Influence of Substrates and Hydrogels on Spearmint (Mentha spicata) Growth and Flowering in a Rooftop Garden

Abstract

Agricultural hydrogels improve water retention in a variety of substrates. However, little is known about their impact on herb cultivation in rooftop farming. To identify the optimal substrate and hydrogel concentration for the growth and flowering of spearmint (Mentha spicata) in rooftop gardens, coir and perlite were mixed in three different ratios of 80% to 20% (v/v; referred to as C4P1), 50% to 50% (C1P1), or 20% to 80% (C1P4). Hydrogels were added into different substrates at different concentrations including 0 (control), 0.25, 0.5, 1.0, or 2.0 kg⋅m–3. Substrate composition significantly affected the growth (P < 0.001) and flowering (P < 0.05) properties of spearmint. The addition of hydrogels into substrates resulted in a significant (P < 0.05) increase in growth parameters for spearmint during the dry season. However, plants grown in C4P1, which has the highest ratio of coir, displayed inferior growth and flowering compared with those of the other two substrates during the rainy season. Therefore, a perlite-based substrate, such as C1P4 with added hydrogel, provides a suitable environment for the cultivation of spearmint in rooftop gardens regardless of seasonal rainfall patterns.