A wave of urban greening is cutting summer heat, but studies from New York City to Austin show that which trees get planted, and where, shapes pollen exposure for allergy sufferers.

As U.S. cities race to expand tree canopy against rising summer heat, researchers are pressing planners to weigh a trade-off that rarely makes it into planting plans: some of the most popular street trees also release large amounts of allergy-triggering pollen. NPR's Short Wave explored the question this week in an episode titled Can planting more trees make allergies worse?, and the research behind it points to an answer that is less about more trees versus fewer trees and more about which ones.

The benefits of trees are well documented. In a review of cooling research, the Environmental Protection Agency reported that tree shade reduced peak surface temperatures on building walls and roofs by 20 to 45°F (11 to 25°C), and that suburbs with mature trees ran 4 to 6°F (2 to 3°C) cooler than new suburbs without them.

For households that deal with seasonal allergies or asthma, the practical question is how to keep those cooling benefits without turning a shady street into a pollen corridor.

Austin and New York Show How Species Mix Drives Pollen

A study by U.S. Forest Service scientist David Nowak and horticulturist Thomas Ogren scored urban forests in 53 cities and states on an allergy index from 1, least allergenic, to 10, most allergenic. Published in Urban Forestry & Urban Greening, the study found an average score of 6.3, with Austin, Texas, the highest at 8.3 and Halifax, Nova Scotia, the lowest at 4.2.

The authors recommended "limiting tree species with high allergenic potential" near people, while noting that pollen is essential for natural forest regeneration.

New York City offers a closer look. An analysis of the city's street trees, posted as a preprint on bioRxiv and later published in Urban Forestry & Urban Greening, found that London plane trees were the largest single pollen producer, at about 29.6 quadrillion grains a year citywide. At a rooftop monitor near Lincoln Center, oak, mulberry, birch and London plane together made up 71% of airborne tree pollen from 2009 to 2012.

The analysis also found that adding 200,000 street trees matching the current mix would change most species' pollen output only modestly. The exception was oaks, where about 25,000 more trees could add roughly 8 quadrillion grains a year once mature.

The Male Tree Question, and the Debate Around It

Some trees, such as cottonwoods and other poplars, willows, and many junipers, have separate male and female individuals — and only the males release pollen. Ogren has argued for years that cities favored male trees to avoid messy fruit and seeds, a practice he calls botanical sexism.

He traces it partly to a 1949 U.S. Department of Agriculture yearbook that advised planting only male cottonwoods along streets, according to the Indianapolis Star. A 2018 report from the American Academy of Allergy, Asthma and Immunology said uniform, male-heavy plantings have produced "homogeneous pollen profiles with a high community prevalence of allergy," the newspaper reported.

The theory has critics. Many common street trees carry both male and female flowers on the same tree, and some city foresters say male-only plantings are rare today. In Indianapolis, sustainability official Lindsay Trameri said species with that potential make up "a very small percentage of those plantings," under 1%. Ogren's broader claim that tree selection doubled allergy rates has not been established by peer-reviewed research.

Weighing Shade Against Sneezes

Researchers increasingly frame allergenicity as one factor among many rather than a reason to plant fewer trees. A September opinion article in Frontiers in Sustainable Cities argued that canopy targets alone do not guarantee healthy urban forests, warning that "Highly allergenic species may increase pollen exposure." The authors called for screening species for health risks alongside heat tolerance, diversity and site fit.

Evidence check: these studies estimate pollen production and allergenic potential, but none directly measured how many additional allergy or asthma cases a planting program causes. The New York analysis relied on one rooftop monitor and pollen equations developed in Ann Arbor, Michigan, and its authors said links between pollen levels and health outcomes remain poorly defined. The heat benefits, by contrast, are well measured — which is why researchers urge smarter choices, not fewer trees.

The people most affected are children and adults with pollen allergies or asthma, residents living along dense rows of a single high-pollen species, and outdoor workers during spring bloom. Neighborhoods with little canopy, often the hottest, stand to gain the most from new trees and should not lose out because of allergy concerns that can be managed through species choice.

Planting Smarter on Your Street

For planners, the New York study's authors suggested avoiding the most allergenic trees near homes, especially London plane, favoring insect-pollinated species and trees with less allergenic pollen, and choosing female individuals of species with separate sexes. Mixing many species also keeps any single pollen source from dominating.

For homeowners, ask a local nursery or extension office which native species suit your yard and produce little wind-blown pollen. Planting near windows, patios, and play areas deserves the most care.

For allergy sufferers this fall and next spring, check daily pollen counts, keep windows closed on high-pollen days, and shower after time outdoors. People with asthma should follow their doctor's action plan.

What happens next is mostly local: city forestry departments set species lists, often with public input. The bottom line is that the choice is not shade or sneezes, since careful species selection can deliver cooler streets without adding as much pollen.

What Readers Want to Know

Do more city trees always mean more allergies?

No. Research shows pollen exposure depends mostly on which species are planted and where, not simply on the number of trees.

Which street trees produce the most pollen?

In New York City, London plane trees were the largest single producer, and oak, mulberry, birch and London plane made up most airborne tree pollen at one monitor.

What is botanical sexism?

It is a term coined by horticulturist Tom Ogren for planting male trees, which release pollen, to avoid fruit and seed litter. Some city foresters dispute how common the practice is today.

Which U.S. city scored highest for urban forest allergy potential?

Austin, Texas, scored 8.3 on a 10-point index in a study by Nowak and Ogren.

Why plant trees at all if they cause pollen?

Trees cut surface and air temperatures, lower cooling costs, and improve neighborhood comfort- benefits the EPA has documented in detail.

What can homeowners do?

Choose native, low-pollen or insect-pollinated species, mix species, and keep high-pollen trees away from windows and patios.