Planting to cool our cities
How urban planting reduces the heat island effect, and the species suited to cooling city schemes
Cities run hotter than the open country around them. Hard surfaces such as tarmac, concrete and brick absorb heat through the day and release it slowly through the night, while dense building layouts trap warm air and limit the breeze that would otherwise carry it away. Add the heat thrown off by traffic, air conditioning and industry, and the result is a measurable temperature gap between the built-up centre and the land beyond it. Researchers call this the urban heat island.
The difference is not small. Major UK cities including London, Manchester and Birmingham have at times measured around 5°C warmer than their surrounding rural areas, and on still summer nights the gap can be wider still. In Manchester, the night-time effect within the urban canopy has been recorded as high as 10°C under settled conditions, and in Birmingham it has been estimated at up to 7°C. The heat that builds up in the day lingers after dark, so city residents get little relief overnight.
This matters most during heatwaves, when the added warmth tips from uncomfortable to dangerous. Research published in the journal Environmental Health attributed 52% of the heat-related deaths during the August 2003 heatwave in the West Midlands to the urban heat island effect, and around 785 deaths in Greater London during the summer of 2018 have been linked to high air temperatures. As summers grow warmer, the pressure on city design to manage heat will only increase.
Vegetation is one of the most effective and affordable ways to bring those temperatures down. Trees, climbers and ground-level planting cool the air around them, shade the surfaces that would otherwise store heat, and soften the hard geometry that makes cities warm in the first place. Where the right species are chosen and given room to establish, the cooling they provide is steady, long-lived, and far cheaper to run than mechanical alternatives.
Why cities trap heat
The materials a city is built from are the starting point. Dark, dense surfaces such as asphalt and concrete absorb a large share of the sunlight that lands on them and convert it to heat, then release that heat slowly once the sun goes down. A street of bare paving and rendered walls can stay warm long into the night, keeping the surrounding air warm with it.
The shape of a city adds to the problem. Tall buildings set close together form narrow canyons that catch sunlight, reduce airflow, and reflect heat between facades rather than letting it escape. Where there is little vegetation and almost no exposed soil, there is nothing to absorb water, no shade to intercept the sun, and no moisture to evaporate and cool the air. The surfaces simply heat up, and stay hot.
A landscape that includes trees, soil and growing plants behaves in the opposite way to bare concrete. It holds water in the ground, shades the surfaces that would otherwise store heat, and releases moisture that cools the surrounding air.
How planting cools the air
Plants cool their surroundings in two main ways, and the larger of the two is shade. A tree canopy intercepts sunlight before it reaches the ground, so the paving, walls and soil beneath stay far cooler than they would in full sun. Research in street settings has found that tree canopies can intercept up to 97% of the incoming solar radiation, which keeps surface temperatures down and stops the slow release of stored heat after dark.
The second mechanism is evapotranspiration. As plants draw water up through their roots and release it as vapour through their leaves, that change from liquid to vapour absorbs heat from the surrounding air, in much the same way that sweat cools skin. The effect is smaller than shading in a tight street, but across a planted area it adds up, and it depends on plants having enough water and root space to keep the process going.
Together these two effects produce real, measurable cooling. Analysis by the World Resources Institute, drawing on a global study of 806 cities, found that greater tree canopy was associated with midday surface-temperature reductions of around 1.5°C, and that every 10% increase in canopy cover lowered local air temperature by roughly 0.3°C. At pedestrian level, some studies have recorded reductions of as much as 12°C in the most favourable conditions. For a city trying to keep streets usable through a heatwave, that is the difference between shade people seek out and pavement they avoid.
Cooling beyond the tree canopy
Trees provide the greatest cooling effect, but they are not the only option, and in dense built-up areas there is often no room for them at ground level. Green roofs, green walls and low planting all contribute, and they reach surfaces that street trees cannot.
A planted roof shades and insulates the building beneath it, and stays dramatically cooler than the bare membrane it replaces. Green walls cool vertical surfaces in the same way. Studies have found vegetated walls running up to 7.7°C cooler at the surface than uncovered concrete in summer, which reduces the heat radiated back into the street and lowers the cooling load inside the building. At the wider scale, urban green spaces have been estimated to reduce warm-season temperatures by 2–4°C compared with surrounding built-up areas.
Ground-level planting plays its part too. A dense mat of foliage over soil keeps the ground shaded and moist, so it absorbs far less heat than open paving and releases moisture back into the air. In schemes where tree planting is limited by services, access or sightlines, layering climbers, shrubs and groundcover across every available surface is often the most practical way to bring temperatures down.
Choosing plants for urban cooling
Cooling is most effective when planting is layered, with a canopy for shade, planting on vertical surfaces, and dense cover at ground level. The species below span all three, and all of them cope with the demanding conditions of an urban site: compacted soil, restricted root space, pollution and long dry spells.
Carpinus betulus
Commonly known as hornbeam, this hardy native is one of the most dependable trees for urban planting, carrying a dense, rounded canopy of toothed green leaves that cast heavy shade through summer and turn yellow in autumn. It copes with compacted ground, pollution and hard pruning, which makes it as useful clipped into a formal street tree as it is left to grow into a broad specimen for parks and squares. Its spring catkins and autumn seeds support numerous insects, small mammals and seed-eating birds. It tolerates almost any well-drained soil in sun or partial shade.
Quercus URBAFLORA® ‘Grace’
A climate-resilient oak that combines English and Iranian oak genetics, selected to cope with the extremes a city throws at it. It grows quickly into a broad-canopied tree of around 18m, tolerating both drought and periodic flooding once established, and its large leaves provide generous shade and cooling. As an oak, it supports a wide range of insects and the birds that feed on them, making it a strong choice for street planting and larger landscape schemes.
Carpinus betulus
Quercus URBAFLORA® ‘Grace’
Tilia cordata
Commonly known as small-leaved lime, this deciduous tree carries a dense, rounded canopy that responds well to pruning and pleaching, so it suits more formal streets and avenues where a controlled shape is needed. Its scented summer flowers are a valuable source of nectar for bees, adding pollinator value to the shade it provides. It prefers a deep, fertile soil and a position in full sun or light shade, and copes well with the demands of urban planting.
Acer campestre
Commonly known as field maple, this hardy native is a smaller tree than the limes and hornbeam, which makes it useful on narrower streets and in tighter spaces where a large canopy would not fit. It carries dense foliage that colours warm yellow in autumn, supporting numerous moth caterpillars while its flowers offer early nectar for pollinating insects. It tolerates most well-drained soils in sun or partial shade, and its compact, adaptable habit makes it a dependable structural choice for confined urban schemes.
Pyracantha coccinea
Commonly known as firethorn, this tough evergreen shrub is easily trained flat against a wall, where its dense, glossy foliage shades and insulates the surface much as a climber would. Clusters of white flowers in early summer give way to bright red berries that feed blackbirds, thrushes and other birds through autumn and winter. It grows in most soils in sun or partial shade, needs tying in to its support while young, and stays dense and well-covered with a trim after fruiting.
Tillia cordata
Acer campestre
Pyracantha coccinea ‘Red Column’
Hydrangea anomala URBAFLORA® ‘Terra Firma’
Hydrangea anomala URBAFLORA® ‘Terra Firma’
A versatile climbing hydrangea that works either as self-clinging cover for walls or as groundcover at low level, giving designers two uses from one plant. The semi-evergreen, glossy foliage holds through much of the year, and the early-summer flowers draw pollinating insects. It grows in sun or shade across a wide range of soils, provided they stay moist but well drained, and its dual habit suits vertical greening and the softening of hard surfaces that store urban heat.
Vinca minor
Commonly known as lesser periwinkle, this evergreen trailing groundcover forms a dense mat that keeps the soil beneath shaded and cool. It produces violet-blue flowers from spring into summer and quickly covers awkward ground where little else will grow. It grows in almost any reasonably drained soil from full sun to deep shade, and a hard cut back in spring keeps the cover dense and even. A dependable choice for the lower layer of a cooling scheme.
Hedera Algerian ‘Bellecour®’
A semi-evergreen groundcover ivy that spreads widely without climbing, which keeps it clear of building facades where traditional ivy can cause problems. Its larger, brighter leaves form dense cover that shades and insulates the ground, suppresses weeds, and provides shelter for invertebrates. It performs in full sun through to deep shade and in a wide range of soils, tolerating both waterlogged and dry ground once established, which makes it adaptable across the varied conditions of a built-up site.
Vinca minor
Hedera Algerian ‘Bellecour®’
Designing planting that keeps cities cool
Good species choices only deliver if the planting is set up to last. The single most important factor for trees is rooting space. A street tree in a small, compacted pit will struggle to establish, draw up little water, and provide a fraction of the cooling it could. Generous soil volume, structural soil where loading is an issue, and reliable water during the establishment years all pay back in canopy and shade later on.
Layering is the next principle. A canopy tree shades the street, climbers green the walls that would otherwise radiate heat, and groundcover keeps the soil cool and moist between them. Each layer reaches a surface the others cannot, and together they cool far more effectively than any one element alone. Pairing planting with permeable surfaces and sustainable drainage helps too, holding water in the ground where roots can reach it and evaporation can continue through dry spells.
We grow all our plants peat-free across the business, and supply at the scale and consistency that large urban schemes depend on. Choosing resilient, well-grown stock and giving it the conditions to thrive is what turns a planting plan into lasting cooling rather than a row of struggling trees.
No single tree or border will cool a city on its own, but the cumulative effect of planting across streets, roofs, walls and open ground is substantial, and it grows year on year as canopies fill out and cover establishes. As summers warm, the green infrastructure cities plant now is some of the most valuable they can invest in.
For more information about planting for urban cooling, speak to the G Team today.