How much can greenery lower the perceived temperature? What is the potential microclimatic improvement, and what is the actual improvement?

The first point is not to confuse three variables: surface temperature, air temperature, and the comfort perceived by the body. The shade of a mature tree can reduce the surface temperature of asphalt and concrete by about 10–20 °C during the hottest hours of the day. Air temperature, on the other hand, drops much less—often by about 0.5–2 °C on a local scale. Perceived comfort can improve by about 2–8 °C, because the canopy blocks direct solar radiation, which is often the dominant factor for a person outdoors. For a well-designed project in Rome’s climate, I would more conservatively estimate a 3–6 °C improvement in comfort under mature tree canopies, a 10–20 °C reduction on shaded surfaces, and a decrease of about 0.5–1.5 °C in local air temperature during peak hours. On a citywide scale, the values become more modest but are by no means insignificant. A study of 93 European cities estimated that increasing tree cover to 30% could reduce the average urban temperature by about 0.4 °C. This may seem like a small figure, but when spread across millions of people and many hot days and nights, it can produce very significant health benefits. The decisive metric, therefore, is not how many trees we plant, but how much healthy canopy and how much actual shade remain after ten, twenty, or thirty years.

Why is there a gap between potential and actual results?

The gap arises because studies often measure mature, healthy, and irrigated trees, whereas real-world projects start with young trees, limited soil, intermittent water, and uncertain maintenance. A drought-tolerant tree is not a tree capable of growing without water. Especially during the first three years, it requires regular irrigation to establish itself. If the soil is compacted or the root mass is insufficient, during a heat wave the plant closes its stomata, reduces transpiration, and loses its very ability to cool the environment when it is most needed.

Other factors that matter include the continuity and size of the canopy, the street’s orientation, ventilation, leaf density, surrounding impervious surfaces, heat generated by traffic and buildings, the time of day the measurement is taken, and the plant’s health and water availability. Future shade, therefore, depends on the unseen conditions designed beneath and around the tree. A small hole surrounded by asphalt may physically accommodate a tree, but it is unlikely to support a mature, healthy canopy for several decades.

What is the role of humidity in this?

Humidity plays an ambivalent role. Evapotranspiration removes heat from the environment but increases relative humidity. In a hot, dry climate, this exchange is generally beneficial, provided there is water in the soil. In an already humid climate with stagnant air, however, the increase in humidity can reduce sweat evaporation and offset part of the thermal benefit.

In the urban Mediterranean, the priority should therefore be to combine shade, ventilation, living soil, and well-managed water, rather than simply adding more vegetation.

What are the other benefits of public green spaces?

Cooling is just one of the benefits. A well-designed green infrastructure can capture and slow down stormwater, promote infiltration and soil recharge, reduce the load on sewer systems, filter out some pollutants, protect the soil from erosion, and provide habitats and ecological connections. It can also provide shade for buildings, pathways, and gathering spaces; reduce the need for cooling; encourage physical activity, play, and social interaction; and support stress recovery and mental health. Public green spaces are therefore simultaneously a climate-regulating mechanism, a water management system, a public health infrastructure, an ecological network, and a social space. When properly designed and maintained, they are an infrastructure that delivers environmental, health, social, and economic benefits.

What factors should be considered when designing public green spaces for the long term?

The initial question shouldn’t simply be “which plant do we like?”, but “what service will this space need to continue providing in ten, thirty, or fifty years?”.

A public green space project is truly sustainable when, over time, it becomes more stable, develops its own ecological autonomy, and continues to generate benefits without requiring an increasing amount of resources. This means designing both what we see and what remains invisible. A tree’s growth depends on the availability of fertile, uncompacted soil, root space, water, drainage, and the ability to develop a canopy consistent with the size of the site. If these elements are not addressed, even the most resilient species can struggle, require constant pruning, or never reach the size needed to provide shade. It is necessary to consider the current climate, but above all, the future climate. It is not enough to simply observe the rise in average temperature. We must also take into account longer heat waves, multi-year droughts, short but very intense rainfall events, extreme winds, new pests, and occasional cold spells. At the same time, we need to analyze the site-specific microclimate: exposure, orientation, ventilation, heat reflected from building facades, the presence of impervious surfaces, and heat generated by traffic and infrastructure. Allergenicity, fruit production, thorns, toxicity, invasiveness, and the ability to provide habitat must also be factored into the design. In the long term, creating diverse plant communities is crucial. A combination of trees, shrubs, herbaceous plants, and groundcover can protect the soil, limit evaporation, capture rainwater, support biodiversity and pollinators, and gradually reduce the need for weeding and mowing. The goal is not to eliminate maintenance—which is impossible in a public space—but to transform it from a series of repetitive and corrective interventions into a more selective, planned, and informed approach. Finally, it is necessary to measure what happens after the project is handed over. One can assess the survival rate after three years, canopy growth, the shade provided during the hottest hours, soil permeability, rainwater retention, biodiversity, and management costs. If the desired results are not achieved, the project must be adjusted. Green spaces are living systems and require adaptive management.

How much does maintenance affect the overall cost of the space?

It can account for as much—or even more—than the initial construction. A proper assessment requires a cost calculation based on the entire life cycle. The first three years are particularly important. During this phase, the following are necessary: irrigation to help plants establish themselves, mulching, checking stakes, selective weeding, formative pruning, and replacing failed plants. Cutting back on this phase risks losing the biological investment before it begins to generate the expected benefits. As a preliminary feasibility guideline—not as a national statistical average—for a robust, low-maintenance public space, it may be prudent to set aside approximately 2–5% of the total initial cost each year for routine management and minor renewals, with a separate, higher budget line item for the first three years. The cost depends heavily on the type of project. Ornamental lawns, formal hedges, seasonal blooms, and complex plantings can significantly increase the frequency of maintenance. Maintenance is reduced above all by designing a system capable of evolving and becoming progressively more stable. The contract that appears to be the cheapest may result in the most expensive garden if it does not adequately fund establishment and care.

What role does landscape design play in preventing fires?

It is important to distinguish between flower beds and small green spaces within the urban fabric and large urban or peri-urban parks, especially when they border agricultural areas, forests, uncultivated land, or settlements. In the first case, the risk may be localized and primarily linked to the accumulation of dry material. In the latter case, consideration must be given to the potential spread of fire over much larger areas, access routes for emergency vehicles, and the protection of buildings and infrastructure. In large parks, it may be more effective to create a mosaic consisting of low-lying areas, strategically mowed strips, separate clusters of trees and shrubs, gravel paths, wetlands, and spaces accessible to emergency vehicles.

What business models can prevent green spaces from being viewed merely as decorative elements?

The key step is to recognize and account for the services provided by green spaces. If a park retains stormwater, reduces the risk of flooding, provides shade, improves health, supports pedestrian mobility, and enhances a neighborhood’s appeal, it should not be funded solely from the budget line item dedicated to green spaces. This makes it possible to develop more comprehensive financial models. A portion of the resources can come from budgets for stormwater management, climate adaptation, mobility, public health, or urban regeneration. Management can be contracted out through agreements in which the contractor is paid not only for the number of mowings or prunings, but for outcomes such as plant survival, canopy development, soil quality, water retention, and biodiversity. In some contexts, green spaces generate credits or economic benefits directly linked to their performance. There are positive examples in the United States. In Washington, D.C., for example, those who build green infrastructure capable of retaining stormwater can generate and sell Stormwater Retention Credits. One credit corresponds to the capacity to retain one gallon of water for one year. Green spaces thus become a measurable part of the urban water system. Other models utilize nonprofit foundations, conservancies, economic districts, concessions, events, and sponsorships. The asset remains public, while a dedicated entity raises funds and ensures maintenance, safety, and programming. In Milan, BAM demonstrates how a partnership between the city government, a foundation, and private entities can integrate maintenance, biodiversity, safety, and cultural programming within a park.

Are sponsorships and public-private partnerships common in certain regions?

In the United States, models such as conservancies, “friends of the park” associations, and Business Improvement Districts are quite widespread. Bryant Park in New York remains a public space, but its nonprofit management is supported by concessions, events, sponsorships, and financial resources raised within the district. Central Park combines private funding and public contributions under an agreement in which the city retains ultimate authority. It represents a shift from sponsoring a flower bed to shared responsibility for a complex public infrastructure. A partnership works when ownership and access remain public, technical standards are verifiable, the contract funds maintenance for a sufficiently long period, and costs and results are transparent. It must also prevent private resources from being concentrated solely in neighborhoods capable of guaranteeing greater visibility or profit. A portion of the funding should also contribute to the upkeep of spaces located in the most vulnerable areas.

What is the role of greenery at different heights within the same spaces?

A multi-layered system utilizes space in three dimensions and seeks to replicate certain relationships found in natural plant communities. Tree canopies provide shade, catch rain, and protect the lower layers. Small trees and large shrubs define spaces, offer blooms, fruits, and habitats. Herbs and groundcovers protect the soil, reduce evaporation and erosion, limit weed growth, and support pollinators.

Even underground, plants can occupy different levels. Root systems with varying depths and architectures utilize water and nutrients in complementary ways, improve soil structure, and can make the system more stable. Multi-layered vegetation can also intercept more particulate matter and slow down rainwater more effectively than a surface composed of a single layer of vegetation. However, this complexity must be designed with the use of the space in mind. In a plaza, dense and open areas should be alternated. Along a busy street, vegetative barriers that hinder air circulation should be avoided.

Which trees should be avoided given current and future climate conditions?

I prefer not to talk about a list of trees to avoid. Countries like Italy have such diverse climatic conditions that the same species may be suitable in a park with deep soil but become unsuitable just a few kilometers away, along a paved road. In parking lots, plazas, and very hot streets, I would avoid the widespread use of species adapted to cool, moist conditions, such as Betula pendula, Fagus sylvatica, Picea abies, and Aesculus hippocastanum. This does not mean they cannot continue to thrive in hilly or mountainous areas, cool parks, or deep soil. Salix alba, Alnus glutinosa, and Populus nigra, for example, make sense where water is truly available, but they cannot be automatically transplanted to the drier, paved parts of the city. I would avoid large-scale new plantings of Betula pendula, Fagus sylvatica, Picea abies, Acer pseudoplatanus, and Aesculus hippocastanum in hot, dry, and impervious sites. This does not mean automatically ruling out these species. Recent studies show that genetic origin also significantly affects drought response. The choice must therefore take into account not only the species but also the origin of the planting stock. Mediterranean species such as Quercus ilex, Quercus suber, Ceratonia siliqua, Olea europaea, Celtis australis, Fraxinus ornus, and Arbutus unedo can provide a useful foundation for central Italy and similar areas, but they are not invulnerable. Even the holm oak, when subjected to extreme drought, poor soil, and compaction, can suffer.

I would avoid monocultures above all else. This means gradually building a population that is more diverse in terms of species, age, structure, and genetic origin. The response to climate change is not to replace one monoculture with a new, resilient monoculture, but to build a diversified biological portfolio.

How do you replace plants in response to rising temperatures? When do you determine that the current ones are no longer suitable?

A city’s vegetation cannot be replaced in a single season. The process involves a gradual transition. The first step is to create an inventory that records species, age, health, stability, root space, irrigation, canopy size, and services provided. This data must then be compared with climate scenarios for 2050 and 2070 and with cities that currently have conditions similar to those projected. At the same time, it is possible to establish experimental plots, use different genetic sources, and monitor growth, water consumption, heat damage, pests, and mortality. The conclusion is reached that a tree is no longer suitable when multiple indicators converge over time: repeated desiccation, progressive canopy loss, high mortality in newly planted trees, unsustainable irrigation needs, increased root failure, excessive maintenance costs, or evident incompatibility with the available soil and space. Mature, healthy trees, however, should not be preemptively removed simply because a model predicts more challenging future conditions. They provide a level of canopy cover and ecosystem services that a young tree would take decades to rebuild. They must be preserved and monitored while preparations for their replacement are gradually made. Native species remain essential for biodiversity and landscape identity. Species or provenances from warmer regions can be integrated into the system, after verifying their invasiveness, resistance to occasional cold spells, and ecological compatibility.

What is the average percentage of investments allocated to the maintenance of green spaces?

There is no truly comparable international average percentage. Budgets include or categorize parks, staff, vehicles, sports facilities, street trees, and capital projects in different ways. However, a 2025 U.S. report indicates that the operating budget generally accounts for one-third to one-half of the total budget for park systems and that management and maintenance constitute the largest expense category. The same report notes that 93% of total investments still come from public entities, while agencies dedicated to parks and recreational activities generally account for less than 2% of the city budget. For each project, it is therefore necessary to distinguish between three types of costs: the cost of initial construction, the cost required to support plant growth during the first three years, and the cost of annual maintenance once the space has reached a more stable state. As a preliminary estimate, one can expect an annual figure of approximately 2–5% of the initial cost for routine maintenance and minor renovations.

What pilot projects could change the perception of green spaces around the world?

The project that will truly change the perception of green spaces will be one in which shade, water, biodiversity, health, the quality of public space, and management costs are all factored into the same budget. That said, a prime example is the OASIS program in Paris. Ten pilot schoolyards have been de-paved and transformed into cool, permeable, and playful spaces that are also accessible to local communities. The project links climate adaptation, childhood, equity, and public space, and has created a model that can be replicated in other neighborhoods.

In Medellín, the Green Corridors use trees, shrubs, and other plants to connect streets, waterways, and stations. The program has also trained gardeners and created jobs, demonstrating that the ecological transition can also generate skills and employment.

Grey to Green, in Sheffield, has transformed parts of an old urban street into a green corridor featuring rain gardens, low-maintenance lawns, pedestrian and bike paths, and sustainable drainage systems. The vegetation was not added merely to improve the street’s appearance, but to collect and filter stormwater, reduce the risk of flooding, promote biodiversity, and make walking through the city center more enjoyable. The city now describes it as the longest green street in the United Kingdom and the largest retrofitted sustainable drainage system in an urban space.

New York’s High Line, on the other hand, has demonstrated how a disused railway infrastructure can become an elevated linear park and create a completely new way to traverse and observe the city. Its value lies not only in its naturalistic landscaping but also in its ability to preserve industrial heritage, create public space where none seemed to exist, and build a stable partnership between government and nonprofit management. It is an iconic model—not necessarily replicable everywhere—but one that has profoundly transformed the international perception of urban infrastructure reuse.

In Washington, D.C., Stormwater Retention Credits transform the ability of green infrastructure to retain water into a measurable and tradable service. It is an important cultural shift from decorative greenery to greenery that generates environmental and economic benefits.

In Italy, BAM in Milan showcases a public-private management model capable of integrating biodiversity, maintenance, safety, cultural activities, and community engagement. It is not merely a sponsored park, but a public space in which landscape design and cultural programming are managed as parts of the same system.