Green Roof Substrate Type and Depth Affect the Growth of the Native Species Dianthus fruticosus Under Reduced Irrigation Regimens

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.