World Making and Phytosociological Associations

These vegetal operations, then, establish how to sense is to transform. By making and remaking atmospheres and soil, plants not only process pollution but also contribute to projects of multispecies world making.15 World-making projects have been discussed not just through pragmatist philosophy but also through the work of feminist technoscience and Indigenous scholars.16 Writing about world making in her discussion of the Matsutake Worlds Research Group, Anna Tsing notes that “each living thing remakes the world” through “multiple time-making projects” that transform landscapes.17 These worlds are in the making through the activities of multiple entities as they contribute to forming distinct environmental conditions.18 Such conditions indicate how organisms are in correspondence with and diverging from numerous other entities, responding to environmental events in different ways and across variable durations.

Vegetal forms of more-than-human sensing can show how these organisms experience pollution and form as distinct environmental subjects and worlds through responses to environmental pollution. Environmental pollution often registers through measurements that exceed acceptable levels or health effects and challenges made to regulators and polluters. Yet pollution also becomes evident through ongoing transformations of organisms and environments. Such modes of environmental sensing highlight how entities and environmental affiliations are affected by and transform through pollution. While in some ways bioindication might become legible through correspondence to an index that provides a systematic way of observing pollution, in other ways it gives rise to more open-ended engagements of incorporating and responding to environmental change. These open-ended or open-air instrumentalisms signal the distributed capacity of organisms and environments to generate different propositions for being together.

As I have discussed throughout Citizens of Worlds, environmental sensing—and the practices and entities involved in monitoring—can make environmental struggles matter in distinct ways. Measuring air quality with regulatory monitors is one way to sense environments. Attending to pollution as it accumulates in organisms, bodies, and ecologies is another way to tune in to environments. These different modes of sensing generate distinct forms of evidence and relevance. They also show how various entities and collectives work through and are individuated by pollution. As Gilbert Simondon has developed the concept, individuation refers to how entities are in-formed in relation to each other and their milieus.19 Subjects, relations, and milieus all have the potential to transform and are not pregiven, although they can be in-formed by sedimentations and inheritances. Here, a “human” is not a fixed entity and can shift in relation to different relations and milieus.20 So too might a “citizen” be encountered as an entity that changes through the collectives and worlds that affect it and to which it contributes.

In this way, subjects—including political subjects—are bound up with multiple other organisms and environments. Research focusing on the symbiotic characteristics of organisms suggests that even the notion of an organism as an individual is fraught with problems. Scott Gilbert, Jan Sapp, and Alfred Tauber have suggested through their work on symbiosis not only that “we have never been individuals” but also that “we are all lichens.”21 Indicator species such as lichens, plants, and many other organisms can provide a way to rethink environmental change through relational distributions of subjects. Such an approach can generate a less anthropocentric rendering of a changing planet, while connecting environmental subjects to earthly conditions that influence them.22 Indeed, if lichens such as Umbilicaria are present, Kimmerer notes, “you know you’re breathing the purest air. Atmospheric contaminants like sulfur dioxide and ozone will kill it outright. Pay attention when it departs.”23 The disappearance of bioindicating organisms can signal the contamination of the atmosphere that affects multiple other organisms and relations, which in turn could also be negatively affected by pollution.

Such a remaking of subjects through environmental affiliations has consequences for how to constitute political subjects and engagements. This distributed and multiple rendering of a subject is far from the universal and static figure that propagates through conventional humanism. A subject necessarily includes all that sustains it. Bioindication is not just an organism signaling the presence of pollution. It also expresses broader environmental engagements that influence earthly inhabitations. Organisms can draw attention to conditions of what Tsing calls the “more-than-human sociality” that informs environmental processes.24 For instance, a forest encompasses the bioindicating characteristics of individual organisms, and materializes and sustains the more-than-human social worlds that are made through these organisms.

More-than-human sociality aligns with the “community approach” to bioindicator-based monitoring, “where indicator values are not assigned to single species or communities, but to groups of species with similar ecological requirements.”25 Rather than monitoring individual entities or variables, these more collective approaches can document how ecological relationships are expressive of environmental pollution and change. The shared “phytosociological associations” and “multidimensional relationships”26 of indicator organisms are an area that has hitherto been somewhat under-studied, however. While scientific texts might chalk this omission up to the lack of a systematic methodology for evaluating community indicators, fledgling methods have been developed that, for instance, indicate air pollution through phytosociological relationships such as epiphytic organisms that grow on trees. Acidity and toxicity, the availability of light and water, and the abundance of nitrogen affect these organismal relationships. Similar to the Umbilicaria lichen, the presence or absence of epiphytes signals broader processes at work in the composition and decomposition of environments.

In other words, bioindication as an expression of ecological communities can be a way to get a sense of the health of environments, since “the replacement of a community by another one can be considered a clear indicator of environmental change.”27 Christelle Gramaglia and Delaine Sampaio da Silva suggest that “sentinel organisms” can signal conditions of pollution while at the same time demonstrating how an ecosystem such as a river operates as a “collective entity.”28 These more-than-human modes of sensing involve other communicative exchanges, as well as distinct expressions of sentience, intelligence, and “meaning” that require connections to numerous ecological operations.29 Rather than designating how a stable nature might be sensed, or bioindicated, by other organisms, more-than-human sensing is instead expressive of multiple environmental inhabitations and relations.30

Vegetal sensing draws attention to how environmental subjects take account of and form distinct experiences of their worlds.31 Connections across organisms and environments are continually remade through the accumulation and dissipation of pollutants. More-than-human sensing signals relationships across organisms, the composition of ecological communities, and the effects of their inhabitations. Such modes of sensing serve as provocations for how to characterize, engage with, and act on environmental problems. If “practice imposes upon its participants certain risks and challenges that create the value of their activity,”32 as Stengers suggests, then more-than-human modes of sensing indicate how risks and challenges cascade across pollution, bodies, environments, and relations. Practices of vegetal sensing recast how pollution becomes evident as well as how it might be acted upon. By incorporating multiple entities into practices of citizen sensing, it could be possible to rework conditions of political possibility while generating alternative strategies for making more breathable atmospheres.

The Political Work of Multiple Citizens

Air-quality gardens draw attention to how more-than-human relations contribute to breathable worlds. Vegetal sensing offers other practices for inhabiting environments and for constituting citizenship. These are multiply constituted modes of citizenship that, drawing on Tsing, attempt to “make common cause with other living beings” by engaging with more-than-humans as “political work.”33 Such political work involves reciprocity and exchange—of sensing, breathing, making, and remaking. Possibilities for more expansive environmental politics surface through tuning in to the world-making projects of other organisms. Air-quality gardens can be spaces for cultivating and pluralizing sensing subjects. They work and rework environmental pollution toward other earthly inhabitations.

Multiple citizens take shape through the “collective potential” generated by these different affiliations, exchanges, and ways of parsing environments.34 Sensing processes and relations give rise to multiple citizens and citizenships that remake environmental struggle and politics. Here, an environmental citizen is not the familiar figure of a responsible consumer–subject amenable to behavior change but rather an opening into other practices and relations of more-than-human sociality and political engagement.

However, the multiplications of citizens and citizenship do not signal a mere concatenation of actors. Instead, they are sites of struggle. Struggle here is not merely or necessarily a political contest that takes the form of “debate.” Instead, it forms through environments at saturation points, species loss, disconnection from land, and contaminated ecologies. The struggles that shape the political capacities and designations of subjects are multiple. Yet this plurality might also move toward “more different ways of being in relation,” as Berlant suggests in the first epigraph to this book’s Introduction. In this way, citizenship is always in process and “produced out of a political, rhetorical, and economic struggle over who will count as ‘the people.’”35 These struggles extend not just to who counts as the people as a collection of humans but also to the possibilities for being in relation with other entities and environments that might make for less destructive worlds. The demos that materializes here necessarily extends to the incorporation of more-than-humans and environmental relations as part of what makes these worlds breathable. Such pluralizations become sites of possibility by transforming the conditions and relations that could be activated through citizens of worlds.

In this regard, Kim TallBear points to the need to “indigenize” fields such as science studies and animal studies, which produce human subjects detached from environments and organisms and separate the organisms they study from their milieus. Drawing on American Indian metaphysics, TallBear seeks to “extend the range of nonhuman beings with which we can be in relation,” which in turn would inform the ecologies we inhabit and the environmental subjects we become.36 Here, TallBear points to how practices for indigenizing environmental subjects can transform social and political engagements by attending to earthly relations. Subjects are not pre-constituted as free-floating cognizers of worlds. Instead, worlds inform and co-constitute subjects. Or as Jennifer Wenzel writes regarding Frantz Fanon’s Wretched of the Earth, “decolonization demands . . . a shift in the valuation and disposition of nature,”37 not as an external entity against which to define humans but as a reworking of humans and more-than-humans toward less extractive and more reciprocal relations.

In this inquiry into how citizens of worlds form and are sustained, multiple modes of citizenship and “membership” materialize. Writing about historical restrictions to Indigenous citizenship, Leanne Betasamosake Simpson explains how nationhood is remade and differently lived within Indigenous worlds, less as a territory of violence and hierarchy and more as a web of relations. Indigenous nations are in relation with plant nations, animal nations, and many other more-than-human nations. A nation is an “ecology of relationships” that distinctly constitutes power and governance.38 Citizens and citizenship are then reconstituted through connections to multiple other entities and milieus, which are formative of political subjects. Human citizens form in relation to councils of plants, animals, and atmospheres, among countless other collectives. They make contributions along with vegetal citizens and plant nations, which form other possibilities for collective affiliation and environmental inhabitation.39

Such formations of environmental citizenship are an attempt to rework political subjects through multiple relations. Plants, among many other organisms, open into other inhabitations and durations.40 They draw attention to environmental attachments and formations of subjects through the signaling, incorporation, and transformation of pollution. The next section describes how Citizen Sense engaged with collaborators to construct and test air-quality gardens. On one level, this initiative could be seen as an idiosyncratic gardening project. On another, through its attention to how multiple organisms respond to air pollution, it offers an invitation to cultivate practices for working with more-than-human citizen sensors toward more breathable worlds.

How to Construct Air-Quality Gardens

The process of setting up two air-quality gardens at the Museum of London in-volved much more than locating plants within planters. The demonstrator gardens drew on local initiatives, from monitoring to gardening, while communicating with and connecting to more dispersed publics about projects for addressing air pollution. The demonstrator gardens did not operate as community gardens in the traditional sense of a sustained cultivation space. Nor were they a project for providing food or contributing to (or reforming) disadvantaged communities through gardening, a topic that researchers have discussed as having complex social and political implications.47 While residents were involved in looking after the nineteen clean-air gardens they installed, Grow Elephant supervised the demonstrator plots during their temporary installation. In this way, the gardens provided a test site for people to learn about air-quality plants, assess how to develop their own gardens, and consider strategies for mitigating air pollution. The gardens also provided a space to study the possible effects of air pollution on plants, while comparing pollution levels from the Dustbox particulate-matter sensor.

While the Phyto-Sensor Toolkit began with the demonstration gardens, it also encompassed a workshop and walk, a physical and online toolkit, and a broader investigation of practices for working with more-than-human sensors.48 Phyto-sensor is a term that the Citizen Sense research group uses to describe the vegetal entities and processes that sense and respond to environmental pollution. The gardens, events, and toolkit explored how plants continually sense and change environments. Because the sensing, bioindicating, and mitigating characteristics of plants are not always immediately evident, each demonstrator garden had a placard noting that the plants had been selected for their “special properties, either absorbing or channeling pollutants, or indicating particular pollutants with changes in their physiology and appearance.”49

The material registers of pollution became more or less evident through the garden, toolkit, events, and different engagements with vegetal sensors. Yet as demonstration spaces, the gardens were zones of inquiry that were focused less on identifying immediate changes and more on learning about more-than-human sensing and the relations it activates. Plants in their milieus became sensors and teachers.50 To this end, the toolkit documents how some plants are especially effective at taking up pollutants. These other organismal and vegetal ways of sensing environments served as a basis for investigating how to cultivate more breathable atmospheres, especially by developing planting scenarios that could be installed in other areas.

We selected plants for the gardens that included species that could bioindicate the presence of pollution, absorb or trap pollutants, or accumulate and transform toxins. Many plants work across these different ways of sensing and responding to pollution. The garden included bioindicating plants such as snowberry (Symphoricarpos albus), which is sensitive to ozone and will show leaf injury and impaired growth when ozone is present.51 Snowberry can also take up heavy metals such as zinc and iron from the soil, but it will show signs of damage and impaired growth if it accumulates these contaminants. Thus the garden included plants that demonstrate how contaminants persist and transform organisms and their environments, often with deleterious effects.

Many of the plants in the air-quality garden trap or channel particulate matter through deposition and dispersal. Plants with hirsute or broad leaves can act as surfaces to capture airborne particles. When planted as a hedge or green screen, plants such as yew (Taxus baccata) can catch particles that wash into the soil during rainstorms. At the same time, many plants are respiring and contributing their own mix of materials to environments, including oxygen and other gases. Yew emits low levels of biogenic volatile organic compounds (BVOCs), which can combine with nitrogen oxide to form ozone. Many urban plants and trees emit BVOCs. For this reason, simply planting more vegetation because it is “green” does not necessarily improve air quality, and can instead exacerbate pollution in areas with high levels of nitrogen oxide that can react with BVOCs.

Plants bioremediate air pollution in ways that are not simple actions of scrubbing and removing the residue of fossil fuels. Instead, they absorb some pollutants while emitting others. They capture particles from the air while carrying them into the soil. They phytoremediate contaminants in the soil but often can only do this when in relation with other plants. They bioindicate the presence of pollutants such as ozone, but this form of sensing and signaling can also demonstrate that plants are likely to die. The air-quality gardens and Phyto-Sensor Toolkit document these bioindicating and mitigating characteristics, showing how plants such as yarrow (Achillea millefolium) can colonize soil in urban areas and provide a surface for particle deposition. Yet yarrow can also become damaged by ozone, a condition that becomes more or less severe in relation to the other species with which it grows.

Because these modes of sensing register through the form, growth, and distribution of plants and are typically more evident when they grow over time in environments, we also set up Dustboxes to monitor real-time pollution levels at several locations within and near the gardens. This Dustbox installation was also a process of consolidating our sensors for wider circulation as a sensor library, which might be used as a mobile and DIY air-quality monitoring infrastructure for installation in London and farther afield. We placed Dustboxes within the demonstrator gardens. We located Dustboxes with local residents. And we located Dustboxes at the official air-quality monitor adjacent to the Beech Street tunnel.52

Because the Beech Street tunnel has especially high levels of pollution in the form of nitrogen oxides and particulate matter, it was an area that local residents, the Museum of London, and the City of London were interested to gather additional data about. In the process of setting up Dustboxes alongside the Beech Street monitor, a City of London air-quality officer who provided access to the official monitor also undertook routine maintenance while at the site. Here, she collected a leftover particulate matter filter saturated with a black soot-like coating. The material residue of particles became evident as a layer of grime, which usually spreads through the air, circulates into the lungs and other organs of passersby, settles on urban structures, clogs instruments, and is taken up by more-than-human organisms.

The Dustboxes provided a way to compare pollution levels throughout the area while also looking at relationships across digital and vegetal sensors. However, the different modalities and durations of these sensing methods quickly became apparent, since moments of elevated air pollution did not correspond to an automatic wilting of plants. Instead, vegetal sensors became a way to tune in to different approaches for cultivating less toxic atmospheric exchanges. In turn, the Dustboxes and other air-quality monitors throughout the area drew attention to where pollution was occurring and suggested how these patterns could inform proposals for developing other gardens in the area.

Distinct patterns of urban pollution began to surface through the Dustbox data and alongside the vegetal sensors. The demonstrator garden at the sheltered entrance to the Museum of London often showed the lowest pollution levels. However, when regional air pollution events occurred, this Dustbox showed levels similar to those of other monitors in the network. Pollution levels at the Beech Street tunnel location were consistently the highest, with pollution concentrations often exceeding the WHO twenty-four-hour guidelines. One resident on an upper floor of a Barbican tower typically recorded lower levels, likely due to the monitor height. Yet during windy conditions, this monitor would register some of the highest levels in the network. At all locations within this central area of London, however, levels of particulate matter were comparatively high overall, with monitors typically registering readings of 20 to 35 µg/m³ over a twenty-four-hour period. When winds circulated from the east (and from regional European locations), levels would often register at least 50 to 70 µg/m³, three to four times above the 2005 WHO guideline of 15 μg/m³ over twenty-four hours.

At the same time, the regulatory monitor in the Beech Street tunnel typically recorded levels of nitrogen dioxide at nearly double the EU twenty-four-hour standard, at 80 µg/m³. Looking more closely at pollution levels using our Airsift tool, we established that nitrogen dioxide was circulating from the nearby intersection and station to the west. At the same time, particulate matter was circulating from the east, both locally at the tunnel and regionally from farther afield. Digital sensors, vegetal sensors, and citizen sensors combined into observational techniques and experiencing entities registering both that pollution was occurring at high levels and that different approaches to addressing this problem might be created by reworking trajectories of sensing and action.53

How to Transform Citizen Sensing through Citizen Gardening

Citizen sensing and citizen gardening began to merge in the search for more breathable urban conditions. In this polluted location adjacent to the entrance of the Beech Street tunnel, the Dustbox and regulatory monitor shared the same airspace as one of the nineteen community-planted clean-air gardens. Existing concrete planters designed within the Brutalist architectural style of the Barbican were situated at the base of the forty-four-story Lauderdale Tower, where residents and volunteers had installed numerous air-quality plants. Small birch (Betula pendula) trees occupied planters, where their waxy leaf surfaces were meant to trap particles, absorb nitrogen oxides, take up heavy metals, and phytoremediate the air and soil. Silverbush (Convolvulus cneorum), a plant with small leaves covered in numerous fine hairs that capture tiny particles, filled planters nearer to the entrance of Lauderdale Tower. The microclimate in this area was not only polluted but also dry and windy, making for conditions within which the more drought-tolerant silverbush was able to thrive.54

Citizen sensing could, in various ways, contribute to and align with citizen gardening and citizen infrastructure. But this juxtaposition of clean-air gardens, air-quality sensors, and highly polluted roadways inevitably raises the question of whether these relatively contained and diminutive planters would be able to mitigate the high levels of pollution occurring in this traffic-clogged area. Regulatory and citizen-led monitoring can establish that high levels of pollution occur and even demonstrate the location of emissions sources. However, the intensity and persistence of pollution often require further measures to address this problem. Air-quality gardens alone do not “solve” the problem of air pollution. Yet these engagements with more-than-human sensing also bring to the surface the dilemmas of how to realize feasible citizen actions with more intractable political processes. Such engagements work toward strategies for intervening within and transforming unbreathable urban conditions.

This inquiry into how to construct air-quality gardens investigates different practices for working with more-than-human sensors, spanning from identifying plants and outlining different planting scenarios for installing vegetation to building community networks. At the same time, these practices for working with vegetal environmental sensors and transforming urban air toward less hazardous and more collaborative environments point to the struggles that arise when attempting to address and mitigate air pollution. Citizen sensing and citizen gardening collide with different formations of power that can surface through practices of making and transforming environments.

From draining swamps to establishing plantations, cultivation is often an expression of power that materializes and reproduces dynamics of resource capture and extraction.55 Planting is a way to reclaim, rework, and establish relations with land and more-than-human entities. Engagement with plants can become a form of resistance or otherwise inhabitation. As researchers and contributors to the Black/Land Project note, practices of urban gardening can form very different ways of engaging with land for Black gardeners, who also see this “as a way to stake a claim to permanency, education, economic citizenship, and community leadership, rather than only as a vehicle for food security.”56 Practices of gardening and inhabiting land can become forms of collective “geo-theorizing,” where environments are made and remade as expressions of self-determination and openings into other ways of relating to land. In other words, this is a way of becoming citizens with land as well as worlds.57

In a similar register to the previous chapter, where people sought to protect green space to stake a claim to self-determination, the Black/Land Project describes how expressions of citizenship materialize through other and distinct ways of being in relation to and becoming with land. In parallel, Kimmerer discusses how gardening can generate practices of reciprocity that not only inform relations with land but also influence political relations and ways of undertaking exchanges with multiple entities. Reciprocity with the land is an invitation to reciprocity with all creatures. Such an orientation amplifies citizenships through the constitution of subjects, relations, and governance practices that work toward mutual exchange rather than domination.58

The cultivation of air-quality gardens signals the presence of air pollution in London, while proposing strategies for how to remake urban environments. Making and remaking environments are practices for attending to other entities, for cultivating their contributions to expand breathable worlds, while establishing and reinventing relations with more-than-human sensors. These are different ways of learning from and engaging with environments. They cultivate other relations that can depart from established inhabitations.59

The intervention that air-quality gardens make, both in the form of the demonstrator gardens we developed in collaboration with the Museum of London and in the form of the residents’ nineteen clean-air gardens, is also worth considering in more depth in this discussion of how to construct air-quality gardens. While these gardens do not instantly absorb the many pollutants in London’s air, they do constitute practices for transforming environments as multiple inhabitations, different durations, and collective exchanges that could generate more breathable worlds.

The Phyto-Sensor garden, events, and toolkits present scenarios for how to construct air-quality gardens and how to transform citizen sensing through multiple citizens’ sensing and planting. These scenarios include how to work with community groups to start a garden project, how to identify sites for installing air-quality gardens, and how to select plants. They also encompass how to monitor pollutants, how to form different planting scenarios (from street trees to green walls), how to monitor along with vegetal sensors, and how to develop more expansive engagements with environments and more-than-human entities.

As collective environmental projects, these propositions for how to construct air-quality gardens resonate with community gardens. Yet even more, they connect with forms of near-future gardening. Many garden plots are now being developed not only as food-based projects but also as spaces for realizing environmental justice. These expanded forms of cultivation can especially be found in community gardens that address and mitigate climate-change inequality, where climate change and increasing urban temperatures make apparent the effects of redlining, lack of vegetation, and urban heat islands.60 One such initiative, Groundwork USA, is working to make more “climate safe neighborhoods” in US cities by using digitized historical maps and data analysis to identify areas at risk of extreme heat and flooding due to climate change.61 These analyses sense and overlay different forms of spatial inequality. Such groups are working toward more breathable urban environments by developing tree-planting initiatives and community gardens, which cultivate green space as a reconfiguration of environmental and political collectives.

Similarly, air-quality gardens could generate other counter-practices of cultivation that rework relations, entities, and environments through shared inhabitations with—and even beyond—pollution. Such gardens do not merely signal that an event has occurred or is occurring. Instead, they also materialize the relations at stake and environments that are harmed when pollution accumulates. The breathability of worlds multiplies through these exchanges that attempt to amplify rather than diminish environmental and social justice. Gardening becomes a sociopolitical practice for cultivating more breathable worlds along with more-than-humans, and a way to address and rework unprecedented environmental change through the co-constitution of citizens and worlds.

How to Aerate a Phyto-Sensor Toolkit

Because the construction of the demonstrator air-quality gardens took place primarily as an institutional collaboration, Citizen Sense hosted a workshop and walk at the Museum of London where the Phyto-Sensor Toolkit could be opened into dialogue with interested museumgoers. The workshop and walk allowed for the further development of the toolkit by drawing on input from participants. While existing residents had made the above-described monitoring projects (and were involved in monitoring with Dustboxes and meeting to describe their monitoring activities), the publics that attended the museum were much more diffuse, with visitors to the City Now City Future exhibition traveling from locations across London, the UK, and the world.

Citizen Sense developed the Phyto-Sensor Toolkit as a resource and guide to aid in the citizen-led research and development of air-quality gardens. The toolkit focuses on how phyto-sensing processes can bioremediate air quality, especially in urban settings. As a pedagogical toolkit, it introduces participants to an array of herbaceous and woody plants, as well as trees, that respond to and mitigate pollution. The toolkit allows people to identify plants in the field while at clean-air garden and green infrastructure sites and to learn more about how they bioindicate or mitigate pollution. Consisting of ten sections, the toolkit first provides an introduction to phyto-sensing and an explanation of citizen sensing. It then gives an overview of the problem of air pollution and describes the Low Emission Neighbourhood initiative and garden. The core content of the toolkit includes a series of plant-identification cards that describe key vegetal responses to air, soil, and water pollution. Also included in the draft version of the toolkit is an outline of the workshop and walk (where the provisional version of the toolkit asked for participants to record comments and propositions for the final toolkit). As part of the walk itinerary, the toolkit introduces the air-quality gardens and air-quality monitoring infrastructure in the Square Mile of London. The final sections of the guide include suggested planting scenarios for constructing air-quality gardens in relation to likely emissions sources, and a list of resources for further researching air-quality plants and gardens.

To test the Phyto-Sensor Toolkit, we held a workshop and walk as part of the Barbican OpenFest, an annual free event to explore the outdoor and indoor spaces around the Barbican. On the day of the walk, a Saturday in mid-March when temperatures might ordinarily be mild, we were instead visited by a cold snap dubbed the “mini-Beast from the East.” Grappling with temperatures of –7 degrees Celsius with wind chill, we worried that people might not attend or be equipped to handle the temperatures that were unusually cold for London. However, many participants did turn up sporting full-body outerwear suitable for extreme winter sports, as well as wearing heavy gloves, woolly scarves, and bobble hats, fully layered up and ready for an investigation of air-quality gardens.

After a brief introduction to the Phyto-Sensor project and an overview of the workshop, walk, and toolkit, we set out to look at air-quality plants in the area. We started our walk at the two demonstrator air-quality gardens at the entrance to the Museum of London. Here, we discussed the different plants installed, their bioindicating and mitigating properties, and how they process pollution in different contexts and in relation to other organisms. Paul McGann of Grow Elephant joined Citizen Sense for the workshop and walk, during which he explained the development of the planters and the installation and care of the plants.

With the frigid temperatures, plants were not yet in full bloom, but many were still identifiable when compared to the field guide in the Phyto-Sensor Toolkit. We struck a balance between lingering to investigate the fine or hairy leaves of plants that capture particles, to walking briskly to the next stop where we might be able to fend off the cold. We walked around the Barbican podium, discussed residents’ initiatives to address air quality, and made our way to the Beech Street monitoring station. As mentioned previously, this enclosed hard space of concrete and traffic tends to have especially high pollution levels. We then looked at how urban architecture, vegetation, monitoring, and urban processes assemble to create more or less polluted environments, and discussed other approaches we could take to construct more breathable conditions.

In the open air, the Phyto-Sensor Toolkit began to unfold as a logbook, plant-identification guide, map, and observational device for recording, considering, and proposing different ways of engaging with vegetal sensors and air-quality gardens. Given the cold temperatures, this unfolding was at times marked by a rapid jog-walk to the next garden. At various stops, we would wave to a stand of Euphorbia and quickly move on to observe lavender, lamb’s ear, and jasmine while hopping in place to learn about clean-air gardens developed by residents as part of the City in Bloom initiative in 2017. We also discussed the proliferation of air-quality plants and gardens in numerous other locations that we would not be visiting, including at the Barbican Tube Station, where a community garden of air-quality plants had been installed to green the urban canyon of the station. On a disused rail track, coral bells and ivy filled planters, where their broad leaves and masses of green surfaces were meant to capture particles and nitrogen oxides. We speculated about how large a planted area would need to be, especially in this area of London, to take up pollutants in an effective way. One participant asked if there was a way to measure plant mass needed in relation to pollution levels. Many people wondered: Was this a feasible strategy, given the intensity and pervasiveness of air pollution in London? Did we need a how-to guide for aerating London’s atmospheres on a wider scale?

As we expeditiously made our way to the Moor Lane pop-up garden, we discussed how air-quality gardens could also be a way to make more evident the role and benefit of vegetation in improving air quality. The Moor Lane garden had developed through a considerable volunteer effort. Populated with corrugated steel pipes that sprouted a burst of air-quality plants, the site provided a striking visual display that collided with the hard concrete and brick edge of the Barbican podium and parking garage. Numerous air-quality plants jostled together in this space, including ivy, ferns, birch, Hebes, silverbush, and juneberry. Plants with smooth but small leaves such as Hebes, as well as vigorously spreading plants like ivy, came together in a display of plants that we discussed and imagined to be sensing, absorbing, capturing, and filtering air pollution. We noted that small trees such as juneberry can mitigate pollutants including particulate matter and nitrogen dioxide as long as their canopies do not mass together and trap pollution at street level. At the same time, this species is particularly beneficial for wildlife, providing berries, pollen, and nectar for birds and insects. The cultivation of urban environments began to draw in multiple other organisms that form ecologies specific to air-quality gardens.

We then made our way to the final stop along our walk, visiting the Barbers’ Physic Garden to consider the restorative and curative qualities of plants. This historic garden was developed in the sixteenth century and was a site to which the botanist John Gerard later contributed.66 The garden was originally developed with plants required by surgeons, and the most recent version of the garden was planted in 1987 on a derelict bomb site. In this series of plots, forty-five different species of herbs lined gravel pathways, with each specimen having a placard noting its distinct properties for treating wounds, bruises, and burns. In this site of former bomb damage, strewn with the ruins of a Roman fort and Victorian walls, we investigated the relationship between plants and health. We considered how plants as curative organisms could contribute to healing environmental health problems. Finally, we asked: Could we make a twenty-first-century how-to guide and Physic Garden for restoring the health of environments?

Once we had returned to the Museum of London, we regrouped over a much needed tea-and-biscuit break before reviewing the air-quality garden walk and beginning a discussion of the Phyto-Sensor Toolkit. Drawing on observations and propositions that participants recorded along the way, as well as generating additional proposals that arose in the course of the workshop, we spent some time thinking and working through what communities would need to construct air-quality gardens in areas of London and farther afield. Paul offered advice on plants and gardening techniques, while we reviewed the difficulties of working with local councils and the complexities of ensuring maintenance of vegetation so that air-quality gardens would not expire from neglect.

The final version of the Phyto-Sensor Toolkit incorporated these proposals and observations from workshop and walk participants. As a how-to guide for air-quality gardens, the toolkit developed through several iterations in the open air. The construction of gardens, the installation of vegetal and digital sensors, the testing and expanding of the instructions and resources through a workshop and walk, and the compiling of the toolkit as a print and online manual available on the Museum of London website all became ways of aerating the toolkit and expanding the designations and practices of citizens and sensing. We developed the toolkit through these open-air modes of inquiry so that it could contribute to additional air-quality gardens and community projects. Just after we published the toolkit online in May 2018, we learned that community groups were taking up the toolkit and using it as a guide for making air-quality gardens, including on Clean Air Day. Aeration is, then, an invitation to incorporate, attend to, and participate in more-than-human sociality and sensing. It presents an openness to expanded and more-than-human modes of sociality that work toward more breathable worlds.

How to Multiply Citizens in Worlds

Plants are participants in urban ecologies, and they contribute to urban environmental communities. The Phyto-Sensor Toolkit tunes in to these expanded processes of environmental sensing that occur across multiple entities. This is another way of thinking about and working with sensing beyond an anthropocentric approach to environments, where organisms and environmental relations are differently brought into view and activated through their interrelations with and experiences of pollution.

As Crawley suggests, “everyone is held within breathing as process.”67 And yet, this breathing involves multiple engagements that are differently situated and experienced. Humans unequally experience exposure to air pollution, which can in turn even affect their capacity to sense. Plants absorb and exchange pollutants, while multiple other organisms are caught up in sensing and processing the effects of polluted atmospheres. Multiple organisms act as sentinels, bioindicators, and more-than-human sensors that express the conditions of environmental toxicity and change. These practices constitute differential and at times collaborative modes of sensing. Such expanded sensing practices offer different ways of cultivating urban air and working toward more breathable worlds by tuning in to the unevenly shared conditions of breathing, sensing, and exchanging atmospheres.

These collective contributions to environmental sensing provide an invitation to cultivate more expansive citizens and worlds in the making as “pluralistic realism.”68 As discussed throughout this study, citizen-sensing practices often challenge the “one-world world” of scientific realism. Different experiences, modes of evidence, and encounters with air pollution materialize through these practices, which do not assemble into a singular epistemological or ontological plane.69 So too does more-than-human and vegetal sensing shift and multiply the worlds of experience that could be brought to bear on the problem of pollution.

Such an approach requires that other worlds of relevance register as significant, rather than attempting to fit these practices into a singular world of experts and amateurs, humans and more-than-humans. Citizen sensing can be an example of a project that is meant to aid an already established trajectory of scientific inquiry.70 Yet this chapter outlines an approach to multiple citizens to indicate how worlds, political subjects, relations, and actions form through different engagements with sensing organisms and sensing technologies. As John Law notes, these are the conditions that challenge a “one-world world,” since practices are a way of activating different realities. Such practices have sociopolitical relevance, since they draw together different collectives, activate different versions of the real, and require contingent and specific strategies for working with and across pluralistic conditions.

Air-quality gardens and more-than-human sensing practices are conditions that might be encountered and cultivated in the middle of things. Pollution and environmental change are unequally bearing down on humans, more-than-humans, and ecologies. Yet these are also conditions that generate different political subjects and political work. Such conditions require strategies for being and becoming citizens of worlds along with multiple other entities in the open air. This turn toward the open air involves attending to concreteness and worlds of experience.71 By working from within the particulars of multiple and differential experiences, it is possible to realize the possibilities of pluralism as what James calls “a translocation of experiences from one world to another.”72 As discussed here, this translocation of experience spans from the vegetal to the digital and beyond. Worlds are not a total surround. Instead, they are vectors of engagement and experience. They form a multiverse or pluriverse, rather than a universe.73 Worlds involve constitutive processes, exchanges, and intensities. They establish conditions of breathability and reciprocity, as well as of unbreathability and subjugation. Environmental sensing unfolds through these distinct yet pluralistic registers to explore and establish how multiple modes of sensing make and remake citizens and worlds.