Rooftop gardening or green roof establishment, one of the afforestation methods for urban greening, has many benefits for the environment, economy, and urban landscape. Various environmental stresses including heat, strong wind, sunshine and drought prevent most plants from growing well with extensive green roof systems. For the establishment of urban rooftop gardening in summer, we evaluated ten ornamental plant species (Evolvulus pilosus, Fragaria × ananassa, Hedera helix, Lampranthus spectabilis, Ophiopogon japonicus, Pelargonium × hortorum, Petunia × hybrida, Thymus serphyllum, Verbena × hybrida, Vinca major) to use for greening with shallow soil (10 cm depth) on the flat rooftop of a four-story building. We measured the area increase in green coverage as a total growth rate, photosynthetic ability of detached leaves, stomatal conductance of an attached leaf, soil surface and canopy air temperature. Thymus, Evolvulus, Petunia, and Fragaria are excellent plant species for rooftop gardening judging from their speedy green coverage with high growth rates (90% in Thymus, 65% in Evolvulus, 60% in both Petunia and Fragaria at maximum for about three months in summer). Thymus and Fragaria showed high performance of water saving with relatively low stomatal conductance under semi-dry conditions and did not need to be replanted every year unlike Evolvulus and Petunia. Evolvulus had the highest photosynthetic activity at 40°C. Petunia and Verbena did not grow continuously but maintained active blooming without increasing their green coverage (about 60%) throughout midsummer. Lampranthus tolerated extremely hot and dry conditions on the rooftop but grew very slowly. As Vinca and Pelargonium died back due to both heat stress and strong wind, we suggested these were not suitable for rooftop gardening. All vegetations showed a 6-8°C lower temperature of soil surface than that of the concrete rooftop during summer nights. We concluded that rooftop greening has a significant effect on cooling a building and saving energy for air-conditioning in summer.
