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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 (Pum₆₅:P₃₀:Z₅); 2) pumice mixed with compost (C) and zeolite in a volumetric proportion of 65:30:5 (Pum₆₅:C₃₀:Z₅); and 3) sandy loam soil (S) mixed with perlite (Per) and zeolite in a volumetric proportion of 30:65:5 (S₃₀:Per₆₅:Z₅). 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 S₃₀:Per₆₅:Z₅ 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 Pum₆₅:C₃₀:Z₅ was larger but not in the summer when the growth index of substrates S₃₀:Per₆₅:Z₅ and Pum₆₅:P₃₀:Z₅ 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 Pum₆₅:C₃₀:Z₅ 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.

The water–air balance of four soilless substrates: 75% sphagnum peat–25% perlite (Ps₇₅:P₂₅), 50% sphagnum peat–50% perlite (Ps₅₀:P₅₀), 50% coir–50% perlite (C₅₀:P₅₀), and a fortified substrate with 60% sphagnum peat–30% black peat–10% perlite (Ps₆₀:Pb₃₀:P₁₀) (in a volumetric proportion) was investigated under two different irrigation methods (drip and sub-irrigation), and its effect on the growth of Begonia ×elatior ‘The President’ was studied. The bulk density, particle size distribution, and water retention curve of the substrates were determined. Furthermore, the water profiles, oxygen (Ο₂) concentration, and Ο₂ diffusion rate of all substrates were determined during a 16-week cultivation period. Plant height, flower production, and both shoot and root dry weights as well as percent growth increase of plants were measured at the end of the experiment. The substrate water profiles showed that the water content was greater and air content was less in substrates of pots irrigated with drip irrigation than with sub-irrigation. The O₂ concentration in all substrates irrespective of the irrigation method was high. The O₂ diffusion rate values of sub-irrigated substrates were greater than those drip-irrigated, and Ps₆₀:Pb₃₀:P₁₀ showed the greatest values. Shoot and root dry weights and percent growth increase of drip-irrigated plants were greater than that of sub-irrigated plants.