Greening cities using "acupuncture": the solution to lower the temperature

The Green Cities Observatory, which he co-chairs in partnership with Hortis, an association of urban green space managers, noted on Wednesday that the 50 largest French cities have more than doubled their average budget per capita dedicated to green spaces in 10 years.

Q: What does vegetation bring to the city?

A: "The greening and renaturalization of cities has effects on both public health, with improved air quality, a significant reduction in perceived temperatures and an increase in physical activity, with pedestrian and cycle paths, parks, etc. It also has effects on social cohesion, because green spaces become meeting places again.

To combat the heat, there is, on the one hand, the direct shade of trees and, on the other hand, the effect of water evaporation from plants, which contributes to the feeling of coolness. Along a tram line, between paved and vegetated surfaces, there is a minimum difference of seven to eight degrees Celsius at ground level. In streets lined with trees and vegetation, you can have differences of at least four to five degrees Celsius in perceived temperature in the shade of the trees. (...) If we multiply a certain number of relevant and coherent patches of vegetation, we could reduce the perceived temperature across the entire urban area by between four and seven degrees Celsius.

And as a reminder, two weeks ago we were experiencing excessive rainfall in a number of areas. Removing impermeable surfaces also helps reduce flooding, sewer system overload, and the discharge of excess polluted water into the natural environment.

Q: How can we effectively green cities to reduce temperatures?

A: "Historically, trees were planted for beautification purposes. When redeveloping a neighborhood, they looked at where the technical networks (sanitation, energy, etc.) needed to go, where cars would circulate and park, and if there were two or three small triangles left over, they would put one or two trees and three flowers there. This scheme needs to be completely reversed.

Planting four olive trees in a large paved square will have no real positive impact on air quality or urban cooling. It's better to create a 20- or 30-square-meter bubble, remove the impermeable surfaces, plant trees, shrubs, and perennials, and multiply these islands in a kind of urban acupuncture approach.

Angers, for example, has implemented an urban continuity strategy: every year they will plant at least a thousand trees in blocks and they will create a network across the entire urban area."

Q: What changes are you seeing in the approach of local authorities to green spaces?

A: "We do not observe a significant increase in the number of square meters of green space per inhabitant or in the number of trees per inhabitant compared to 2023, but we do observe an improvement in management: consideration of soils, maintenance of plants, search for an effective ecosystem.

During the 2020 municipal election campaign, there were a lot of empty promises and a kind of race to see who could plant the most trees. In the campaign for the last election, the proposals were much more substantive, well-structured, and strategic.

Among the 50 largest French cities, seven out of 10 have implemented policies to support the greening of gardens and sidewalks, through permits to plant vegetation at the base of walls, for example, and communication with residents.

Adding greenery is particularly necessary in peripheral neighborhoods, built between the 1950s and 1970s according to a logic of urban densification and vertical expansion, where the ground floors of buildings were reduced to car traffic and parking. Today, all major urban programs are renovating and rehabilitating these spaces.