Adaptive Response of Petunia × hybrida Plants to Water-Scarce Urban Substrates

The use of hydrogel and biostimulants holds great potential for plants’ adaptation to stressful urban conditions, increasing their tolerance to drought stress. In this study, we investigated the plant performance and anatomical response of Petunia × hybrida hort. ex E. Wilm., cultivated under different substrate volumes and compositions, hydrogel amendments, and biostimulant treatments, as well as their interactions under drought stress. Namely, the plants were planted in pots with a substrate depth of 7 cm and 10 cm and cultivated under different combinations of organic (peat) and inorganic (perlite) substrates. Moreover, half of the plants were subjected to hydrogel and biostimulant treatments. Different watering intervals (24–96 h) were applied in combination with exposing the plants to direct sunlight for 8–10 h. The results showed that a larger substrate depth, along with hydrogel and biostimulant amendments in a mixture of perlite and peat, helps plants adapt to dry conditions when grown in shallow substrates, providing optimal water availability and thus contributing to the physiological adaptation of plants to water deficit. The study clearly demonstrates that substrate selection and irrigation frequency must be jointly optimized to ensure resilient urban greening. Hydrogels stand out as essential amendments, enabling significant water savings by extending irrigation intervals without compromising vascular growth or drought resilience. These water-efficient substrate strategies are vital for sustainable urban vegetation management, especially as cities face increasing environmental pressures and the imperative of climate adaptation, thereby supporting multiple Sustainable Development Goals.