cf. ⁶ ‘Summer Glory’	Crassulaceae	C	Europ.-Caucas.	10	[158]
Sempervivum L. ‘Reinhard’	Crassulaceae	C	-	10	[152]
Sesuvium verrucosum Raf.	Aizoaceae	C	America	5	[134]
Sideritis athoa Papan. & Kokkini	Lamiaceae	C	Macarones.	7.5, 10, 15	[19,138]
Sideritis hyssopifolia L.	Lamiaceae	C	NW-Medit.	5, 10	[37]
Silene conica L.	Caryophyllaceae	T	Paleotemp.	5, 10	[37]
Silene gallica L.	Caryophyllaceae	T	Eurimedit.	10	[131]
Silene vulgaris (Moench) Garcke	Caryophyllaceae	H	Paleotemp.	5, 10	[153]
Spartium junceum L.	Fabaceae	P	Eurimedit.	10, 13	[87]
Sporobolus pungens Kunth	Poaceae	G	Subtrop.	15	[120,141]
Stachys byzantina K.Koch	Lamiaceae	H	E-Asia	10, 14	[132,152]
Sternbergia lutea (L.) Ker Gawl. ex Spreng.	Amaryllidaceae	G	Medit.-Mont.	20	[130]
Teucrium chamaedrys L.	Lamiaceae	C	Eurimedit.	14	[132]
Teucrium fruticans L.	Lamiaceae	NP	Stenomedit.	19	[77]
Thymus caespititius Brot.	Lamiaceae	C	Iberian Peninsula	15	[151]
Thymus marschallianus Willd.	Lamiaceae	C	Eurasiat.	14	[132]
Thymus pseudolanuginosus Ronniger	Lamiaceae	C	S-Europ.	15	[151]
Thymus serpyllum L.	Lamiaceae	H	Eurasiat.	11 ± 1	[135]
Trifolium arvense L.	Fabaceae	T	Paleotemp.	10	[131]
Trifolium campestre Schreb.	Fabaceae	T	Paleotemp.	10	[131]
Tuberaria guttata (L.) Fourr.	Cistaceae	T	Eurimedit.	20	[130]
Verbascum blattaria L.	Scrophulariaceae	H	Cosmopol.	10	[131]
Verbascum thapsus L.	Scrophulariaceae	H	Europ.-Caucas.	15, 20	[36]
Veronica prostrata L.	Plantaginaceae	H	Eurasiat.	14	[132]
Zoysia matrella (L.) Merr.	Poaceae	H	E-Asia	7.5, 15	[161]

¹ Researchers considered that substrate layer thickness of extensive green roof was from 5 to 20 cm [158]; ² the names of the species have been corrected based on the indications of “World flora online” [162]; researchers preferred to keep the names indicated by the authors of the papers, except when multiple synonyms of the same species were used; in this case, researchers have only reported the correct botanical name; ³ according to “World flora online” [162]; ⁴ according to The Life Forms of Plants of Raunkiaer [163]; H = Hemicryptophyte, NP = Nanophanerophyte, G = Geophyte, T = Therophyte, C = Chamaephyte, P = Phanerophyte; ⁵ references are mainly related to Pignatti et al. [164]; ⁶ “cf.” means that, according to the authors, the cultivar cannot be affirmed with certainty but the phenotype is compatible.

This entry is adapted from the peer-reviewed paper 10.3390/plants12233985

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