Extensive green roofs are a promising technology for reintroducing lost flora in degraded urban environments, but further research is needed for their application in semiarid regions. Therefore, research was undertaken to determine the effects of substrate type and depth and the amount of irrigation during a drought period on the establishment, growth, and physiology of the native species Dianthus fruticosus sub. fruticosus . Treatments included two substrate types [a soilless substrate (Pum ₅₀ :Per ₂₀ :C ₂₀ :Z ₁₀ ) or a substrate with soil (S ₁₅ :Pum ₄₀ :Per ₂₀ :C ₂₀ :Z ₅ ), in which Pum = pumice; Per = perlite; C = compost; Z = clinoptinolite zeolite; and S = sandy loam soil, mixed in a volumetric proportion indicated by their subscripts], two substrate depths (7.5 cm or 15.0 cm), and two irrigation regimens during drought [15% or 30% of pan evaporation (E _pan )]. Measurements included substrate characteristics such as particle size distribution, dry and saturated bulk density, water characteristic curves, and in situ determination of substrate moisture during drought stress. Plant growth was determined based on biometric measurements such as growth index (GI) and dry weight and physiological indicators such as SPAD, chlorophyll _a+b , and carotenoid contents. It was found that substrate moisture during drought was increased in the soil substrate compared with the soilless substrate as a result of its better water retention capacity in low tensions. Dianthus fruticosus sub. fruticosus growth was promoted by the deep substrate (15 cm) throughout the entire study, whereas substrate type and irrigation during the drought period did not have an effect. Similarly, leaf dry weight was increased in the deeper substrates, whereas shoot and root dry weights were similar in all treatments. SPAD was found to be a more sensitive method than chlorophyll and carotenoid analysis and revealed an interesting sequence of treatment influences on D. fruticosus sub. fruticosus physiology that depended on the climatic conditions and stress imposition. More specifically, during establishment, both substrate type and depth affected growth with the soil substrate and deep profiles yielding higher SPAD measurements. Soon after the initiation of drought, the deep profiles had higher SPAD values than the shallow ones, whereas in high-irrigation regimens and, to a less extent, deeper profiles provided increased SPAD values after the middle of drought imposition. Chlorophyll and carotenoid levels reduced during the drought stress period, but very limited differences were detected between treatments. It was concluded that D. fruticosus sub. fruticosus is a very promising native plant for use on extensive green roofs in the Mediterranean region, and its growth was better in a substrate depth of 15 cm. However, its growth was sufficient even with a 7.5-cm substrate depth and irrigation of 15% E _pan .

Green roofs have received increased interest as a result of their environmental, social, and economic benefits. The present study aims to evaluate suitable, lightweight substrates for the installation of semi-intensive-type green roofs and their effects on Lavandula angustifolia growth and physiological status under Mediterranean climatic conditions. The study was conducted in field containers (1.2 × 1.2 m), and treatments included the use of two depths (20 cm or 30 cm) and three different substrates: 1) pumice (Pum) mixed with peat (P) and zeolite (Z) in a volumetric proportion of 65:30:5 (Pum ₆₅ :P ₃₀ :Z ₅ ); 2) pumice mixed with compost (C) and zeolite in a volumetric proportion of 65:30:5 (Pum ₆₅ :C ₃₀ :Z ₅ ); and 3) sandy loam soil (S) mixed with perlite (Per) and zeolite in a volumetric proportion of 30:65:5 (S ₃₀ :Per ₆₅ :Z ₅ ). Each experimental plot was planted with four plants of L. angustifolia . The physical and chemical characteristics of the substrates and the in situ substrate moisture levels were determined. Plant growth was determined by the growth index and root dry weight, whereas the plant physiological status during the stressful summer period was determined by chlorophyll _a+b content and stomatal resistance. It was found that the bulk density of all of the substrates was less than 0.8 g·cm ⁻³ , whereas the water retention of substrate S ₃₀ :Per ₆₅ :Z ₅ was greater than the other two substrates at all tensions. Substrate depth was the most influential factor; therefore, the deeper substrates (30 cm) provided higher growth indices and root dry weight, increased total chlorophyll content, and reduced leaf stomatal resistance compared with the shallower substrate depth of 20 cm. Differences between substrate types were smaller compared with those of substrate depth. In spring, the growth index in substrate Pum ₆₅ :C ₃₀ :Z ₅ was larger but not in the summer when the growth index of substrates S ₃₀ :Per ₆₅ :Z ₅ and Pum ₆₅ :P ₃₀ :Z ₅ exceeded that of the substrate amended with compost until the end of the study. In late summer, the plant total chlorophyll content decreased in all substrates, whereas substrate Pum ₆₅ :C ₃₀ :Z ₅ exhibited increased stomatal resistance compared with the other two substrates at the same period. It was concluded that L. angustifolia could be grown successfully in semi-intensive Mediterranean green roofs at a substrate depth of 20 cm, but its growth and tolerance to the harsh summer conditions would improve at a substrate depth of 30 cm.