Botanical Sexism: The Claim, the Evidence, and the Debate

Botanical sexism is the claim that modern cities made their own allergy seasons worse — not by accident, but by planting. According to the theory, landscapers and municipalities spent decades preferring male trees and shrubs, because males drop no fruit, seeds, or pods on sidewalks. Those same males, however, produce all of the pollen. The result, proponents say, is urban air loaded with more pollen than a naturally mixed landscape would ever produce. It is a memorable phrase and a genuinely interesting argument. It is also contested — other scientists argue the effect is narrower than claimed, applies only to some species, and cannot explain the broader rise in allergies.

Key takeaways

• Botanical sexism is horticulturist Thomas Ogren's claim that decades of preferring male trees and shrubs for tidy urban planting loaded city air with extra pollen; he set it out in Allergy-Free Gardening (2000) and a 2015 Scientific American essay.
• Ogren's OPALS scale rates plants from 1 (least allergenic) to 10 (most), and he notes it has been used by cities, health departments, and USDA urban foresters.
• Critics, including Jennifer Hu's 2021 analysis in Slate, argue the theory can apply at most to the minority of urban trees that are dioecious — most street trees, like oaks, are monoecious and bear both sexes on the same tree.
• The McGill Office for Science and Society, reviewing the claim in 2024, found little direct empirical evidence that male-biased planting measurably raised citywide pollen counts.
• Both sides agree that individual planting choices have local consequences, and that urban pollen burdens are also rising for reasons — longer growing seasons chief among them — unrelated to the sex of any tree.

Botanical sexism: the claim, as its proponents make it

The term was coined by horticulturist Thomas Leo (Tom) Ogren, who developed the argument across books including Allergy-Free Gardening (2000) and set it out for a general audience in his 2015 Scientific American essay, "Botanical Sexism Cultivates Home-Grown Allergies". Ogren's version of the claim has several parts. First, many popular landscape species are dioecious — separate male and female plants — including willows, poplars, aspens, ashes, silver maples, pistache, mulberry, and junipers. Second, the nursery trade propagates these as clones, and the clones sold for tidy urban planting are overwhelmingly male. Third, Ogren argues this preference was encouraged from the top: he points to the 1949 USDA Yearbook of Agriculture as advising the planting of male trees to avoid seed litter. Fourth, he argues female plants are not merely neutral but actively helpful, since a female flower traps pollen out of the air in the process of being pollinated.

OPALS: Ogren's plant allergy scale

To make the claim usable, Ogren created OPALS, the Ogren Plant Allergy Scale, rating plants from 1 (least allergenic) to 10 (most). He notes that OPALS has been used by cities, health departments, and USDA urban foresters in their Urban Forest Effects model. Some US cities have gone further and adopted pollen-control ordinances restricting the planting of the most allergenic male clones — a policy step Ogren cites as the claim translated into practice. His narrower concept of "proximity pollinosis" — that the plants closest to where you live, work, and sleep matter most — is part of the same framework.

Botanical sexism under scrutiny: the critics' case

Critics do not dispute the basic botany: in dioecious species, only males shed pollen, and nurseries do sell male clones of some species, ginkgo being the famous example. What they dispute is the scale. A widely shared analysis by botanist and science writer Jennifer Hu, published in Slate in 2021 under the headline "The complicated truth behind 'botanical sexism,'" argued that the theory rests on a thin and partly misquoted documentary base, and that it can apply at most to the minority of urban trees that are dioecious. Most street and landscape trees, critics note, are monoecious — bearing both sexes on the same tree, like the oaks that dominate many city canopies — so no planting-sex choice exists for them at all. The Wisconsin Department of Natural Resources' forestry service made the same point in its 2024 explainer, "Botanical Sexism: Fact or Fiction?", sorting urban trees into monoecious, dioecious, and "it's complicated" categories and concluding the truth is more nuanced than the slogan.

Grass and ragweed: the pollen the theory cannot explain

Critics add two further points. The biggest drivers of pollen misery for many people are not trees at all: grass pollen dominates summer, and ragweed — an annual weed nobody plants on purpose, as Ragweed Season describes — dominates fall in cities like Tulsa, Wichita, and Memphis. A theory about tree planting cannot explain those. And the McGill Office for Science and Society, reviewing the claim in 2024, found little direct empirical evidence that male-biased planting measurably raised citywide pollen counts — the trend line Ogren needs — as distinct from the well-documented effects of longer, warmer seasons, which are covered in the broader record traced by The History of Pollen. Some reviewers have also rated OPALS as less accurate than alternative allergenicity scales. The reference summary of the debate reflects this split: the theory "has found some scientific acceptance," while other scientists "have also been critical of it, stating that it only applies to certain trees and is not as widespread as Ogren alleges."

Where the two sides actually agree

Strip away the slogan and there is a shared middle. Both sides accept that individual planting choices have local consequences: a strongly allergenic male tree of a dioecious species planted directly outside a bedroom window in Dallas will raise that household's exposure, whatever the citywide statistics say. Both accept that species selection by cities and nurseries rarely considered allergy potential at all until recently, and that considering it is reasonable. And both accept that urban pollen burdens are rising for reasons — longer growing seasons chief among them — that have nothing to do with the sex of any tree.

The genuinely open question

What remains genuinely open is the headline claim itself: how much of a city's pollen load, if any, is attributable to male-biased planting. That is an empirical question the current published evidence does not close, and this article will not close it either.

What this means for an individual sufferer

For an individual sufferer, the practical upshot does not depend on the verdict. Your exposure is dominated by your city's actual counts — which Pollen Atlas publishes daily, city by city, in the daily city rankings and on each city page — not by the population genetics of its street trees. If you are choosing what to plant at home, low-allergen selections are a sensible tiebreaker between otherwise equal options; if your symptoms are significant, an allergist can identify your specific triggers, and you can find an allergist through the Pollen Atlas directory. This article is general information only and is not medical advice.

Frequently Asked Questions

What is botanical sexism?

Botanical sexism is the claim, in the term coined by horticulturist Thomas Ogren, that cities made their allergy seasons worse by planting mostly male trees and shrubs of dioecious species, because males drop no fruit or seed litter but produce all of the pollen. Other scientists argue the effect is narrower than claimed, and the question is not settled.

Are male trees worse for allergies?

According to proponents, yes: in dioecious species only male plants shed pollen, and Ogren argues the clones sold for urban planting are overwhelmingly male. Critics respond that most urban trees are monoecious — bearing both sexes on the same tree, like oaks — so no planting-sex choice exists for them at all.

What is OPALS?

OPALS is the Ogren Plant Allergy Scale, created by Thomas Ogren to make his claim usable, rating plants from 1 (least allergenic) to 10 (most). Ogren notes that OPALS has been used by cities, health departments, and USDA urban foresters; some reviewers have rated it as less accurate than alternative allergenicity scales.

Did the USDA tell cities to plant male trees?

Ogren points to the 1949 USDA Yearbook of Agriculture as advising the planting of male trees to avoid seed litter. Critics, including Jennifer Hu's 2021 Slate analysis, argue that the theory rests on a thin and partly misquoted documentary base, so that part of the claim is disputed.

Conclusion: botanical sexism, weighed fairly

Botanical sexism is best understood as a real mechanism carrying a contested conclusion. Proponents, led by Ogren, can point to documented male-clone planting in dioecious species, a rating scale adopted by public bodies, and cities that have legislated on it. Critics can point to the minority share of trees the mechanism can even touch, the absence of direct citywide evidence, and the inconvenient fact that grass and ragweed ignore the theory entirely. Neither side's evidence licenses certainty. What you can be certain of is the air outside your door today — so check your city's free pollen counts and forecasts at PollenAtlas.com, for your neighbourhood and worldwide, and let measured data settle the part of the question that affects your morning.

Sources cited in text: Ogren, "Botanical Sexism Cultivates Home-Grown Allergies," Scientific American (2015), and Allergy-Free Gardening (2000) — the proponent case, including the 1949 USDA Yearbook claim and OPALS; Hu, "The complicated truth behind 'botanical sexism,'" Slate (2021) — the critical case; Wisconsin DNR Forestry News, "Botanical Sexism: Fact or Fiction?" (2024); McGill Office for Science and Society analysis (2024). Claims are attributed to their makers throughout; this article is general information only and is not medical advice.