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The theory of ‘botanical sexism,’ or planting ‘male’ instead of ‘female’ trees, debunked

Adobe, Ally Rzesa/Globe Staff

It is a tantalizing — or at least amusing — idea: For half a century, urban foresters in league with the US Department of Agriculture have engaged in “botanical sexism” by preferentially planting “male,” rather than “female,” trees. The result has been worsening allergies across the developed world, due to the overabundance of pollen-spreading males.

Or so claims Tom Ogren, a California man who holds a master’s degree in agricultural science and describes himself as “essentially a horticultural epidemiologist.”

“When you only plant males,” Ogren said in a phone interview, “you’ve got a problem.”

If he were just one man with an unusual theory, the story would end here. But he has legions of fans and his ideas have, ahem, taken root around the world, appearing in reputable news outlets, including NPR, The Guardian, and Axios, to name just a few. Last year, after a social media post, the phrase “botanical sexism” went viral, which perhaps should come as no surprise. Here was a theory that tapped into anxieties about sexism and sex itself, and also gave people something to blame as they suffered every springtime from allergies.

But there’s a problem. Legions of experts — allergists and tree botanists among them — say Ogren’s theory just doesn’t add up.


“What he’s saying actually doesn’t make sense,” said Richard Primack, a Boston University biology professor and expert on tree reproduction.

Another skeptic, agronomist Sarah Taber, put it somewhat more colorfully in an exasperated Twitter thread last year. “So this is [expletive] right off the bat lol,” she wrote.

Ogren’s theory boils down to this: Starting in the 1960s, Dutch elm disease swept across the United States, killing off the elm trees that lined American streets. As city arborists replenished the urban tree population, they made a fateful decision, Ogren claims — they planted almost exclusively “male” trees. Male trees produce pollen and females don’t. The result, Ogren says, is that city air in springtime now holds an unnaturally high concentration of pollen.


Today, Ogren contends that the “gender balance” among city trees is out of whack.

And the consequences, he says, have been devastating: a sharp rise in the rates of allergies and asthma during the past half century and exacerbated hay fever symptoms in cities across the country.

Usually avuncular, Ogren showed flashes of frustration in the phone interview about criticism of his work, referring to his detractors as “every two-bit [expletive] telling me, ‘You don’t know any botany.’” He said botanical sexism — “probably the only original thought I ever had in my life” — has been suppressed by powerful interests, including real estate brokers and the garden nursery industry. “The powers against me are big time,” he said.

A gardening writer and teacher, Ogren laid out his theory in a 2004 book, “Safe Sex in the Garden,” and it soon captured the public’s — and the media’s — imagination. “Out in the countryside, you basically have sexual diversity of plants,” Ogren said in a 2006 “All Things Considered” segment aired on NPR, whose host, Robert Siegel, gamely took the bait.

“But once you’re in the city, it’s a man’s world when it comes to trees,” he intoned.

In the years since Ogren’s NPR appearance, experts have disputed the foundations of botanical sexism, including the notion that trees fit into a neat gender model. A large share of tree species, numerous allergists and tree botanists said, are “monoecious”; that is they sprout both male and female flowers on the same tree. (”Trees do not respect the human gender binary,” wrote Taber, the frustrated Twitter agronomist. “Trees are super [expletive] queer.”) Only a small minority of tree species — the “dioecious” ones — can be said to have two distinct genders.


Ogren knows this, he emphasized, and said he has a solution. We should cultivate special varietals of monoecious trees that only produce female flowers and, thus, zero pollen. But Primack said that approach might not be feasible and noted that planting only female trees could create its own problems. Female trees shed fruit and other byproducts that can cause problems in a city, such as producing extra food for rats.

Also, in regions like the Northeast, we’re surrounded by trees, and the vast majority were not planted by humans. The approximately 40,000 currently living trees planted by Boston’s city arborist, for example, represent “a drop in the bucket” of the allergens Bostonians inhale, said allergist Leonard Bielory.

There is also scant evidence that city arborists are deliberately discriminating against female trees, as Ogren contends. A bewildered spokesperson for the Boston Parks & Recreation Department said, “We do not have a policy of planting one gender of tree versus another.”

But boring facts seldom stop a sexy idea. In the years since Ogren’s NPR appearance, his concept has appeared in The New York Times (in an op-ed by Ogren), Scientific American (another Ogren op-ed), the Detroit Free Press, the Canadian Broadcasting Corporation, Toronto’s National Post, Wired UK, NBC Washington, and many other outlets.


Last year, the theory made the jump to new media and, presumably, an audience with Generation Z.

“You’re sneezing and congested all day because of botanical s*xism,” a TikTok user named @elliebotoman wrote in text superimposed onto a video that received nearly half a million likes.


tom ogren is credited for the term after finding only male trees planted in his community #environmentalscience #urbanplanning #climatechange #tree

♬ original sound - Jazzy Fizzle

“So the patriarchy is also responsible for my allergies???” TikToker @sagey.babey wrote in response.

The viral TikTok video prompted Taber’s exasperated Twitter thread. In her point-by-point debunking, she gave special attention to Ogren’s theory that the flowers of female trees remove significant amounts of pollen from the air.

As Ogren asserted in an interview: “The beauty of a female tree is not just that it does not produce pollen. It also traps pollen.”

To which, Primack, the BU tree botanist, said, “That is just completely wrong.” Female flowers capture such a minuscule portion of the pollen in the air — “like one out of a million pollen grains,” he said — that there’s no way they could impact allergies.

Ogren’s journey to botanical feminism started with what he now regards as a sexist act of his own. Decades ago, he read a book about psychosomatic illnesses, which claimed that most allergy sufferers were women because they were, “whatever, scatter-brained or hysterical,” he said of the book’s thesis, which he now finds preposterous. But at the time, when Ogren’s wife complained of severe allergies, he told her to buck up.


Until one day, while teaching gardening at a prison program, he had a sort of epiphany. He had his students sniff several flowers. After passing around a bottlebrush flower, about a third of the class was sneezing violently “and some still were at the end of the week,” he said.

“As soon as that happened,” he said, “I knew, Whoa, this stuff is not psychosomatic. This stuff is legit.”

Ogren embarked on a mission to remove allergenic plants from his backyard in hopes of relieving his wife’s symptoms. But he couldn’t find a good reference on which plants were most likely to cause allergies. So he wrote “Allergy-Free Gardening: The Revolutionary Guide to Healthy Landscaping,” published in 2000.

It introduced an “allergenicity” rating system, dubbed the Ogren Plant Allergy Scale, which some gardeners still use today. Dr. Clifford Bassett, a New York allergist, said the system is “a helpful tool ... for those who suffer from seasonal allergies.”

He also discovered what he now presents as a sort of smoking gun of the botanical sexism plot. “When used for street plantings, only male trees should be selected, to avoid the nuisance from cottony seed,” says the 1949 USDA Yearbook of Agriculture, in reference to the plains cottonwood tree. (The book makes similar recommendations for other species and, in at least one case, advises planting only the female variety for a species known to have problematic males.) The USDA followed through on its 1949 directive, Ogren says, by widely distributing male-only “cultivars,” or clones, of maple trees.

Like any good conspiracy theory, experts said, botanical sexism contains some truth at its core.

Allergies have indeed gotten worse in recent decades. But experts said that discrimination against female trees is not a cause. “One of the main factors is air pollution,” Primack said. Global warming also plays a role.

There is also something to the idea that planting decisions can impact allergies, just not nearly on as vast a scale as Ogren claims. In the Southwest, for example, in regions where there is little or no natural forest, tree selection could be more impactful, Primack said.

But what if a city implemented the maximal version of Ogren’s plan? What if every town and city in the Boston area replaced every human-planted tree with trees that had only female flowers? Would that bring the allergy sufferers among us some relief?

“Not unless you were willing to destroy the Arnold Arboretum, the Wompatuck State forest, Plymouth state forest, and all of the natural habitat” surrounding Greater Boston, said Dr. John Costa, medical director of Brigham and Women’s allergy and clinical immunology practice.

Even then, he said, “You’d still be feeling the effects of ... a competing allergen that you can do nothing about, which is the grasses.”

Mike Damiano can be reached at mike.damiano@globe.com.