From the production of food products to the regulation of water flows, urban agriculture provides a number of services. Scientists from the French Institute of Agronomic Research and the AgroParisTech university have shown that rooftop gardens represent an interesting tool for urban waste management, for producing food products and for retaining rainwater runoff. The results of a study were published Dec. 19 in the journal Agronomy for Sustainable Development.

In cities, several challenges are present for making urban areas more sustainable, including food supply, sensitivity to heat waves, waste management and the risk of flooding linked to the impermeability of soils. These problems are a call to rethink the idea of cities and urban agriculture as a possible solution among several others to these challenges, as it is possible to use this as a tool to encourage ecological functioning and generate ecosystem services. Despite financial constraints and risks of urban soil pollution, deploying initiatives like rooftop gardens appear to be an important potential tool in the development of urban farming and nature in cities.

The scientists in the study demonstrated that protection levels in rooftop gardens are comparable, or even better than those of traditional family vegetable gardens, and near to those of professional market gardens in organic agriculture in the region around Paris. The yields are very similar among the different types tested, Technnosols [soils whose componants come from human action] composed of composted urban green waste and ground wood proving to be as productive as commercial potting soil, if not even more so.

On the South Bank during the 60th anniversary of the Festival of Britain.

The three types of Technosols tested (two composed of composted urban green waste and ground wood with and without earthworms, one composed of potting soil) retain three fourths of the incidental rainwater, contrary to bare roofs. Regardless of the type of Technosol used, the amount of heavy metals present in the vegetables is significantly lower than the required norms for acceptability.

Finally, the organic waste used in the rooftop gardens decomposes and gradually releases its beneficial components. In taking into account all of the inputs (rainwater and additional watering) and outputs (drainage), the researchers found that the Technosols retain more nitrate than they give off.

These results show that low-tech rooftop gardens, which are easy to maintain, allow for a recycling of organic waste and produce vegetables without having to resort to chemical fertilizers all while capturing rainwater runoff. Integrating these productive rooftops represents a real opportunity to develop cities of a high level of service to the ecosystem, even if it may still be necessary to do more research on the precise type of soils used.