Lavender is meant to help you sleep, but it keeps Nicola Spence awake at night. One of the many pests and diseases that worry England’s chief plant health officer is Xylella fastidiosa. Lavender species are welcoming hosts for the bacteria.

Xylella has not yet arrived in the UK but it has devastated olive groves in Italy and is spreading through Europe. It would be a serious threat to British fruit growers, and native oaks and elms, drying them out and eventually killing them, so the Royal Horticultural Society (RHS) has banned imported lavender and several other plant groups from the Chelsea Flower Show.

“If we found Xylella at Chelsea, we’d have to close the whole thing down,” Spence says. It’s not an idle threat. “In the build-up, we’ve got two inspectors on site for two weeks checking everything with the RHS biosecurity team.” She pauses. “We found some lavender last year. It was on the bottom of a trolley and someone had forgotten.” The plants were tested and were disease-free, but the incident underlines the fragility of Britain’s biosecurity.

We meet on the frontline of the UK’s biosecurity border, at Provender Nurseries in Kent. The nursery, a main grower and distributor of plants and trees to garden centres and landscapers, hosts an inspection point for the Animal and Plant Health Agency (Apha). Lorries come here directly after crossing the Channel and unload their plants inside a secure warehouse – even the ground is specially sealed – for officers to examine.

Plant health and the environment are now considered to be integral to the UK’s national security. It is not just making sure that Xylella does not infiltrate Britain’s gardens. It is also ensuring that pests and funguses do not devastate crops, as Phytophthora infestans did in 1845, causing the Irish potato famine.

The variety of threats is enormous. A stray lavender tray may seem banal, but in the US, a caver’s boot was responsible for a disease that is devastating agriculture. The fungus Pseudogymnoascus destructans was accidentally imported about 20 years ago on the footwear of cavers returning from Ukraine to the US. It causes “white-nose syndrome” in several species of North American bat, wiping out at least 90% of some populations. Beetles, caterpillars and other crop pests that are usually eaten by the bats have thrived. In response, American farmers have increased pesticide use by more than 30% – a factor that may be linked to higher infant death rates.

Even more concerning for UK security services is the possibility that a pathogen might be introduced deliberately. Last year, a Chinese couple were arrested after allegedly smuggling into the US a wheat-destroying fungus that was described by American attorney Jerome Gorgon Jr as a “potential agroterrorism weapon”. In 1989, a disease broke out among cocoa plants in southern Bahia in Brazil that was reported to have been deliberately introduced to attack the plantation owners.

“There is concern in the counterterrorism and homeland security network that plant products could be a risk,” Spence says. “We haven’t seen any plant pests and diseases used for deliberate release, but it’s obviously something that we think about and are aware of.”

Some households in the UK received mysterious packets of seeds in the post last year, sent from China. It was a resurgence of a phenomenon seen in 2020, when thousands of packets arrived unsolicited. Spence’s team at Apha investigated and found they were harmless – they appeared to be part of a supposed “brushing scam”, where vendors use the delivery as evidence of a sale so they can post fake reviews online – but “things like that can pop up from nowhere”, she says. “If you were intentionally wanting to spread something, it’s a pathway.”

Natural threats such as Xylella are more pressing concerns, and the types and variety of them are growing. Spence is the first chief plant health officer, appointed in 2014 after the post was created in response to the emergence of ash dieback, a fungal disease that is on its way to killing as many as 80% of Britain’s ash trees.

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She retires on Friday, leaving behind an agency that has grown from 100 inspectors to about 400, working at border control points across England. The risk register she launched a decade ago has expanded from 600 to 1,400 pests and diseases, and between five and 10 new threats are assessed each month. Her replacement, who will have to face all this, is yet to be announced.  

“We’ve definitely seen an increase in risk in numbers of pests and diseases affecting plants and trees over the past 30 years,” Spence says. “I would say there’s been a sharp increase. It’s driven by climate change because some pests and diseases can establish [themselves] now if they get here.” Milder or wetter winters also allow some pests to survive that would have died previously.

Unsolicited seed packets and importing diseased plants are two pathways, but there are others. Spence reels off wood packaging – particularly wooden shipping pallets, hitchhiking bugs, fungus spores blown across from France, and even furniture imports.

That includes “headboards from very reputable furniture retailers”, Spence says. “You take it apart and it’s riddled with longhorn beetles, which can get into our native trees.”

Keeping disease out seems like an impossible task, but Spence says inspecting at border points and monitoring the environment with the help of the Forestry Commission reduces risks.

“We’re even using detector dogs, as well as asking the public to look out for things,” she says. “We’ve got helicopters, drones – we fly over a million hectares [almost 2.5m acres] of forest every year and take photographs to look at the canopy condition. We use satellites with spectral filters, which can tell a diseased plant or tree, what species it is and whether it’s under stress. And then we send our person or our dog [to inspect it].”

That is assuming the inspector or dog can reach the tree. One emerging problem is rewilding, she says.

“I worry about that. We would rather woodlands were managed, because then we can see early warnings of pests and diseases,” Spence says. “We have found some pests and diseases in neglected, wilded patches that have just been left alone.” In south-east England, Apha helicopters have spotted damage to trees that is evidence of eight-toothed spruce bark beetles, which bore into trees and eventually kill them, causing destruction in rewilded areas.

“[But] they’re impenetrable. So we know where they are, and then can’t get to them.”

People have bought small, one-acre plots of woodland with the best of intentions, she adds. “They think, ‘Oh, this would be lovely for my family.’ And then we find a disease and, of course, they’re totally unprepared for that.”

After she stands down this week, Spence, an RHS trustee, will “spend more time with plants and trees” and the people who tend them. At Chelsea next year, she will be a carefree attendee.

But, please, don’t bring any banned lavender.

Illustration by Fine Art Images/Heritage Images via Getty Images

Photographs by Andy Hall for The Observer, FlowerPhotos/Universal Images Group via Getty Images