Optimizing Green Roof Substrates with Biochar and Frass for Improved Plant Performance

The urban heat island (UHI) effect, driven by rapid urbanization, can be mitigated through green roof systems, which offer thermal benefits such as reduced surface temperature and increased thermal insulation. However, their effectiveness is limited by nutrient-poor substrates, low water holding capacity and weed intrusion. This study evaluated the potential of palm kernel shell (PKS) biochar and black soldier fly larvae (BSFL) frass as sustainable additives to improve substrate quality and plant performance. Three combinations of biochar and frass (20%, 25%, and 30%) were incorporated into a soilless green roof medium and compared with a commercial substrate and a standard control. Arachis pintoi (pinto peanut) was selected as the model plant species. Growth performance was assessed via shoot number, chlorophyll content, percentage coverage, survival rate, and substrate characteristics (pH, moisture, and temperature). Substrates with 30% biochar and 20% frass, and 25% biochar and 25% frass, significantly improved plant growth and coverage compared to commercial substrate. Nutrient analysis showed the former enhanced micronutrient availability, while the latter increased macronutrient content. Although commercial substrate exhibited superior overall nutrient levels, the identified combinations present viable, low costs alternatifs for enhancing green roof vegetation. Future studies should extend the experimental duration to confirm long-term effectiveness under variable environmental conditions.