The 6 essentials of urban agriculture on rooftops

Faced with the densification of cities, which will accommodate nearly 66% of the world's population by 2050, rooftops represent an opportunity to reclaim under-exploited areas. Roofs constitute up to 32% of the horizontal surface of a city and can be converted into spaces for renaturation and creation of services such as food production, rainwater retention, carbon storage and recycling waste.

1. Varied production systems

The particular feature of agricultural production systems installed on terraced roofs is their location “above ground”, that is to say in an artificial environment. These arrangements can take various forms.

Growing trays are a system where the substrate is contained in various supports such as wooden boxes, geotextile or “smart pot” bags. The advantage of this technique is its rapid implementation, its modularity and its relative reversibility.

Crop beds concern a system similar to conventional green roofs, but allowing agricultural production. Their use can be compared to growing in the ground, but placed on a roof. It is a system that optimises the growing space while facilitating maintenance.

Hydroponics, aeroponics and bioponics are high-tech systems: all parameters (water oxygen, pH, temperature, etc.) are controlled to optimise plant growth through centralised computer management. Hydroponics represents a closed loop cultivation system where the roots of the plants are constantly in contact with a water circuit enriched in nutrients. In the case of aeroponics, the roots of plants are bathed in a permanent mist of nutrient solution. When we talk about bioponics, we mean that the nutrients inserted into the circuit are organic and biological.

Agricultural production systems installed on rooftops can be based on different techniques. Growing trays correspond to a system where the substrate is contained in various supports such as wooden boxes or geotextile bags. Crop beds can be compared to growing in full ground, but set out on a roof. Hydroponics, aeroponics and bioponics are high-tech systems: water oxygen, pH, temperature and other parameters are controlled to optimise plant growth through centralised computer management.

2. The four project families

Urban rooftop agriculture is extremely multifaceted, not only in terms of the growing techniques used, but also in terms of project forms.

Urban micro-farms are characterised by their multifunctionality: they rely on agricultural production to develop other initiatives (special activities, workshops, events, etc.) around themes linked to food, the environment and the rediscovery of nature. The diversification of activities allows them to multiply sources of income.

Shared gardens, a historical form of urban agriculture, are spaces maintained by a group of individuals who collectively produce food and other plants. Among these collective rooftop gardens, there are several forms: gardens shared by the inhabitants of a building or a neighbourhood, corporate vegetable gardens, educational vegetable gardens, therapeutic gardens.

Productive urban farms are companies or startups where the majority of income comes from the sale of the produce. To ensure the viability of their activity, they often offer services (training, visits, activities, etc.). The sale can be made in the form of baskets distributed through short supply chains, in organic outlets or grocery stores, on sites or in restaurants.

Restaurants with vegetable gardens follow the “farm-to-table model”, whereby fruits and vegetables grow on the rooftop of a restaurant, hotel, hospital or business where a cook, restaurateur or other manager uses the produce to supply the restaurant, canteen or bar downstairs. These arrangements make it possible to bring added value to cooked meals thanks to quality products, while enhancing the company’s image.

3. A particular microclimate

In the heart of a dense city, the temperature can be up to 3°C higher than on its outskirts. On a roof the temperature is even higher because of the increased exposure to the sun and the characteristics of the materials used in the roof. However, some roofs are in the shade of nearby taller buildings. It is therefore important to be familiar with the exposure of the building as well as the areas of shade before setting up a project involving crops.

Roofs are exposed to stronger winds than on the ground which can have an impact on crops and the substrate: dispersal, drying out, crops being swept away because their roots are only loosely anchored in the substrate… Likewise, for farmers and the visiting public, it is not always pleasant to be exposed to gusts of wind. It therefore seems useful to provide for improvements or equipment during the design of the project (on a new structure as on an existing structure) in order to reduce wind pressure.

4. Specific crops for our roofs

The urban heat island effect in the city makes it possible to consider growing exotic plants such as okra or acmella oleracea. On rooftops, the growing system is the main factor that determines which plants can be grown.

Hydroponic systems generally favour crops that are simple to grow and that sprout fairly quickly: leafy vegetables (salads, spinach, chard, etc.) and fruit vegetables (tomatoes, peppers, courgettes, etc.). However, the successive improvements of the systems and feedbacks show that it is possible to grow an increasing diversity of vegetable crops.  

For boxes and growing trays, the greater the height of the substrate, the greater the diversity of growable vegetable crops. On the other hand, the depth of the substrate makes it easier to withstand temperature variations and the high heat present on the roof.

Is pollution characteristic of these spaces?

The question that often comes to the fore is that of pollutants. Is produce grown on urban roofs bad for our health? To answer this question, we need to look at the three sources of pollution that exist in the city: water, air and growing substrates.

Most urban agriculture projects are supplied from the city’s drinking water network; which eliminates any possible risk of water pollution. In the case of water supply by rainwater recovery or filtration of a wastewater network, it must be subject to regular checks.

Air pollution levels depend on the distance from the emitting source, major roads or industries, and the height of the building. The first campaigns to analyse the presence of contaminants in fruits and vegetables from cultivated roofs and, in particular, that carried out on the rooftop of AgroParisTech on the 5th floor of the building in the 15th arrondissement of Paris, highlighted values below the regulations in force. For a site near sources of pollutants, it will still be necessary to carry out regular monitoring and perhaps adapt the type of plants grown.

6. Rooftop breeding

Animals in cities offer a number of functions that plants, grown in urban agriculture, cannot provide, such as the ecological maintenance of urban spaces or the production of natural organic fertilisers. However, although it may appear relevant, the introduction of animals into cities poses many more regulatory problems than do crops. Their production is subject to product sanitary regulations, as well as animal welfare and health standards.

L’Urban beekeeping activities continue to be set up on roofs, whether on that of the Paris Opera, the Galeries Lafayettes department store or the headquarters of major companies or in private homes. With the Labbé law and the “0 phyto” measure in municipalities, life seems quieter for bees in the city than in the countryside with food all year round thanks to the differentiated management of green spaces. But once again, the multiplication of hives must not compromise this positive state. We are beginning to see feeding problems as well as health problems for these urban bees, especially in Paris.

L’ Aquaponics represents the assembly of aquaculture and plant culture, all in a closed loop system. It is interesting today, because it makes it possible to reduce the expenses related to water by 80-90% compared to a traditional holding and requires little or no mineral inputs essential to the plants. Fish droppings become a source of food and a natural fertiliser for plants. Water saving, effluent management: aquaponics represents an ecological model of production. For this technology to participate significantly in the production of food, there remain many challenges, both technical and socio-economic.

We are increasingly seeing henhouses spring up in urban areas which are sometimes part of programmes to combat food waste in certain municipalities. Although it is still unusual to encounter chickens on a roof, valuing these unused spaces in small rearing areas in the heart of the city can be an opportunity to relocate animal products close to home.

To find out more: Fanny Provent and Paola Mugnier, Agriculture urbaine : comment aménager une toiture-terrasse, the lab recherche environnement VINCI ParisTech, Eyrolles editions.

The co-authors will present this first practical guide from the lab recherche environnement (ed. Eyrolles) on 21 September 2020 on the occasion of the Lab Evening “Urban and vertical farmers”

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