Paris is actively committed to creating urban forests to combat urban heat islands, improve air quality, and promote biodiversity. To this end, the municipality has adopted the definition of “urban forest” coined by renowned landscape architect Gilles Clément, namely a certain number of trees with undergrowth plants such as shrubs, perennials, and ferns. These urban forests are most often created in squares where hardscaping is prominent, carefully selected after an in-depth study of the subsoil. However, the French capital is limited in its ability to create green spaces because the city's underground is very congested.
Main initiatives underway
Inaugurated in June 2024, Paris's first urban forest was created on Place de Catalogne near Montparnasse station in the 14th arrondissement. The city of Paris has transformed a former roundabout with very little vegetation into a green space. The new green square now has 470 trees and 4,000 m² of wooded area, including an 860 m² clearing. The cooling effect of the vegetation has reduced the ambient temperature by 4°C.
Inaugurated in September 2024, the city of Paris' second project, called “Bois de Charonne” in the 20th arrondissement, covers 3.5 hectares along the Petite Ceinture. It includes more than 7,500 trees planted on the former railway line and a small pond promoting biodiversity.
Finally, the town hall square is currently undergoing a major transformation: 49 large trees are being planted in the ground, and around 100 small deciduous trees are being integrated into the fountains and the square, along with 20,000 plants to beautify the space. It is not possible to plant more perennial plants because there is a car park under the square.
Two other projects are in development: at the Petite Ceinture in the 20th arrondissement, a future 35,000 m² forest will replace a former disused railway station, creating a new green space between the Cours de Vincennes and Rue Volga. A greening project is also planned for Place du Colonel Fabien, in the same spirit as that of Place de Catalogne, aiming to reduce the square’s prominent hardscaping by introducing greenery.
Focus on Place de Catalogne
On a sweltering day in June, I had an appointment with Eric Porcher, head of the City of Paris irrigation department, at Place de Catalogne in the 14th arrondissement. The buildings on the square, mainly social housing, were designed by the famous architect Ricardo Bofill. A veritable urban forest stands in the center of the square, and the cooling effect is immediately noticeable.
The city of Paris has transformed the former roundabout in the center of Place de Catalogne into a green space. “The square, which used to have a lot of hardscaping, was adorned with a fountain in its center, which has been replaced by 470 planted trees and 4,000 m² of wooded area, including an 860 m² clearing,” explains Eric Porcher.
The new green square is divided into four distinct areas: the lawn, the swales and perennial beds, which are open to the public, and the forest, consisting of large trees and undergrowth plants.
Photo credit : Irrigazette - Urban Forest Place de Catalogne
A customized irrigation system
The lawns are watered with classic Rain Bird 5004 sprinklers and the perennial beds with Hunter MP Rotators. “The lawns are surrounded by swales, which form small natural sloping streams that collect rainwater and flow into a water tank located in the technical room under the square,” explains Eric Porcher.
The forest was ultimately opened to the public, even though this was not the initial plan. The undergrowth plants, including ferns, shrubs, and perennials, are watered with Rain Bird 5004 sprinklers. As for the 450 trees, including oaks, cherry trees, and hornbeams, they are watered with special sprinklers designed for trees with root balls and aimed at filling the basin. “We installed Hunter CN 25 bubblers mounted on one or more sprayheads depending on the size of the root ball,” explains Eric Porcher.
It should be noted that there is no drip irrigation on the square, only sprinkler irrigation, because rats gnaw on the pipes, and this is the case throughout the city of Paris.
Eric Porcher adds: “The installation has two zones and five solenoid valves depending on the available flow rate.”.
Rainwater is collected for watering
The city of Paris has no shortage of water. It has at its disposal a potable water network, drawn from multiple sources to secure its supply (from the water table, the Seine, the Marne, various springs more than 100 km from Paris, etc.) and a non-potable water network, composed of water from the Seine, the Marne, and the Saint Martin Canal. However, water consumption for watering green spaces and supplying fountains accounts for a significant proportion of the city's water consumption. A water conservation plan was voted on last fall, and the parks department has therefore been instructed to reduce its consumption.
As we saw above, rainwater is collected by the swale system and redirected to a technical room located under the square. The room already existed because it was used to supply the fountain in the center of the square. Mr. Porcher explains: “The swales and lawn are watered with potable water, and the forest with non-potable water that is filtered and treated by a UV lamp reactor.”
Irrigation based on agronomic data
The Liebearth programming system, designed by an engineer based in Vienna, was chosen to manage irrigation at the square. Mr. Porcher briefly how it works: “This system allows you to install an infinite number of moisture sensors (five here) and to program them on a non-calendar basis, where the sensor starts and stops the watering. And the system goes even further with predictive weather data and a connected rain sensor.” He adds: “It's an agronomic approach to watering, which means that we adjust the amount of water by increasing or decreasing the frequency of watering, rather than increasing the amount; this works better for plants and means less water is wasted.”
Conclusion
The Place de Catalogne experiment has been a success. Of the 450 trees planted last year, all but one have survived.
The city of Paris is now tackling the Place du Colonel Fabien project, which will be developed on the same principle and should be ready next winter.
These urban forests aim to reduce temperatures in the city by creating cool islands, improve air quality by capturing CO₂ and filtering fine particles. Another advantage is that they promote biodiversity by providing habitats for various species and offering residents spaces for relaxation and walking. The City of Paris plans to plant 170,000 trees by the end of the current term of office, thereby reinforcing its commitment to ecological transition.
