Within the framework of the European research project Interreg NWE GROOF (Greenhouse to Reduce CO2 on RooFs), experiments were carried out by ASTREDHOR, one of GROOF’s partners, on four cropping systems in order to determine their technical and economic relevance in an urban context.
Presentation of the project
Between March and October 2019, four species were tested (peppers, chard, lettuce, basil) on four growing systems within the ASTREDHOR Est-Horticole station. The four growing systems experimented consist of a hydroponic system (Goponic), an aeroponic system (GHE Aeroflo), a system with substrate and drip irrigation (JB Hydroponics) and a vertical system (Tootem). Note that, in parallel, another partner of the GROOF project, Les Jardins de Gally, is carrying out similar experiments, that are not illustrated here.
The aims of these experiments are threefold
First of all, helping to choose the best growing system for the four GROOF’s pilots of greenhouses planned to be built by the end of 2020 and located in France, Luxembourg, Belgium and Germany.
Secondly, guiding and offering robust results to the 10 project leaders of rooftop greenhouses selected during the open call for a one-year coaching by GROOF’s partners.
Thirdly, offering efficient agronomic solutions on rooftop cropping systems to the entire urban farming community.
The results of these experiments focus primarily on the agronomic quality of the production systems (in particular the yields and commercial qualities of the production). Thus, for lettuces (Lucrecia RZ variety), the GHE Aeroflo system with 3.6 kg / m² of marketable production for the growing season is the most suitable. For basil, pepper and Swiss chard, the JB Hydroponic system offers the best yields (3.5 kg / m², 16.3 kg / m² and 9.2 kg / m² respectively). Despite the good agronomic results of the JB Hydroponic system (with biochar substrate and drip irrigation), the total weight of the system remains significant (approx. 110 kg / m² saturated with water) compared to the others (approx. 50 kg / m²) which can cause problems for certain roofs with limited bearing capacity. Especially since after a growing season, additional materials are required with this system in order to compensate soil compaction, which is not necessary with hydro / aeroponic systems. The study also details the advantages and disadvantages, from installation to maintenance, of these systems (radar representation to help farmers to decide), and proposes an economic approach through maintenance costs (energy consumption, water, fertilizers) as well as a spatial approach by considering different installation scenarios in a standard greenhouse of 400 m².
In the continuation of the tests carried out within the framework of the French research program CasDar RT Techn'AU about urban materials from recycling for the creation of substrates suitable for urban agriculture, the results of these tests on production systems adapted to rooftop greenhouses production will help future agri-urban operators to select the best production tool suited to their urban context. However, knowing that a non-negligible part of building greenhouses projects has a social and educational scope, future research programs, focusing on cultivation systems that are easy to implement and maintain, would also have an interest in being developed.  More info (in French) : https://www.astredhor.fr/data/info/148424-CR601.pdf
Photo 1: Lettuces are a good product for urban farming production: easy and fast to grow and ready to be sell fresh. Systems such as GHE Aeroflo are perfectly adapted for these plants in a rooftop greenhouse context. Crédit : Solène Batard – ASTREDHOR Est-Horticole.
Photo 2 : Substrate and drip irrigation growing system, such as the JB Hydroponic system, provides good agronomical results for peppers, basil and Swiss Chard. Crédit : Solène Batard – ASTREDHOR Est-Horticole.