Any hay fever sufferer will be able to tell you when spring has arrived. The change in season is often accompanied by a runny nose, itchy eyes, and for those who have it, worsening asthma. From spring right up until the end of summer, an estimated 13 million people in the UK with a variety of hay fever allergy triggers will feel the assault of pollen on their systems. But many sufferers are saying that they’re struggling more this year than they have before.
Could botanical sexism be to blame? Horticulturist Tom Ogren says that a rise in hay fever could be explained by city landscapers opting to plant male trees instead of female ones. This, he says, means that there’s more pollen in our cities, leading to a worsening of hay fever symptoms.
Taking close-up photographs of ash trees in his neighbourhood in San Luis Obispo in California around 35 years ago, Ogren began to notice something strange about each and every tree. “Every single one I found was male. I couldn’t get a picture of a female ash tree because I couldn’t find any. None,” he says.
Trees can be one of three sexes. They can be monoecious, which means they bear both male and female flowers on a single tree, or they can be dioecious and bear exclusively either male or female flowers. Although many hay fever sufferers are also sensitive to grass pollen, tree pollen released by male flowers is one of the major culprits behind the condition. Ogren says that there is usually a balance between the three sexes, but that balance has changed over the last few decades.
Ogren started observing a strange preference for male trees everywhere he went. “It wasn’t just in my town,” he says. In Toronto, Canada, he noticed that the honey locust trees, which populate the city and which usually have long branches with seed pods on them, had no pods at all, while in England, Ogren noticed that virtually all of the willow trees were male. “Thirty years ago, 40 years ago, most of those weeping willows were female clones, but they don’t live that long, and so they get replaced, and they’ve all been replaced with golden willows, which are all now male clones,” he says.
Curious about the reason for urban cities preference for male trees, Ogren began conducting some research. In a library he stumbled upon the US Department of Agriculture’s 1949 yearbook which states: ‘When used for street plantings, only male trees should be selected to avoid the nuisance from the seed’. As the note in the USDA yearbook hints, male trees do not produce fruits, seeds or pods like female trees do, and therefore are preferred because they keep city streets cleaner. But what male trees do produce instead is pollen.
Ogren says that early city landscapers would cut the male branches from trees grown naturally from seeds and use these male cuttings to regrow purely male cultivated trees without the mess. “Now when I look at Italian Cypress trees, say 30 years old or less, you never see any seeds on them, no matter how big they are. They’re male top to bottom,” he points out. “In the past, the female flowers would trap some of that pollen and remove it. But those female flowers aren’t even there now.”
With pollination, trees either rely on insects to transport pollen to their destination on female flowers on other trees of the same species, or rely on the wind to transport them there. Single-sex trees are reliant on wind pollination. But when streets are filled with male trees, there aren’t any female trees nearby to capture the pollen. “Landscapes that are highly diverse, where they use many different species are often quite allergy-friendly, even though there may be a sprinkling of very allergenic trees used in that landscape,” says Ogren.
Pollen levels are often at their highest closer to the tree. In one 2005 study looking at pollen spread from Scots pine, researchers from Madrid found that half of the pollen produced fell within 11 metres of the tree, while just seven per cent of the pollen made it further than 200 metres. According to Allergy UK, the number of people suffering from hay fever has increased in the past few decades. Just 10 per cent of people in the UK suffered from hay fever in the 1970s, but this figure has risen to 20 per cent today.
Beverley Adams-Groom, chief pollen forecaster at the University of Worcester says that trees don’t usually produce pollen in their early years, but as they get older and then reach maturity, they produce more and more., “If a lot of allergenic trees are planted in a neighbourhood, then the pollen load will increase over the years,” she says, adding that many of the main allergenic trees are monoecious rather than single-sex. “We have seen this happen at our own pollen station in Worcester over the last 25 years, which is located in a suburb where lots of allergenic trees have been planted during this time.”
Simon Creer, professor of molecular ecology at Bangor University says that the planting of trees for landscaping purposes could aggravate hay fever, and points out that silver birch trees are on occasion planted in lines in municipal areas. “They create a striking visual effect, but of all the trees, I understand that birch is the big bad wolf when it comes to people becoming sensitive to birch pollen,” he says.
In order to remedy this, Ogren created the Ogren Plant Allergy Scale (OPALS), which numerically ranks tree species based on their allergenicity. In 2017, the Birmingham City Council made an allergy-friendly garden display at the Royal Horticultural Society’s Chelsea Flower Show, tagging every single plant at its display with an OPALS rating. OPALS is now also used by the USDA’s urban forestry programme, as well as the American Lung Association.
And cities’ predilection for male trees only becomes more alarming when other factors like climate change and increases in air pollution are taken into account. A 2019 study in the Lancet indicates that climate change could be lengthening the pollen season. The researchers looked at airborne pollen data stretching back 26 years in 17 sites across the globe, and found that the rising temperature was associated with an increased pollen load over time. Other studies have shown how air pollutants could damage pollen’s cell wall, forcing the allergen to be released into the atmosphere. “Every time these things burst, they become much more allergenic,” Ogren says. “The worse the air pollution is in a city, the more important it is that we use common sense.”
While a lot of research has been conducted into tree species and their pollen spread, there has been little research into grass pollen, which Allergy UK says is the most common hay fever allergen. Creer has been leading the PollerGEN project, which is looking to identify whether it was the high load of grass pollen, which was exacerbating hay fever or, like tree pollen, different grass species. “When you actually look at the allergenic content of different species of grass, the range of allergens that they produce and the amounts that they produce differ,” he explains. “Knowing that you’re being assaulted by a combination of over 170 different species of pollen in different relative amounts throughout the year, I was really curious to try and find out more.”
But if we are to fix botanical sexism, Ogren says that cities and the nursery industry need to begin taking action, and that starts with labelling plants and conducting pollen control ordinances across towns and cities. “We can’t just keep doing things the same as the way we did before,” he warns. “Whether it kills you or not, many people are being made to feel uncomfortable or miserable and that’s just not good.”
Alex Lee is a writer for WIRED. He tweets from @1AlexL
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