Wherever I may go today, if the opportunity arises, if I have an hour to spare in an unknown city, a complacent drift takes me along the streets, towards these placid chlorophyllous enclaves encircled nowadays by the procession of motor vehicles and overlooked from on high by the residences and towers that peek up around them, showing through the foliage of cedars and catalpas.
I see in these vegetal Noah’s Arks so many modest treasure chests, beaten on all sides, abused, compressed by the tide of urbanization, but whose explosive vegetal deflagration will one day re‑seed the abandoned cities (Gracq 1985, p. 39).
Might this day, dreamed by Julien Gracq, have finally arrived? Everywhere today, there is nothing but talk of greening cities—you need only look at the manifestos of the mayoral candidates in Paris in 2020, each “greener” than the next, to be convinced of this. The galloping global urbanization (since 2008, according to UN statistics, more than half of humanity is a city dweller) makes it essential to reflect upon the links between cities and nature, especially vegetation. Does the ecological future of the planet rest on cities and their inhabitants, as Valérie Chansigaud suggests in a recent book  (2017, pp. 144–145)? In any case, it is certain that the urban world is becoming a central object for thinking on the environmental transition.  This ecological role  assigned to the city and the urbanistic consequences that follow are new and not at all obvious. In order to better understand the issues at stake, I propose, by focusing on the presence of vegetation and on the developments that accompany this presence, to go back to their origins, in order to gain a clearer view of the traditions of thought and the practices that they are shaking up in Europe.
A sustainable city through nature—but without ecology
Since the 19th century, in its relationship with plant-based nature, thinking in urban planning has been based essentially on two principles. The first, which goes back to well before 1800, considers the city as a place of anti-nature, of artifice—this is undoubtedly one of the primary characteristics of the Western city. Not that the vegetal has no place there, of course—but it must be ordered, organized, wanted in some way to be accepted. It is therefore considered, by engineers such as Alphand, for example, who sees it as the raw material of the art of gardens, as an element of urban furniture—the flower box on the sidewalk, the trees lining a street—or as a part of the urban structure when it comes to parks or gardens. Its “naturalness”  is, in a way, ignored, and all the more so since the imperial expansion, the progress of transportation, the opening to the world, particularly from the 1830s onwards, allowed the importation of exotic plants from all over the world, which were acclimatized. There are few limits, therefore, to the imagination of the landscape gardener: greenhouses, fertilizers, and the regular replacement of flowers free the decorative choices made from many environmental constraints.
The other principle of the dominant urbanistic thought, this time born with the industrial revolution and the unprecedented urban expansion that took off in the 19th century in Western Europe, considers the city as a problem: its artificiality becomes harmful. Pollution attacks the bodies of city dwellers exhausted by the rhythm of industrial agglomerations and the noise that surrounds them; the absence of beauty, the harmful attractions of the places of perdition, would ruin the morality of the poorest, of this numerous mass of which one fears—in Paris, traumatized by the revolutions which follow one another, or in London, whose East End is perceived as a place of perdition—the moods and the uprising. In these conditions, natural elements, and particularly plants, appear as a remedy: we must offer to the sick bodies and souls the appeasement of the forests, the countryside, the seashores; failing that, bring as much greenery as possible into the cities, to erase the nuisances, to reduce the afflictions. The public authorities are responsible for this and see it as one of the sources of their legitimacy.
The plant is therefore a tool of urban planning: it decorates and heals, and sometimes structures space. It does not have its own existence: olive or chestnut trees, local or exogenous flora, it does not matter, as long as it fulfills its function, decorates a square, softens the minerality and hostility of the city. We do not think, or very little, about the environment that it must occupy. Ecology, whose word was coined in 1866 by the biologist Ernst Haeckel, but which only organized itself as a science at the turn of the 20th century, focused on “natural” environments; during its first decades of existence, it ignored the city, the place of artifice and culture. Certainly, one wonders about the most favorable pedological conditions for the growth of trees in particular: the death of two thirds of the coniferous trees planted in the sandy soil of the Bois de Boulogne by Alphand in the 1850s, and the tree die-offs caused by gas leaks in the Parisian soil at the same time, have revived this imperative. Similarly, in cities with a particularly polluted atmosphere, we try to choose species that can resist, such as the solid geranium for example. Nevertheless, this does not make a systemic thought of the plant without its environment.
The indifferentiation between plants reached its peak with the notion of green space, which gradually emerged between the two world wars and was imposed after 1945. The garden contained at least a qualitative, ornamental dimension; it carried the seeds of an often fertile landscape thought. The green space, on the other hand, reduces the plant presence to a spot, a colored surface whose destination is so diverse that it becomes indeterminate. It blossoms to allow sporting activities, the need for which is perceived by the city dwellers; it becomes a place of rest, recreation, games, the lungs of the city. The public authorities accompanied this movement: the famous circular of February 8, 1973, sent to prefects by the minister Robert Poujade, defined green space and set a target of 10 m² (108 sq. ft) per citizen. The large housing estates built until the mid-1970s embody the misdeeds of this approach: the large area of greenery, poorly maintained and often poorly thought out, does not play the restorative and beneficial role that was expected of it. The post-war decades also saw the start of an unprecedented urban sprawl that continues today (from 1962 to the present day, urban density in France has fallen by a third) (Mathis and Pépy 2017, p. 11). The gardens of this suburban tide, again, take little interest in the environment in which they develop; impeccable lawns are born in a profusion of fertilizers and pesticides, while one chooses plants from catalogs according to one’s desires, not without succumbing to the effects of fashion, which are also found in the urban planning projects carried out by the public authorities. From the “Kleenex gardens” evoked by Nathalie Blanc (2004, p. 605), where each plant is discarded when it withers to be replaced by another raised in a greenhouse, to the enkagazons of the Palais Omnisports de Paris-Bercy studied by Bernadette Lizet (2008), the logic is indeed that of a “plant kit” (Mollie 2009) detached from its natural environment, chosified—properly speaking deracinated.
Toward an urban ecology
The rise of environmental concerns in the 1970s began to change the situation. Nathalie Blanc shows, for example, that it was only at this time that the notion of urban ecology, which had been born in the 1920s with the sociological tradition of Chicago, integrated nature in the sense of a living environment—in particular, in France, with the work of Paul Duvigneaud. The city then becomes a living environment where man and nature interact. In the same way, the Unesco program “Man and Biosphere,” launched in 1971, proposes to study, in the city, the interactions between societies and natural elements (Blanc 1998, pp. 293–295). Under these conditions, an interest in the “urban ecosystem” (Duvigneaud 1974) can be born and the view of the plant is modified. The plant is no longer an isolate that can be transported and transplanted at will; it is part of a social and natural whole that must be taken into account.
This advance is all the more important as awareness of urban biodiversity is growing. Already in the 19th century, amateur naturalists, reacting against the increasing artificialization of the urban environment, had proposed urban floras, for Paris or Vichy, for example. In the second part of the 20th century, Paul Jovet, a researcher at the Muséum National d’Histoire Naturelle (French National Museum of Natural History) in Paris, embodied this interest with his work on the wild plants of abandoned Paris metro lines, and those of the streets of Athis-Mons, in the southern Paris suburbs (Lizet et al. 1999). It is, however, only in the 1970s that this marginal research begins to bear fruit in the landscape and urbanistic thought, to develop then through the works for example of Gilles Clément. The idea of the Garden in Movement, where spontaneous flora is accompanied and not constrained, is exposed in an eponymous book in 1991 and applied at the opening of Parc André Citroën in Paris in 1992. In 1988, a “wild garden,” Jardin Sauvage Saint-Vincent, was inaugurated on the northern slopes of Montmartre hill; in 1997, a second such garden, Jardin Naturel Pierre Emmanuel, opened at the southern end of Père Lachaise Cemetery under the impetus of landscape architects Agnès Bochet, Laurent Gérard and Virginie Formigé and the ecologist Sylvestre Voisin. It is a return to a qualitative and not quantitative way of thinking: the plants are chosen for their adaptation to the environment, their endogenous character, not without also thinking about the landscape dimension of the spaces where they are planted.
This movement, which was embryonic at the time and still today is far from involving all municipalities and garden designers, accelerated in the 1990s (White 1998), supported by the public authorities, in the wake of the 1992 Rio Earth Summit which, through Agenda 21, promoted the notion of sustainable development in which cities became a key player. More than a thousand French local authorities, and not the least (Nice, Bordeaux, Nantes, etc.), have adopted these principles since 1993 through environmental charters, planting commitments, more respectful management of natural environments, etc. At the European level, the Aalborg Charter adopted in 1994 commits the signatory cities to implement actions to make urban environments more sustainable and to ensure that they participate in the ecological transition. This objective is facilitated by the establishment of the Natura 2000 ecological network, initiated by the European “Habitats” directive of 1992: if most of the protected areas are outside the city, the creation of green and blue networks (initiated by the Grenelle laws of 2009 and 2010), which now contribute to structuring the urban space, should facilitate the movement of fauna and flora. From now on, following various legislative texts passed since the 1990s (such as the Solidarity and Urban Renewal Act of 2000, for example), French municipalities must take green spaces into account in their urban plans and integrate them at various levels, from local to regional. The changes are considerable: the management methods as well as the understanding of vegetation in cities have been transformed in the space of 30 years.  Since the 1980s, cities such as Rennes or Orléans have practiced differentiated management of their green spaces, weeding—but without phytosanitary products—the most prestigious places, and leaving more room for spontaneous plants elsewhere. As in Montpellier, endogenous plants such as Kniphofia or Stipa tenuissima (finestem needlegrass), better adapted to climatic conditions, are chosen to reduce water consumption and facilitate maintenance. While there is no shortage of experiments at all levels of the territory, small cities sometimes lack the financial and human resources to implement such transitions; the legislator then plays a strong incentive role, as with the Labbé law, which has prohibited the use of phytosanitary products in public spaces since 2017 and private spaces since 2019.
The city at the environment’s peril? 
These transformations call into question the city as it has been defined for the last two centuries. Can ordinary, local nature, left—to a certain extent—free to move about, still play the role of remedy for the urban ills that it has been attributed until now? By handing the city over to the vegetal savages, by making it lose the orderly character that should inspire moderate and civilized human behavior, is there not a risk of de-urbanizing it? In fact, in spite of the growing action of ecologists who are working to restore ecological functioning in the city without a utilitarian vision, the insistence on the ecological dimension of urbanism over the last 30 years, particularly at the political level, is based on a well-understood interest on the part of city dwellers, which is fully in line with the expectations of previous decades. The association Plantes et Cité, in a 2014 study, identifies a dozen “services” provided by nature in the city, grouped into three categories (Laille et al. 2014). The benefits for humans include health and well-being, social ties and identity: all in all, this is a fairly traditional vision (and quite accurate, of course) of what urban vegetation can provide in social, physiological and psychological terms. A second set of issues concerns the benefits for the economy, around urban agriculture, the valorization of waste and buildings, etc. A third group, finally, concerns natural balances, but, apart from biodiversity, its elements are again related to human beings: thermal regulation (fight against heat islands), air quality, water flow, etc. The triptych refers of course to the three pillars of sustainable development: social, economic and environmental. As before, therefore, plant nature is placed here at the service of human activities and interests; simply the attention given to it is both more global and more refined in the relationships it weaves with urban societies.  In this sense, Nathalie Blanc suggests revaluing the city as a living environment, ceasing to disqualify it in order to implement an ecological requalification—which would not only be instrumentalized (2007). Michel Vernes, for his part, denounces the conception of a nature-remedy that he sees as an element of “de‑urbanism” (1984): can the emergence of ecology in the city offer another conception?
In any case, it certainly modifies the place that city dwellers occupy in the definition of urban planning policies: greening is no longer only the prerogative of public authorities, who decide on a park or a square, but requires the action of all citizens. Indeed, there can be no biodiversity without ecologically responsible vegetation of private spaces and without the support of citizens: the choice of endogenous plants, the abandonment of pesticides and chemical fertilizers are some of the means, not always clearly stated, that allow progress in this direction. The city can then enter into a more horizontal mode of governance, breaking with previous practices.  Here again, of course, there is still much to be done: developers or individuals, and even some local authorities, do not always play the game of vegetation and management that is more attentive to environmental conditions.
Ultimately, ecological urbanism calls into question the artificiality of the city, and thus challenges the very notion of the city as approached by the dominant architecture since at least the Enlightenment. Many authors have warned of the dangers of the all-vegetal city: the city, which would be defined by what it is not (neither the forest nor the countryside, etc.), would lose its soul (Berque 1997); it would dissolve in urban sprawl and the gardens that accompany it (Calenge 1997), which are often anti-ecological. The ecological city, through its attention to biodiversity and the environment, cannot be thought of as a monad—which it has certainly never been. But more than ever, it is conceived as part of a whole, of a territory, of a country, whose environmental conditions determine its “natural” development, the fauna and flora found there, and the methods of managing the elements. But then, where does the city begin and end? How can it be defined in its singularity? We should probably not imagine a clear boundary between the city and what it is not, but rather gradients of urbanity, which make it possible to identify it at its core and to link it to its environment (Fleury 1996). This is the case for the in-between area that is the green belt, between the dense city center and the agricultural outer ring: “This space not specifically urban, nor rural (IAURIF 1995) is precisely that in which the city is born, where rural space becomes urban: it is in a way a vascular cambium in the botanical sense” (Fleury 1996, p. 46). The city would retain what is necessary for its existence in terms of artificiality, while at the same time fitting into an ecosystem that it would obey without denying itself.
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