As residential and commercial areas expand in southwest Florida, green spaces are decreasing in urban environments. Rooftop gardens provide improved storm water management, reduce heat-island effects, reduce electricity consumption, expand wildlife habitat and increase biodiversity. However, there is little information on appropriate soil composition for green roofs in southwest Florida or native species performance in such novel ecosystems. The objectives of this research are to determine:
- whether the addition of expanded shale to the soil significantly changes soil moisture or temperature.
- whether the addition of expanded shale to the soil increases plant growth.
- performance differences in five native species under different soil substrates and radiation regimes.
Nine green roof boxes, located at the Naples Botanical Garden, were each planted with 5 individuals each, of 5 plant species, (Ipomea imperati, Spartina patens, Liatris spicata, Schizachynum scoparium and Sesuvium portulacastrium). Three replicate boxes were constructed of three soil compositions. Plants were grown for 9 months and plant cover, final biomass, rainfall, and soil and air temperature were monitored. At the conclusion of the experiment, leaf-level gas exchange of I. imperati was also measured. Additionally, the performance of I imperatihas been monitoried on a recently completed rooftop garden in Naples, FL.
Results/Conclusions
Plant productivity was reduced with increasing shale content even though soil moisture content was higher (p<0.05). This reduction is likely due to stress induced by higher soil temperatures (often >10˚C higher than the no shale treatment) (p<0.05). During the wet season, our data indicate the dark colored shale increases soil temperatures midday resulting in apparent heat stress in the plants and reducing aboveground biomass production. At the onset of the dry season, the shale’s moisture holding capacity proved essential when air temperatures were lower and there was a lack of rain.
Green roofs need to establish rapidly to hold the soil in place; in this study, Liatris spicata does not appear to be an appropriate green roof species due to its slow growth in this environment. In the established residential green roof, we found that differences in wall color on the roof created different thermal regimes moving from east to west across the rooftop impacting plant performance. These differences also changed seasonally. Future studies must consider southwest Florida wet/dry season dynamics in microclimate, and trade-offs in substrate capacities including: water holding capacity and temperature impacts.