Evaluation of Growth and Green Coverage of Ten Ornamental Species for Planting as Urban Rooftop Greening

Abstract

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.

The Effect of Irrigation Treatment on the Growth of Lavender Species in an Extensive Green Roof System

“…Low stomatal conductance values in all lavender species also suggest the presence of a drought defense strategy. Our results agree with the findings of Sendo et al [59] that on hot summer days, in an extensive green roof system with plants not being subjected to water stress, the drought-tolerant species Fragaria × ananassa, Thymus serphyllum, Evolvulus pilosus, Ophiopogon japonicus, Vinca major and Hedera helix had significantly lower stomatal conductance than the nondrought tolerant species Pelargonium × hortorum, Verbena × hybrida and Petunia × hybrida. In northeast Italy (Trieste), Salvia officinalis grown in 14 cm deep substrate of an extensive green roof system that received natural precipitation and irrigated only during prolonged drought periods showed a stomatal conductance value of 15.1 mmol m −2 s −1 and 83% desiccation of the shoots in August that recovered by 40% after autumn rains [60].…”

An Experimental Study in Simulated Greenroof in Mediterranean Climate

“…Thymus serpyllum, when irrigated, reached high coverage values of about 50%. This growth performance has been confirmed by two separate studies: Provenzano et al (2010), in a Mediterranean environment, on an 8 cm deep substrate, reported coverage of 85% some months after winter transplanting; in Japan a trial was conducted on a 10 cm deep substrate, receiving 5 litres of water every two days, the coverage increased by more than 70% between May and September, reaching a final value of 90% (Takahiro et al, 2010). In this trial, rain fed plants reached 4% coverage.…”

Mediterranean Green Roof Simulation in Caldes de Montbui (Barcelona): Thermal and Hydrological Performance Test of Frankenia laevis L., Dymondia margaretae Compton and Iris lutescens Lam

“…It is important consider that a combination of different species is more effective than monocultures in favouring the thermal and hydrological properties of a green roof [20,21]. For this reason, many studies have attempted to understand how certain species influence the specific aims of a green roof, selecting and testing several species, different respect to Sedum genus, and in certain environments [4,[22][23][24][25][26]. Several studies, instead, have focused the attention on the evaporation and photosynthesis activity of the species in order to define the thermal properties and energy performance of the green roof [27][28][29].…”