2.3. Promote vegetation

Vegetation is an interesting solution to remedy the UHI phenomenon.

First of all, it makes it possible to limit the storage of heat during the day: 1) it is a living tissue capable in a certain way of regulating its temperature thanks to transpiration; 2) it stores very little heat; 3) it provides shade to the underlying surfaces in particular thanks to the crown of the trees; 4) it limits the maximum of surface temperature (Figure 22) .

Then, at night, it cools quickly and the vegetated areas create localized islands of freshness.

Problems:

- The vegetation must be in good health, especially during the hottest period of the year. It is therefore necessary to provide it with the water it needs for its photosynthetic functioning.

- It consumes floor space, which takes up space for other fittings.

- It represents a cost for its establishment, its maintenance (pruning, cutting branches, lawn mowing ...) and monitoring by specialized services

- It requires good soils

- The crown of the trees prevents infrared radiation emitted by the ground from escaping properly during the night, which limits nighttime cooling

The more areas the city contains with natural soil and vegetation, the lower the maximum heat island (Figure 23).

Vegetation can be introduced into the city in different forms: areas of lawns, low vegetation (shrubs), rows of trees in the streets, groves, cultivated gardens, urban parks, forests, green facades, green roofs.