Freshwater scarcity and saline groundwater are major constraints for maintaining green roofs in coastal areas. This study evaluated the response of Jacobaea maritima subsp. sicula, (Sicilian silver ragwort) a drought-tolerant coastal ornamental plant, to tap water and seawater irrigation under Mediterranean summer conditions. Plants were grown in 10 cm-deep green-roof modules and subjected to six irrigation regimes: tap water, seawater, or alternating tap water and seawater, each applied at 4- or 8-day intervals, with irrigation volumes equal to 60% of cumulative reference evapotranspiration (ETo). Growth, relative water content (RWC), chlorophyll index (SPAD), and leachate electrical conductivity were monitored to assess plant performance and salinity responses. Seawater irrigation caused rapid substrate salinization, leaf dehydration, and plant death within one month, while alternating seawater with tap water also failed to sustain survival. In contrast, tap water-irrigated plants maintained high RWC, chlorophyll content, and stable visual quality throughout the experimental period, even with deficit irrigation at 60% ETo every eight days. These findings demonstrate that J. maritima subsp. sicula is well suited for freshwater-irrigated extensive green roofs in semi-arid regions, providing reliable performance under infrequent irrigation and limited water supply. However, seawater or high-salinity irrigation should be avoided. Future research should explore mixed freshwater-seawater irrigation regimes with a higher freshwater proportion, aiming to reduce total freshwater consumption while sustaining plant survival and esthetic performance.