The great tree debate: which varieties are best suited to our changing climate?

From the moment a tree is planted, it tells a story. Not just about the weather — as it twists in the wind, bends in a storm, drops leaves in a heatwave — but about the people who planted it, the times they lived in and their influences.

The 2020 Plant Atlas, released this week by the Botanical Society of Britain and Ireland (BSBI), tells a complicated and alarming story about our planting habits, both past and present. The 20-year study found that more than half of our native species are in decline and we now have more non-native species in the wild than native ones — not in volume but in variety.

Dr Kevin Walker, head of science at the BSBI, says he wasn’t surprised by the findings. “We’ve introduced tens of thousands [of species] from all over the world into our gardens as ornamental plants.”

Aggressive agricultural and forestry practices, habitat loss and climate change are the main factors blamed. But while farming practices can be improved and habitats protected, does the pace of climate change mean that we must accept the need to embrace non-native plants, specifically trees — that can cope with our changing weather?

We only have about 40 native tree species in the UK, and many are struggling. Aside from climate change, there is the ever-present threat of pests and diseases; from Dutch Elm disease to Ash Dieback and oak processionary moth. With every new disease the number of native species it seems wise to plant, especially on scale, gets smaller.

Nick Phillips, principal forestry policy advocate at the Woodland Trust, says that the only way to fight these diseases is for anyone planting a tree to make sure they are only buying plants grown in the UK. “If you’re going to plant a tree the number one thing that you have to ask is where the tree has come from,” says Phillips. “We’re potentially losing more hectares of trees due to pests and diseases than anything else.”

But a number of horticulture and arboriculture professionals are putting forward the argument for more ecologically driven ways of planting — suggesting that in order to ensure the trees we plant today thrive and provide shade for all species in our heating climate, we need to plant trees from other parts of the world.

Arboriculturist and author Tony Kirkham says when choosing tree species for our gardens it’s time to think outside the box. “I’m not anti-native, but if we just stick with natives we are just going backwards.” Rather than worrying about a species’ origin, Kirkham says we should focus instead on its eventual height and spread; “we are going to need canopy cover for shade and cooling — some trees just aren’t going to survive as a single specimen open to all the weather elements.”

When we think of non-native species, it’s easy to jump to the extreme and assume that everything non-native is going to be invasive. One might point to the untold damage still being caused by Rhododendron ponticum, which has taken over vast areas in Scotland and Wales, blocking out the sun and smothering native species. The BSBI highlights other problematic species, such as Cornus sericea, which invades wetlands, and Lodgepole Pine, which collects in bogs and heaths.

The BSBI urges caution when planting non-natives. But that doesn’t mean not planting anything non-native at all; so how do we justify this sort of intervention and mitigate any damage done by moving plants and trees around?

Errol Fernandes, head of horticulture at the Horniman Museum and Gardens in south-east London, points to history for some lessons here. “Plant species have been moving all over the globe for millennia. The tectonic plates shifted and with them so did flora and fauna. As birds and animals move and migrate, they transport seed and for as long as people have walked the earth, they have carried plants with them.”

In 2021 Fernandes and his team planted a Miyawaki-inspired micro-forest alongside the South Circular Road boundary of the museum’s gardens. Some 900 saplings went in, with 35 species being represented in a space a little over 400 square metres. Yet instead of adopting the traditional Miyawaki method of planting exclusively native trees, the plant list included a mixture of tree whips that instead were chosen for their ability to complement the site’s unique topography, with areas subject to being very wet in winter and dry in summer.

“This idea of native and non-native is fascinating for so many of us right now,” says Fernandes. “We know that there are intrinsic links and interdependence between certain plant and animal species that have evolved alongside each other,” he continues, stressing that it is important not to exclude native species from our plant selections.

As well as considering what plants we can introduce from abroad to cope with the UK weather as it changes, we should also look at how our existing tree stock is evolving, says the writer and garden designer Darryl Moore.

“Plants have been around a lot longer than we have on the planet, they’re obviously pretty adept at adapting,” says Moore, whose recent publication Gardening in a Changing World: Plants, People and the Climate Crisis considers our approach to plant selection in great detail, with a focus on naturally occurring plant communities and what we can learn from them.

“That’s one thing that people don’t think about when they talk about the future of planting,” he says. “Plants have strategies to deal with the situations they find themselves in. Some may not adapt quickly enough. Others will adapt and carry on.”

Phillips agrees on this point, highlighting the importance of genetic diversity within planting. On a larger scale, an underused method of ensuring this, he says, “is to support trees to breed and spread their seeds themselves, allowing natural colonisation”.

If we just stick with natives we are going backwards. We are going to need canopy cover for shade and cooling — some trees just aren’t going to survive all the weather elements

“Trees are not very adaptable as individuals but as a habitat they can be extremely adaptable because they have that genetic diversity. The ones that can cope with the changing climate will survive. The ones that are struggling will die out and you are getting that natural selection speeding up.”

In the south of England an increasing number of gardeners are finding inspiration for hot, dry summers in Mediterranean-style planting. But both Kirkham and Moore point to the limitations of these schemes — for example, they may not be best suited to a site in the UK that experiences heavy winter rainfall or weeks of snow. Instead, they suggest, gardeners should look to the east of the US and the Eurasian steppe for planting ideas.

“Many temperate trees that would grow well in our climate closely resemble our native species in appearance — size, shape, habit, leaf,” says Kirkham. We’re probably familiar with the species but not the particular variety, he suggests. So much so that if you were to replace them with these different varieties most people wouldn’t even be aware that they were walking in a woodland of non-native species.

Neither, Kirkham proposes, would our native wildlife be able to distinguish the difference. “I am sure that many of our native biodiversity such as birds, mammals, fungi and invertebrates wouldn’t be put off using them as habitat, not knowing that they were non-native.”

Although the Woodland Trust supports non-native tree planting in the right context — for example, for creation of timber, or in an urban setting where conditions are extreme — they still maintain that planting exclusively native woodland is essential to the survival of ecosystems.

“We particularly favour native trees where they’re appropriate because of their wildlife value,” explains Phillips, although he also acknowledges that there has been much more research done into native plants and their role in ecosystems than non-natives.

The RHS, meanwhile, has conducted research into plant-dwelling invertebrates that suggests that while it’s important to have a majority of native species in a plant collection, near-native species still support a similar amount of invertebrates. Other factors to consider when planting for wildlife are things like leaf cover and planting density.

If you’re going to plant a tree the number one thing that you have to ask is where the tree has come from

As for mammals, birds and fungi, we still don’t know enough about the interdependence of certain species on certain plants but if we take the long view, the health of a tree or woodland could arguably be the most important factor in sustaining an ecosystem.

An example of an alternative tree choice, Kirkham proposes, would be the oriental beech, Fagus orientalis, from the Caucasus regions. “This looks very similar in appearance to our native beech (Fagus sylvatica),” he says, “but will be more at home in the south of England as it comes from the hotter regions with cold winters.”

Kirkham also suggests that some of the American oaks from the eastern states of the US, such as Quercus palustris, Q rubra or Q coccinea, would be at home in the UK and will eventually make large shade trees, as will yellow poplar (Liriodendron tulipifera) and black walnut (Juglans nigra).

Another area where Kirkham and Moore agree is on the need to be more ambitious with our planting, watching and learning as we go and taking more risks. “It’s always a work in progress,” says Moore of choosing plants.

In turn, resilience will come from diversity. “You’ve got to try things out and build things up so they have layers like a natural plant community,” Moore says. “Plants are really incredible life forms that have adapted over this period of time. They’ve been through an awful lot in terms of the planet’s history, and now we’re trying to understand what they do.”

It’s easy as a gardener, when choosing a plant, to be totally overwhelmed by the options and factors that you have to consider; from soil type to shade and rainfall. And now that our weather is becoming increasingly unpredictable, that is more true than ever. So how do we as gardeners, and humans, find a role for ourselves and avoid being nihilistic?

“We’re so good at demonising ourselves,” says Fernandes. “Where does that leave us if we feel like, at every corner where we make an intervention, we’re wrong? I feel that we should be considering ourselves as keystone species [ . . . ]. We can choose to be a positive force for good or a force that only causes damage. Seeing ourselves as part of an ecosystem, learning from nature and not assuming that we know everything. The lesson is ahead of us.”

One area where the relation between tree provenance and the wider cultural landscape is contentious is through the intertwined history of plant hunting and colonialism. The Victorians in particular were famed for gathering plant trophies and were responsible for introducing the most notorious of all invasive plants, Japanese knotweed, to the UK in the middle of the 19th century.

Fernandes also voices concern over the very words used around non-native plants. “Sometimes the language expressed in relation to species and horticulture is toxic — alien species, non-native, invasive,” he says.

“We could then take a short side step into thinking about migrant communities, people who perhaps may not always feel welcome in our parks and gardens. We could take that thought further and start to think about why it is that the horticultural profession lacks diversity across the board.”

Encouraging more consideration of the trees we choose and their origins is powerful. Our gardens do not exist in isolation and, as ever, we have much to learn from them. Challenging the status quo and thinking in new ways about our plant choices should not only help make our gardens more sustainable in the long run but also strengthen the culture around gardening.

With a bit of thought and an open mind, we can plant trees today that will thrive and be resilient in our changing climate, telling a positive story about us to future generations.

Find out about our latest stories first — follow @FTProperty on Twitter or @ft_houseandhome on Instagram