Effect of Hydrogels in Three Substrates on Growth and Ornamental Quality of Apple Mint (Mentha suaveolens) in Unirrigated Green Roofs

Green roofs have positive effects on urban settings, leading to improvements in aesthetics and energy consumption of buildings. However, green roofs are extremely difficult environments for plant growth and survival because water availability fluctuates dramatically and is often limited between dry periods and rain events. We analyzed the effects of different concentrations of a water-retention additive, hydrogel, in green-roof substrate mixtures on the growth and ornamental quality of Mentha suaveolens. Coir and perlite were mixed in the ratio of 80:20 (coconut coir dust to perlite, v/v; referred to as C4 P1 ), 50:50 (C1 P1 ), or 20:80 (C1 P4 ) at a substrate depth of 20 cm. Hydrogel (hydrophilic polymer: medium, w/v; dry weight basis) was added to the substrate mixtures at 0 (control), 0.25, 0.5, 1.0, or 2.0 kgm-3. All plants were watered every two days in the first week until they were well established, and then were not irrigated. The substrate with high coir substrate content increased the growth of Mentha suaveolens under drought conditions, but decreased the ornamental quality in rainy conditions. This study revealed that substrate C4 P1 , which has high concentration of coir, is not recommended for Mentha suaveolens in the rainy season. The addition of hydrogel increased the growth and ornamental quality of Mentha suaveolens in substrates C1 P1 and C1 P4 under drought condition. Moreover, substrate C1 P4 with 1.0 kg m-3 added hydrogel was optimal for Mentha suaveolens growth on green roofs, regardless of drought and rainy conditions.