Three giant redwoods tower over Wakehurst’s Elizabethan mansion like skyscrapers. Yet at 40 metres (131ft) high, these are almost saplings – not even 150 years old and already almost twice as high as Cleopatra’s Needle.

“At the moment they’re some of the tallest trees in the UK and they are starting to poke above the forest canopy. But if they grow to their full potential, they’re going to be three times taller than most trees,” says Dr Phil Wilkes, part of the research team at Wakehurst, in West Sussex, an outpost of Kew Gardens. One or two of these California imports would be curiosities, such as the 100-metre high redwood that was stripped of its bark in 1854 and exhibited to Victorian crowds at the Crystal Palace in south-east London, until it was destroyed by fire in 1866.

But there are more than just a handful of giant redwoods in the UK, and Wakehurst has many more than these three. The Victorians were so impressed that they brought seeds and seedlings from the US in such large numbers that there are now approximately 500,000 in Britain, according to a Forestry Commission estimate that includes coastal redwoods and dawn redwoods (a separate Chinese species) as well as the giant redwoods. Experts at Kew think most of the UK trees are giant sequoias, the official name for giant redwoods. California has about 80,000 giant redwoods, as well as also having coastal redwoods and a few ornamental dawn redwoods imported from China.

When Wilkes and his fellow researchers at Kew and University College London highlighted the numbers last week, they provoked a wave of interest, and visitors to Wakehurst’s gardens have talked of little else.

“People are often worried that they’re an invasive species, but they seem to be pretty benign,” Wilkes says. “There’s no evidence they’re self-seeding.”

This could be because they are juveniles not yet ready to reproduce – redwoods live up to 3,000 years – or because their cones only usually open up in the heat of a forest fire. That means each of the UK’s trees was probably planted. It also explains how half a million giants have managed to hide in plain sight.

“They were prized possessions,” Wilkes adds. “Quite often they were planted at a manor house and they planted a driveway with rows of redwoods. And these houses have burned down or been demolished but the redwoods still exist.”

Redwood aficionados have charted some of the locations, from people’s back gardens to parks and suburban streets where homes have been built around the trees. But as the redwoods have grown, so have opportunities for conflict, such as in Canons Drive in Edgware, north London, where some residents are fighting to protect an avenue of giant sequoias under threat from insurance companies concerned about roots undermining the houses.

There are no such concerns at Wakehurst, where the redwoods are in several parts of the extensive grounds and arranged phytogeographically, with plants and trees laid out according to their continent of origin, so visitors can walk through the gum trees of Australia down into a North American valley.

In the redwood glade, sprinkled with sunlight and rain and birdsong, Wilkes’s semi-permanent smile turns into a beam. “The feeling of being in a forest, anywhere in the world, is just second to none,” he says. His work involves using satellite data and Lidar – light detection and radar – to create 3D laser images of trees, a way to measure the size and mass of trees more precisely than the traditional method of measuring their trunks’ circumferences.

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“You go out in these vast expanses of forest and they’re just a different world when you get under the canopy, off the beaten track. It really pulls you in. It’s so captivating – it’s one of the most complex environments you can work in, and it’s really satisfying.”

Trees have an enduring appeal to people. Last year, researchers at Derby University found that people value trees more highly than their neighbours, while forest bathing – a western interpretation of the Japanese practice of relaxation known as shinrin-yoku – has grown in popularity.

Perhaps the unique appeal of the redwoods is their scale; the oldest of them existing before the English language and the tallest of them, at 115 metres, higher than St Paul’s Cathedral. And possibly they would have been felled, like most of England’s forests, had they been discovered by the Elizabethans who built Wakehurst and believed the task of man, as the historian Keith Thomas put it, was to “level the woods, till the soil, drive off the predators, kill the vermin, plough up the bracken and drain the fens”.

In the more ecologically enlightened 21st century, Wilkes identifies a different risk: that the desire to find fixes for the climate crisis will lead to rash choices.

“In Wales, they are planting them as a way of offsetting your carbon emissions. Is putting a plantation of redwoods in the Brecon Beacons [Bannau Brycheiniog] the right thing to do?”

He is dubious, saying native broadleaf woodland has many more benefits than just as a carbon store.

“Urban trees are not valued, but actually they do have a lot of value,” he says. “Carbon is one way you can give them a value, but it’s probably one of the least important things they provide – they cool cities, there’s flood mitigation, health impacts, biodiversity. They’re not a means of offsetting carbon. Decarbonisation is the only way.”

California’s teetering giants

For millions of years the world’s tallest trees have graced California peaks and coastlines, growing through centuries of changes. The towering redwoods that first took root in groves through the Sierra Nevada mountain range are as resilient as they are stunning.

But those landscapes have seen significant shifts over the past century and the forests have suffered. Spurred by the climate crisis, devastating droughts and scorching temperatures have added new stressors for the redwoods, particularly the famous giant sequoias, which now struggle to bounce back after big wildfires.

Part of the problem stems from California’s gold rush era, when settlers descended with an overzealous appetite for good timber, cutting down much of the old-growth forests. They also suppressed indigenous land management techniques, which included setting “healthy” fires that cleared out the forest. A century of fire suppression created an overabundance of vegetation that set the stage for larger, more catastrophic blazes.

Robbed of the most resilient ancient trees, forests now face a devastating cycle: the trees that die leave more fuel for dangerous fires. Vulnerable trees are also increasingly under attack from native bark beetles, insects that feed on their spongy red trunks until they topple. Scientists estimate that roughly a fifth of California’s remaining giant sequoias have died in recent years due to this combination of factors, including one particularly severe 2020 wildfire that wiped out up to 10,000 mature trees.

There are efforts underway in California to protect them as federal agencies, states, and indigenous communities work to bring good fire back to the land, and seed decimated landscapes with new trees. Threats from global heating continue to mount and changes are outpacing mitigation work.

Along with their picturesque stature, the trees are also crucial to maintaining healthy ecosystems in California by capturing carbon dioxide, providing a cooling effect when temperatures spike, and they are vital habitat for other forest creatures. When they disappear, the landscapes will be forever changed, along with the plants, animals, and people that have grown to depend on them.

Gabrielle Canon, San Francisco

This article was amended on 17 March 2024. The subheading was amended to specify giant redwoods rather than all redwood trees. Some text in the article was also amended to clarify the makeup of the 500,000 redwood trees in the UK.