Britain’s overheating cities have the fewest trees
Recent research from Ordnance Survey, the Met Office and 4 Earth Intelligence has named the British cities most vulnerable to extreme heat. Portsmouth, Leicester, Exeter, Bristol and Nottingham are projected to warm the most by the end of this century.
The headline figure is the one that stopped me. By 2040, 71 cities across England, Scotland and Wales will reach high heat vulnerability. Almost two-thirds will reach the most severe level by 2100.
I grow trees for a living at our nursery and I read this report differently from how a planner or a journalist might. To me, a heat vulnerability map highlights the areas of the UK that need our trees to help the most. For gardeners up and down the UK, it should be a call to action to invest in shade, green space & living cover.
The research makes that explicit. The cities most at risk of extreme warming are those with the least tree cover, green space and waterways.
What the research says
The OS heat vulnerability index compared current temperatures with the structural characteristics of Britain’s built environment, then projected those forward using climate data. It is a serious piece of work, and the findings are specific.
Portsmouth holds the highest heat vulnerability now and through to 2100, with 83% of British cities already in the high vulnerability category.
Leicester will see the biggest rise in the rankings, climbing from tenth today to fourth by 2100.
The City of London and the City of Westminster are among the most heat-retentive urban environments in the UK, with the highest built-environment score and the least effective natural cooling of any city.
The mechanism behind these numbers is simple physics. Concrete and asphalt absorb solar radiation during the day and hold onto that heat through the night. Hard surfaces act like radiators. Cities can be up to 10°C hotter than the surrounding countryside because of this effect.
The UK recorded its first ever 40°C temperature in 2022. Heatwaves are projected to increase in both frequency and intensity. The report tells us where that heat will bite hardest.
Trees make a measurable difference
The cooling effect of trees is commonly treated as a pleasant side benefit. The data says otherwise.
Weather measurements in Leeds demonstrated that urban treescapes reduce harmful heat exposure by up to 96% compared with nearby grey spaces. Areas with tree cover ran around 1.4°C cooler on average than built-up streets, and up to 5°C cooler during the hottest summer periods. When Leeds city centre hit 34°C, nearby urban woodlands were more than 6°C cooler.
The health consequences are measurable too. Research found that London’s urban forest helped avoid 153 heat-attributable deaths between 2015 and 2022, including 16 during the 2022 heatwaves. Areas with higher canopy coverage stayed 2.0°C cooler at night during those heatwaves.
Increasing tree coverage by 10% would reduce urban heat island-related mortality by a further 10%, according to that same London research.
Night-time cooling matters most. Heat becomes dangerous when the body gets no overnight recovery. Trees deliver exactly that relief, lowering minimum temperatures when hard surfaces are still releasing stored heat into bedrooms.
A mature oak (Quercus robur) or lime (Tilia cordata) is a piece of cooling infrastructure that improves with age, supports pollinators and costs a fraction of any engineered alternative.
Nearly half of urban areas lack tree cover
Research commissioned by Friends of the Earth found that nearly half of neighbourhoods in English towns and cities have less than 10% tree cover. Trees and green spaces lower night-time urban temperatures by up to 5°C, yet over a third of neighbourhoods lack adequate access to green space.
The Portsmouth findings add a difficult layer. The city’s most heat-exposed districts, including Somerstown, Portsea, Southsea and Fratton, overlap with areas of socioeconomic deprivation. Limited green space and extensive hard surfaces concentrate the risk among people with the fewest resources to escape it.
The places that need shade most have the least of it.
That finding should shape where planting effort goes. Canopy in an affluent suburb is welcome. Canopy on a street with none is urgent.
What you can do with a garden and a spade
A single garden tree will not cool a city, I will be honest about that. Tens of thousands of garden trees, planted street by street across Portsmouth, Leicester, Bristol and Nottingham, shift the canopy figures that this research says matter.
Urban tree cover is built plot by plot. Councils own a minority of urban land. Gardens, front drives and small landscapes make up a huge share of the space where canopy can actually grow. If you own any of it, you hold part of the answer.
When I help someone choose a tree, I start with three questions. What space do you have. What do you want from it. What are you trying to achieve over the next thirty years. The right tree for a small Oxford terrace garden differs from the right tree for a half-acre plot outside Cheltenham.
Practical starting points
Small gardens: a rowan (Sorbus aucuparia) or crab apple (Malus) gives shade, blossom and berries for birds in a 4 to 8 metre footprint.
Medium plots: a field maple (Acer campestre) or silver birch (Betula pendula) delivers meaningful canopy within a decade.
Larger spaces: native oak, lime or hornbeam build the kind of dense summer shade the Leeds measurements recorded.
Plant on the south or west side of the space you want to cool. Deciduous natives give you shade in summer and let winter light through, which keeps heating bills down too. Environmental benefit and practical self-interest sit in the same decision.
Position matters as much as species. A tree shading a patio, a south-facing window or a child’s bedroom wall earns its place every July.
Think long term
The projections in this research run to 2040 and 2100. Trees work on the same timescale. A tree planted this autumn will be casting useful shade by the 2030s and substantial shade by 2040, exactly when the index says all 71 chartered cities reach high vulnerability.
That timing is the whole argument for acting now. Canopy cannot be bought off a shelf when the heatwave arrives. It has to be grown, and growing takes years. Every season of delay is a season of cooling lost.
At our nursery we grow in peat-free compost, harvest rainwater and deliver by electric van, because the way a tree reaches you should match the reason you plant it. The same logic applies to the bigger picture. A credible response to urban heat has to be built with the same care across supply, planting and aftercare.
If you live in one of the named cities, or anywhere with more paving than canopy, you can act this season. Measure your space, think about what you want standing there in 2040 and beyond, and plant the tree that fits. Your garden becomes part of the cooling infrastructure this research says we need, and the people who live on your street decades from now will feel the difference on the hottest day of their year.
