PAP 45 : For the love of beautiful foliage

Planting the city of today

Caroline Mollie, December 2020

Le Collectif Paysages de l’Après-Pétrole (PAP)

Concerned about ensuring the energy transition and, more generally, the transition of our societies towards sustainable development, 50 planning professionals have come together in an association to promote the central role that landscape approaches can play in spatial planning policies.

Caroline Mollie is a landscape architect and author of the book Des arbres dans la ville, l’urbanisme végétal, which has just been republished in June 2020 by Actes Sud. She invites us to ask ourselves about the place of trees in urban space.

The slow decomposition of trees, a few hundred million years ago, has greatly contributed to the production of gas and oil, which we have used and abused over the last century at the risk of unbalancing the planet’s climate. Today, trees are our best allies in absorbing the CO2 resulting from the combustion of fossil fuels. Where and how can we plant them to establish this virtuous circle?

Documented by a great deal of scientific research in the fields of climatology, biodiversity, but also human health and well-being, the beneficial role of trees is recognised worldwide. It concerns all living environments, and in particular metropolises that are experiencing increasingly intense and frequent heat peaks. Urban dwellers are now unanimous: our urban landscapes must give trees a place of honour. Their ability to absorb the CO2 that is largely responsible for pollution and urban heat waves makes them the potential saviours of our cities.

Post-oil cities will therefore have to reserve a significant place for trees. In response to a growing demand for nature, more and more cities are making remarkable efforts in terms of planting. However, the viability of the major greening programmes that they announce can be questioned if they are started hastily. In fact, all too often, we see practices that are contrary to the interests of the trees as well as those of the city and its inhabitants. The beauty of trees creates the wonder necessary for any deep attachment. This wonder is coupled with a form of respect for their life energy: the beauty of a tree cannot be dissociated from its vigour. When a « beautiful tree » is well in its space, it breathes health. It has not been mistreated, its nature as a living being has been respected. Precipitation is not a good counsellor. It tends to ignore or even flout these two fundamental and complementary characteristics of trees, their beauty and their condition as living beings.

Trees are beautiful

The beauty of the trees is unanimously acclaimed. Sung by poets, honoured by painters, they are one of the rare elements of our environment whose beauty is a consensus. Who doesn’t marvel at a beautiful tree, a wide canopy, a vigorous and slender branch or a spectacular flowering? When they interact harmoniously with architecture, trees play a full part in the beauty, elegance and identity of the city. They create places of well-being, pleasure and conviviality. However, sensitive criteria such as beauty, harmony and wonder have lost their place in urban projects over the last few decades. Plantations considered superfluous or inconvenient have been sacrificed or abused in favour of functional imperatives such as housing, work, travel and leisure. The quality and health of the plant heritage of cities has suffered as a result. Beautiful trees or groups of trees free from any pruning are not frequent. The crowns, which are too often mistreated, have difficulty decorating, embellishing or shading our everyday landscape. This indifference to the beauty of plants persists today, even though environmental arguments have begun to prevail here and there over the imperatives of construction or traffic. But, often massive and hastily planted, new plantings invade squares, streets, avenues, facades and roofs without the city gaining in elegance and without public spaces offering the city-dweller extra pleasure and comfort. As if aesthetic considerations suffered from the same lack of interest as in previous decades, the tree has become an alibi or decorative object.

Announced and illustrated with enticing computer graphics, how can these displayed exploits embellish the city? Is it conceivable that announcements aimed at doubling, tripling or even tenfolding the number of existing trees could improve the urban landscape? What should we think of the multiplication of plantations of mature trees carried out in a hurry? What can we say about the inflation of tubs planted with Mediterranean trees, particularly palm and olive trees, which are clogging up the roads even in the northernmost cities? The legitimate greening of cities must aim to create beauty and well-being, to imagine happy spaces 1. It must be the subject of projects explicitly aimed at embellishing the city in a harmonious dialogue with architecture. Trees can be magnificent and evoke beautiful emotions, they can create multiple living spaces, provided that their planting and management are part of an overall project.

The history of our cities is rich in examples of green urban planning projects that have been able to solve urgent and very concrete problems with an expressed intention of beauty and pleasure. Examples include the 18th century beautification plans in Nîmes and Toulouse, for example, which led to the extension outside the walls of previously fortified, overcrowded and unhealthy cities, and then the Paris promenades designed to air and cleanse the capital and later the garden cities. These projects, which intimately associated plants and trees with urban development objectives, have endured over time, marking their era and offering people pleasant places to live and beautiful cities, recognised as such throughout the world 2 .

Trees are living beings

The growth of the trees over time blossoms into beautiful foliage that generously dispenses their benefits to our cities. A tree reaches its optimum efficiency once its crown and root system are fully developed, in accordance with the characteristics of its species. It is then that it can have a positive influence on the urban climate by purifying the air by absorbing carbon dioxide, which it transforms into oxygen to allow us to breathe, and into carbon to build up. The tree cools the atmosphere by releasing the vapour and water it draws from the soil. It also lowers the ambient temperature by shading floors and facades.

The tree is a source of wonder when it is adult and well developed. Then it plays its full role in embellishing the city and creating places of well-being. The effectiveness of trees against urban heat waves is directly linked to their good growth. Current practices do not favour it. Caught in a real planting fever, trees are requisitioned en masse to air-condition the city. All sorts of feats are imposed on them, such as climbing up building facades, undergoing drastic root cutting to be inserted in planting pits, tubs or cramped planters. They are extracted from their natural environment, mutilated and transported over long distances to miraculously reconstitute urban forest massifs.

It is wrong to imagine that the multiplication of plantations will solve the problems facing cities. If they continue, as is too often the case, to be piled up, amputated, constrained, contained, hung and thirsty, trees will vegetate and lose their health and longevity. Their growth energy will be thwarted, making it impossible for them to develop beautiful crowns. The benefits they could provide under good planting conditions are annihilated. The lack of consideration for them is a sham. These bad practices result in significant costs and an increased carbon footprint. They reflect a loss of knowledge and know-how.

All plants, especially trees, are programmed to live in real soil where their root system develops horizontally and from which their trunks and framework rise vertically. To have a real effect on the atmosphere, they must grow tall and healthy as adults. Any form of pruning or pruning, no matter how well done, will represent a trauma that alters their health and reduces their longevity, whereas the life of a tree can extend over several centuries.

What are the foundations for 21st century plant urban planning?

It takes at least twenty to thirty years for a plantation to produce the desired effects. It is therefore thanks to our predecessors that our cities today are provided with the appreciated foliage that we would like to see established more widely. Urban conditions are difficult for trees: all the more reason to pay the utmost attention to their well-being in space and time with the common sense and wisdom of the gardener. What steps must we take today to continue to enjoy a beautiful heritage and be able to pass it on to the next generations?

It all depends first and foremost on a few measures of common sense. For a tree to give its best, its vital needs must be met.

To do this, it is important to lay the foundations today for a new cultural pact that will provide a response to the climatic and ecological problems that are affecting our cities. This would be an opportunity to invent new models of green urban planning adapted to each country and each region according to their history, culture and climate, and meeting today’s needs for conviviality, mobility, housing and activities.

A few examples : Quantity in Milan and quality in Melbourne

Since 2012, Melbourne has been implementing an urban reforestation strategy. Its heritage of 70,000 trees is in decline and a loss of 40% is expected within 20 years. Faced with the challenges of climate change, the authorities have decided to increase the proportion of wooded areas from 22% to 40% by 2040 by planting some 3,000 to 3,500 trees per year 4. More than just multiplying the number of trees, it is a question of planting the right trees in the right places, encouraging the development of beautiful crowns that can regenerate the air as well as possible and provide maximum shade. This original and ambitious programme is accompanied by measures to improve urban biodiversity, the health of the trees and the permeability of the soil. Particular attention is paid to raising the awareness and involvement of city dwellers in this programme. A team of « urban forest dwellers » has been formed to promote communication with the public.

In contrast to Melbourne, the response to the climate challenge in Milan is a mass planting programme. In July 2019, the mayor announced the planting of three million trees by 2030 to reach the score of 2.5 trees per inhabitant. The plantations will be carried out in the areas with the highest thermal levels with the financial participation of companies. Milan plans to plant 100,000 trees per year for a population of 1.4 million inhabitants.

Such a rate implies a massive and immediate introduction of probably adult trees planted in tubs in public areas, with little effect on atmospheric regulation. Conversely, in Melbourne, 3,000 trees will be planted each year for a population of 4.5 million inhabitants: over time, one can hope to walk in the streets under a shady, cool and airy canopy.

The Melbourne programme seems to guarantee a balanced cohabitation between the trees and the inhabitants. For its part, a « reforestation » programme, however attractive it may be with its intention of renaturation on a large scale, risks being established without associating the creativity that the city calls for, that melting pot of inventiveness where the sensitivity of the artist combines with the skills of the gardener to create something wonderful.

The imitation of the example of Milan is currently the most frequent case. Each city has its own « tree plan » and competes in a race for numbers in favour of « urban forestry ». The « ready-to-plant » mini-forests announced in the city of Paris with a lot of publicity are, as such, highly questionable and will only have a limited effect on the surrounding neighbourhoods. What will remain in the long run of this planting inflation, if all these trees are installed in disregard of their basic needs?

The planting fever has contaminated the creativity of designers who compete with each other in imagination to signify their commitment to the cause of urban climate. But few projects are in keeping with the time of the plant and take its requirements into account.

Adventures and climbs: the « Vertical Bosco » in Milan

As such, Milan offers a questionable experience with the « Vertical Bosco », a set of two entirely green towers designed and built by architect Stefano Boeri. The images of this vertical forest have been taken up all over the world and have met with enormous success. This project is currently being implemented in several countries. The project impresses but raises an ethical question. These building facades offer very limited development volumes for the crowns of the trees, and impose narrow troughs in which it will be impossible for them to grow normally. The maintenance of such an ensemble, like all plant walls, is very expensive. It requires, among other things, the regular intervention of aerial pruners to contain the crowns. Remember that there are plants « programmed » to grow vertically, climbers and sarmies. In a few years, they can convert a wall into a mosaic of leaves and flowers. Vegetated facades are fashionable images, they are called « saleswomen ». Apart from being a form of plant mistreatment, they can become a constraint for inhabitants who do not have a green thumb and complain about mosquitoes or lack of light. Trees are not made to climb buildings and live vertically, nor are they made to float.

A floating forest: the « Park Archipelago » in Copenhagen

The « Archipelago Park » project in Copenhagen is another example where aesthetics outweigh ethics. The idea is to introduce mature trees on small floating and movable islands in Copenhagen harbour. This entertaining project is to be carried out from 2021 onwards. It has already been awarded two international design prizes. Cut from the ground, inserted in watertight bins, the trees sail according to demand and opportunities to create temporary « fun ». Under such conditions, their good development is impossible and their premature death assured. Once again, this project conveys an idea of the tree as an interchangeable decor, which has no choice but to submit to the constraints of the project.

Pots everywhere in the city

The trees in pots that are multiplying in the public space of our cities are a « scam » 5, another form of imposture to aesthetics and life. Raising trees in pots as Louis XIV’s gardeners did for the orange trees of Versailles requires time and know-how. In the same way as in the art of bonsai trees, the growth of the crown and roots must be pruned regularly, and to do this, the trees must be dug out, repotted and the soil must be kept moist. As they are planted today in pots, tubs or planters, the trees have no chance to develop. They will have no effect on the ambient atmosphere and will clutter up the urban space instead of creating beautiful shady areas.

In Marseilles, two examples close to each other bear witness to the contrast between this practice and successful planting. In 2005, plantations were planted on Place Bargemon, which was completely renovated. Part of this square is a slab on a car park: about fifty 1.5m high cast iron pots planted with olive trees were placed there. Fifteen years later, the trees have not grown, their sparse, stunted crowns show all the signs of a slow decline. One is struck by the sterility of this place, in the immediate vicinity of which there is a small esplanade that was planted on the same date with soap trees (Koelreuteria paniculata). Today they form a bright, flowery canopy 12 m high. It would be interesting to develop this comparative assessment by collecting the opinions of the local residents and establishing in each case the costs of installation and maintenance. There is nothing to prevent a car park slab from being planted, as long as certain rules are respected.

Pruning and pruning

To conclude this plea in favour of beautiful foliage, let us mention the issue of pruning, which alone would deserve a long development. Pruning is a real scourge in France. They have become a common habit as soon as the slightest inconvenience arises, such as clutter, shade or falling leaves. They damage the beauty as well as the health of the trees 6. A simple rule applies: the less you touch a tree, the better it is.

Although many cities are now engaged in excellent tree management programmes, pruning is still far too frequent and often unjustified 7. Pruning does not do the tree any good, but tires it and makes it vulnerable. Forcing it to draw on its reserves to rebuild branches or roots, these interventions create entrances for rot and disease. Severe pruning on a decaying or less vigorous tree finishes it off. Pruning does not make the tree secure, but rather weakens it. The rot will penetrate the branches and then the trunk. By digging them out, it alters their mechanical resistance, which is the cause of accidental falls. Finally, pruning does not solve the problem of excessive shade on facades; on the contrary, it strengthens them, since after severe pruning, the tree emits a quantity of shoots that will form denser masses of foliage than before.

Nevertheless, it is still possible and sometimes advisable to prune the trees as part of the monitoring of their evolution. A minimum of precautions, know-how and « planning-management » remain essential. Training pruning is necessary to adapt the tree’s architecture to the constraints of the public space, to raise the crown to the desired height, to respect traffic patterns or to overhang networks, for example. They are carried out on young branches, every year in the nursery and a few years after planting. The maintenance pruning of the crowns should favour the freest possible growth habit. They allow a crown to be thinned or aerated when shade problems arise, or to remove damaged branches or dead wood before it falls to the ground. These prunings are the work of specialised climbers-pruners who know how to respect the silhouette of the tree and avoid drastic cuts. Architectural pruning is required in environments with a historical character. They must be carried out very regularly, sometimes annually, and require a substantial budget. It is often difficult not to give in to fear or to the « precautionary principle » when faced with a tree that is a little bulky or too close to a façade. But before taking action, at the risk of weakening a tree, it is preferable to think, take your time and, if necessary, call in specialists 8.

For a new art of cities with trees

In the context of climate change, our societies have recognised the need to change our relationship with nature. This is demonstrated by the vogue for « wilderness » among hikers and snowboarders, the development of urban agriculture to raise awareness among urban dwellers of the shortcomings of globalised consumerism and the possibility of eating in a close territorial and social circle. The ideological vogue for « urban forests » is part of this context of general awareness and alert 9.

There is no doubt that, moreover, the tree has its place in a rethought urban planning. Post-oil urban planning will have to recompose the spatial influence of the peri-urban area and, in particular, promote its greening so that plot fences can resume their « urban » functions, combining pleasure for leisure and the preservation of biodiversity. Trees have found their place in town planning since the 17th century in Versailles, the first parkland town. Its ability to repair the urban planning of megacities and invent a sustainable city remains a particularly urgent task.

  • 1 Gaston Bachelard, Gilles Hiéronimus, Poétique de l’espace, PUF, Paris 2020.

  • 2 We studied these themes in our book Des arbres dans la ville, republished by Actes Sud in March 2020.

  • 3 Reference to the famous Télérama article published in February 2010.

  • 4 Official programme on the website of the city of Melbourne (EN).

  • 5 As Francis Hallé says in his book Du bon usage des arbres, un plaidoyer à l’usage des élus et des énarques, Actes Sud, Arles 2011.

  • 6 The city of Lyon, for example, estimates that 80% of the decline of its heritage is due to brutal pruning.

  • 7 In the opinion of specialists, any cutting of branches with a diameter greater than five centimetres should be avoided, as it could open the way to rot and pathogens and weaken the tree. On the other hand, pruning carried out annually or every two years on young twigs is quickly covered with a scarred bulge. They are traditionally carried out on fruit trees to encourage fruit production, on willows for wicker production and on ornamental trees in a heritage context to form curtains or canopies.

  • 8 Circle of quality in arboriculture:

Association of tree trimmers and climbers:

French Society of Arboriculture:

  • 9 The use of this term suggests that the forest can be transported into the city by transplanting adult subjects. In the city, the environment is artificial, the tree must be introduced into it in a spirit of garden art, following the example of the promenades de Paris or the cités jardin.