New research reveals garden trees for the future

New collaborative research from Frank P Matthews and the RHS reveals the ability of garden trees to deliver a broad range of benefits under the UK’s changing climate.

Of 19 popular trees largely chosen by gardeners for their aesthetics and tested by the charity’s scientists at RHS Garden Wisley, all of them were found to deliver environmental benefits in at least one area, whether that be in cooling, flood mitigation, carbon capture or support for wildlife. With the UK set to see hotter summers over the long term, the deciduous trees in the study provided highly effective cooling via transpiration and shading when in leaf, with ‘Callery’ pear (Pyruscalleryana ‘Chanticleer’) closely followed by flowering cherry (Prunus ‘Fugenzō’ and red maple (Acer rubrum ‘Brandywine’). Italian cypress and western red cedar (‘Cole’s Variety’) were the most effective evergreens. Effectiveness was linked to large dense canopies and high water uptake.

Callery' pear (Pyrus calleryana 'Chanticleer — Callery pear Pyrus calleryana ‘Chanticleer

Cherry Prunus 'Fugenz — Cherry Prunus ‘Fugenzō’

Acer rubrum 'Brandywine' — Acer rubrum ‘Brandywine’

The cherry maintained its capacity to transpire and cool even when temperatures were around 35° C and humidity was low, while the other trees in the experiment plateaued or declined. The cypress (Cupressus sempervirens ‘Garda’), holly (Ilex aquifolium ‘J.C. van Tol’) and pear provided the most transpirational cooling relative to canopy size – a useful attribute for smaller gardens.

On account of its dense canopy, the cypress also proved the most effective for canopy rainfall storage, wind shelter and screening.

As autumn marks the start of tree planting season, the partners recommend that gardeners consider what services they might need trees to perform in their garden and select varieties accordingly, also taking into account soil conditions and garden location.

Top performing trees for…	Description
Cooling via transpiration	Prunus ‘Fugenzō’ – handsome tree, that goes on cooling even in the heat and alongside sufficient soil moisture being present
Cooling via shading	Acer rubrum ‘Brandywine’ – attractive garden tree with wide planting potential
Canopy rainfall storage	Thuja plicata ‘Coles variety’ – other Thuja just as good, makes a large tree but can be clipped or used as a hedge
Soil saturation mitigation	Crataegus x media ‘Crimson Cloud’ – good garden tree, easily pruned, pollinator friendly, likely to tolerate wetter soils in summer where it can transpire away the water
Carbon sequestration	Pyrus calleryana ‘Chanticleer’ – makes a sizeable but upright tree. Fruits and flowers of some varieties valuable to wildlife. Many with some tolerance to warm and somewhat dry conditions
Wind shelter and screening	Prunus lusitanica ‘Myrtifolia’ – Evergreen so winter shelter, and can be grown as tree or hedge, very robust
Pollution mitigation	Corylus avellana ‘Red Majestic’ – masses of foliage to mop up pollution, like all hazels is easily pruned and makes a fair informal hedge
Pollinator support	Malus ‘Scarlett’ – good garden tree with abundant spring flowers and like all apples supports a wide range of insects

Dr Mark Gush, Head of Environmental Horticulture at the RHS, said: “[MBG1] In considering potential trees to grow, gardeners are increasingly mindful of looking beyond just the aesthetic, towards species that will also provide a range of benefits while being resilient to changes that we are now experiencing in the climate. Long-term detailed and rigorous field monitoring studies like this one are extremely valuable in providing evidence-based guidance on this complex topic.”

Dr Mark Gush, Head of Environmental Horticulture at the RHS, said “In considering potential tree species to establish in their spaces, gardeners are increasingly mindful of looking beyond just the aesthetic, towards species that will also provide a range of benefits while being resilient to changes that we are now experiencing in the climate. Long-term detailed and rigorous field monitoring studies like this one are extremely valuable in providing evidence-based guidance on this complex topic.”

Nick Dunn, Managing Director of Frank P Matthews tree nursery, said: “This important project has offered an opportunity to accurately endorse garden trees both native and non native as key contributors to a broader environmental benefit than was previously accepted”.

Lis Larsen, Lead Researcher, said: “This project aimed to show how garden trees offer different benefits, and how this knowledge can support more mindful tree selection. By thinking carefully about what we need in our own outdoor spaces, we can also contribute positively to the wider neighbourhood.”

The partners established six replicates of the 19 semi mature tree taxa, at its Field Research Facility at RHS Garden Wisley. The trees were chosen based on their prevalence in gardens¹, compact size and anecdotal evidence of their potential. Two separate 2-year trials were conducted with 9 taxa monitored between 2021/2022 and 10 taxa between 2023/2024. All were connected to sap flow sensors and soil moisture probes to quantify their water uptake and cooling ability under irrigated conditions, while their leaf area, stem growth and photosynthetic rates were also measured to establish their wind screening and pollution mitigation potential, carbon sequestration rates and pollinator support.

For the complete guide to the environmental benefits of the trees tested see ‘Ecosystem services of garden trees’ and for the paper published in Frontiers in Environmental Science visit: Frontiers | Impact of environmental drivers on ecosystem service-delivery of small garden trees in a temperate climate.