Growing media affects edible plant production and leachate on a simulated rooftop farm

Background/Question/Methods

Green roofs, rooftops that have growing media and plants on them, provide a number of benefits including increased biodiversity, decreased building energy use, storm water mitigation, increased rooftop lifespan, and mitigation of the urban heat island.  Food production may also be a potential benefit of green roofs. In recent years, urban agriculture, farming in small plots in and around urbanized areas, has become increasingly popular, yet space for urban farms remains a barrier and roofs have been proposed as a possible space. Although there is a growing body of literature focused on ‘traditional’ green roofs planted with Sedums and other succulents, little peer-reviewed literature exists investigating agricultural rooftops. Growing edible crops in the harsh rooftop environment, and in nutrient poor growing media presents novel challenges. We used a two-way factorial design to evaluate production potential and storm water retention properties of four different plant species Amaranthus dubius, A. tricolor, A. cruentus, and Portulaca oleracea and three different substrate types: potting soil, extensive growing media, and GaiaSoilÔ (a lightweight green roof growing media). Custom lysimeters were constructed and installed on a rooftop at Fordham University in Bronx, NY.  Each plant species were grown in pairs in separate lysimeters for 6 weeks. 

Results/Conclusions

Unsurprisingly, the most nutrient rich potting soil showed the greatest amount of biomass accumulation across all plant species, and leached 8x more total phosphorous than extensive growing media; however, potting soil did not leech a greater amount of total dissolved nitrogen than extensive growing media. Of the four species evaluated, P. oleraceea produced the greatest amount of edible biomass across growing media types, while A. curentus produced the greatest edible biomass of Amaranthus species in extensive growing media. Our study demonstrates that across growing media types, having plants increases the quantity of water retained compared to unplanted plots, but specific plant species had no affect.  The presence of plants in growing media decreases leeching of total dissolved nitrogen and total phosphorous from potting soil and extensive media while having no affect on GaiaSoil.  These results stress the importance of growing media selection in green roof design and the trade-off between production and mitigating storm water quality.  We have also shown that plant species seems to play less important role in storm water management than growing media selection, although testing more plant species is recommended.