How do we green up cities – and why do we need to?

Cities around the globe are facing a double-edged sword of climate change and urbanisation. We are already seeing the effects around the world, through loss of life from heat and extreme weather events, and a rise in health issues linked to environmental quality factors such as air pollution and noise.

Greening of cities, in appropriate, sustainable and forward-thinking ways, has unique potential to reduce the severity of many of the key issues associated with climate change and urbanisation. The critical role of plants in making our urban environments more resilient cannot be underestimated. First we need to understand what the issues are, how plants can be used strategically, effectively and sustainably, and how we can implement greening in practice to create positive change on a global scale.

During the Third International ISHS Symposium on Greener Cities in September 2024, 150 scientists, engineers, planners and other stakeholders from 15 countries and five continents gathered at RHS Wisley to share latest research, advice, and effective strategies for implementing urban greening at multiple levels.

Why do we need to green up cities?

Our world is overheating, and no more so than in cities, where nearly 60% of the global population now live – a figure that’s expected to rise to nearly 70% by 2050, according to the UN. In the UK, more than four fifths of the population already live in urban areas.

Urban areas are particularly vulnerable to the effects of climate change. Cities are on average warmer than their surrounding landscapes, which magnifies global heating effects, while they are also more strongly impacted by extreme weather events such as storms, thanks to the lack of absorbent surfaces. In addition to the risks associated with heating and flooding, increasing urbanisation also enhances the health risks posed by city-specific environmental issues such as air pollution and noise.

The problem of increasing heat

Heat in cities is already a huge public health issue. During summer 2023, there were nearly 3000 heat-associated deaths in the UK according to the UK Health Security Agency, with more than two thirds of those being people aged 65 and over.

In Maricopa County, the vicinity including Phoenix – the USA’s hottest and fifth largest city – heat-related deaths have risen nearly tenfold over the last decade, with half of these fatalities estimated to be unhoused civilians living on the streets of Phoenix. Many of the world’s major cities are becoming more and more uninhabitable for those who are most vulnerable.

Warming in cities is also expected to cause issues with failing infrastructure, power outages, and shortages of food and resources.

Air pollution is a health issue in cities

Air quality is currently the UK’s, and the world’s, largest single environmental health risk, according to Professor Fraser Torpy, speaking at the Greener Cities symposium.

“Urban air is innately polluted: some of the most common harmful compounds affecting air quality in cities are carbon monoxide, nitrous oxides, sulphur dioxides, volatile organic compounds and ozone, levels of which regularly exceed global guidelines,” he says. These compounds have been linked to a range of life-threatening conditions, such as stroke, cancer and COPD (Chronic Obstructive Pulmonary Disease), as well as respiratory infections and asthma.

Air pollution is estimated to be responsible for seven million deaths per year worldwide – the same number lost their lives to the Covid pandemic – more than £24 billion per year in health costs, and a mass surge in poor health, lost productivity and low workplace performance, according to Professor Torpy. Expenditure on healthcare related to poor air quality is expected to make up 1% of total global GDP by 2050.

Flooding is becoming more common

Flooding is becoming an increasingly common natural disaster in cities around the globe, thanks to factors associated with both climate change and urbanisation.

Climate change is increasing the frequency and severity of extreme weather events, leading to more frequent heavy downpours. Warmer air holds more moisture, giving greater potential for more intense rainfall – this particularly affects cities on account of the urban heat island effect (temperatures in a city are higher than in surrounding countryside). In coastal areas, flood risk from intense rainful is often compounded by rising sea levels, meaning there are multiple sources of excess water contributing to flooding. 

Urbanisation makes flooding more severe as areas where plants and trees once grew have become paved, concreted or tarmacked over. This means the excess water cannot be absorbed and has nowhere to go, so run-off is rapid and extreme, leading to drainage systems becoming overwhelmed. The dense populations associated with urbanisation also means the effects of flooding can come at a higher human cost.

Noise is another health risk in cities

In Europe, noise is second biggest environmental cause of human ill health after air quality. The greatest contributor is road traffic noise, making increasing noise levels part and parcel of urbanisation. 40% of UK residents are exposed to road traffic noise, according to Dr Jo Webb speaking at Greener Cities.

Noise is more than just an annoyance: long-term exposure is linked to conditions such as cardiovascular disease, sleep disturbance, mental health issues, and impaired cognitive development in children. The need for noise control cannot be overlooked in an increasingly densely populated and increasingly urbanised world.
 

How can greening help?

Urban greening, through introducing plants and green spaces into cities in targeted and effective ways, has unique potential to help alleviate all of these key environmental issues associated with our growing, warming cities.

Shading by street trees significantly reduces temperatures in city streets, while both trees and other greening features such as green walls, green roofs and hedges improve air quality. Many forms of greening have potential to mitigate flooding by reducing runoff – whether that be by interception of rainfall by leaves, infiltration of water into vegetated areas, or capture and use of water in irrigating green walls and other planted features.

The uptake of water by plant roots and evaporation through leaves helps to dissipate excess water that reaches the ground – a benefit that can be harnessed by sustainable drainage systems (SuDS) such as swales, which use a plant-centric design to alleviate flood risk and waterlogging. All of these greening features will also benefit wildlife in urban areas; a factor that cannot be overlooked in the face of a global biodiversity crisis.

Speaking at the symposium, Dr Federica Larcher emphasises that low-cost nature-based solutions need to be at the heart of urban development. The RHS has already called on UK Government for gardens and green spaces to be integrated into the core of the UK’s strategy for new developments. However, Professor Torpy feels we need to go much further: that in order to increase their future resilience and make them inhabitable long-term, we need to retrofit our cities with plants – the right plants, in the right places, and in the right numbers.

Ways of greening cities

While space can be very limited in cities, meaning it can often be hard or unfeasible to retrospectively incorporate substantial greening areas such as gardens and parks, there are a number of greening methods that can be employed in cities across the world with minimal changes to existing infrastructure.

Street trees

Importance of considered selection: both in street trees and for all other methods of greening, plant selection is key for ecosystem services (benefits to humans and our environment that are delivered by plants, such as cooling, improvement of air quality and flood mitigation) to be provided effectively. At Greener Cities, Professor Ross Cameron cited that the cooling benefits that plants bring to building walls have been shown to vary by at least 15C depending on species.

Water use by trees is highly variable depending on species. Professor Diane Pataki emphasised that species commonly planted as street trees can often be some of the most inefficient or thirsty species. For street trees that will cope with lower water availability and require less irrigation, it can be worth looking to Australian and Mediterranean species.

The value of trees for biodiversity needs to take into account all wildlife, not just pollinators. Dr Andy Hirons and his team are developing a tool that assesses overall benefit to biodiversity taking into account resilience, future potential for susceptibility to ‘pests’ and other metrics.

Native vs. non-native: Professor Cameron feels that when choosing trees against the backdrop of climate change, we cannot rely solely on native species. Street trees are a long-term investment and therefore need to be resilient to climate change. “To ensure our city trees are resilient, we need to be looking at trees that are native to climates that are comparable to the climates we expect to experience 50-70 years from now,” says Kevin Martin, Arboretum Curator at Kew.

Additionally, Professor Cameron has found that for some plants, natural, wild-type species have been shown to be more resilient than highly bred versions (cultivars) of these species.

Seed provenance: plants of the same species adapt over time to the specific conditions in which they are growing, meaning that where seed is collected is key in conferring how well adapted the resulting tree will be to its environment.

“It is predicted that the English oak will struggle across much of its current range in the future, so seed provenance from outposts of the current range is important in raising young trees that are pre-adapted to our future conditions,” says Dr Andy Hirons.

Green walls

Green walls have the potential to mitigate the negative impacts of urbanisation on biodiversity and ecosystem services, according to Dr Guilia Daniele. They can reduce building temperature, increase biodiversity and improve air quality.

There are two main types – green facades (plants growing directly on walls) and living walls (plants mounted onto walls). Green façades are cheaper and are lower maintenance, while living walls give better coverage and plant diversity.

Green walls can be retrofitted onto buildings, but since irrigation is a common challenge associated with green walls, urban ecologist Gary Grant reports that the best systems are those that include rainwater capture systems within the infrastructure in order to irrigate the green wall.

Professor Cameron points out that for a simple, low-cost green façade, Hedera helix has been shown to reduce the wall temperature of a building by as much as 26C.

Green roofs

Green roofs bring similar benefits to green walls, particularly in terms of air quality, biodiversity and often providing a wellbeing space for people with limited access to nature, and likewise are feasible to incorporate into densely urbanised areas where space for greening can be very limited.

Green roofs can also be retrofitted, but Dr Peter Irga points out that the weight implications of some types of green roof mean these would ideally be considered at the point of building design in order to optimise the impact that can be achieved.

Hedges

Hedges are often very effective at pollution capture, owing to their dense, linear form and location along roadsides, according to Dr Mark Gush. However, potential benefits of urban hedges can be diverse beyond capturing air pollution.

Research by Dr Jo Webb has found that large, dense hedges, especially those with large, oval leaves, can significantly reduce perceived noise levels when planted as a barrier. Meanwhile Dr Tijana Blanusa has found that hedge planting in schools brings a number of benefits, such as improving children’s engagement and attitudes towards nature. Professor Charles Laubscher is exploring the use of African plum as a multifunctional hedge that offers an alternative vitamin-rich food source in South Africa to not just provide ecosystem services, but also enhance food security in urban areas.

Conversion of brownfield sites

Perennials are any plant living for at least three years. The term is also commonly used for herbaceous perennials which grow for many years (To compare: annual = one year, biennial = two years).

perennials

What is being done around the world?

In Turin, green walls have been installed as part of city centre redevelopment, which help to provide all-important ecosystem services in a dense urban environment, including pollution capture, and showed that minimal urban greening interventions have a substantial impact on citizens’ wellbeing.

In Sydney, greening an office block roof increased the efficiency of the rooftop solar panels by keeping temperature closer to optimum and led to a 9-fold increase in biodiversity compared to an identical non-greened roof; a figure that is thought to be an underestimate. A banded bee was found on the roof, which is the first time this species has been reported in Sydney.

In Seoul, rooftop greenhouses are being built on commercial high rise buildings for urban farming, which also reduce greenhouse gas emissions via the exchange of energy between office and growing space.

Across the USA, the National Wildlife Federation’s yard certification schemes targets domestic gardens by giving homeowners a list of suitable plants from a national database, resulting in certified yards having the highest diversity of wildlife and lowest water use. 

Challenges and barriers

The biggest barriers to urban greening are cost and maintenance, but there are also other concerns, such as around fire safety when cladding buildings with green walls. There are social barriers to greening, such as apprehension about having a green roof or wall, and logistical barriers, such as space.

Cost: the initial output costs of urban greening projects can be high, owing to the resources, materials, plants and labour required. Examples of low cost greening projects include conversion of brownfield sites using existing materials and well-adapted plants, but high-cost projects such as installation of green walls and green roofs will often become more feasible if they are built into the original plan during building design. The capital cost can be often be followed by high costs of maintenance.

What we need to do, according to the experts

At the Greener Cities symposium, Professor Torpy and other speakers expressed a belief that no matter how much research is done, in order for effective improvements to be seen in practice, researchers must work collaboratively with policymakers, developers and industry in an integrated approach. The consensus was that this approach will need to be radical and will need to draw on high-quality, widely available information that equips planners and developers to prioritise plants and greening.

On a domestic scale, through the sharing of knowledge and awareness, there was encouraging news that everyone can be empowered to make a collective difference in whatever space they have.

We are converging on an ambitious vision for the future of green cities, but we are not persuading enough people to get involved. The more people get involved, the faster we can achieve our goals for green cities

We need to work together – across society

Availability of quality information is key

We have already seen that species selection is key to long-term resilience – so we need quality information to inform plant choice to exist and to be publicly available.

Plants – and provision for their maintenance – come first

We are in a climate emergency, and a radical amount of greening and novel approaches will be needed on a global scale.

“Plants are a universal core component of cities, and greening is not happening quickly enough. We need to move away from green walls being a prestige system to being a utility system that can mass-treat air to provide to lots of people,” says Professor Torpy.

Preservation of mature trees needs to be prioritised over planting of new trees

We need to focus on conserving our existing urban trees: replacing existing trees by planting new ones is no substitute for the ecosystem services provided by mature trees, according to Professor Stephan Pauleit, who reports that an 80-year-old tree provides these benefits to the human environment ten times more efficiently than a 20-year-old tree.

Kevin Martin feels that management techniques may need to be reviewed: large street trees are historically often pollarded, whether or not there is a safety reason for doing so. This leads to a cycle of maintenance whereby the tree needs to keep being pollarded. Since the canopy cannot reach its full potential, this drastically reduces the cooling effect and other ecosystem services provided by the tree, and leads to much higher maintenance costs.

Everyone can make a difference

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