Longer Growing Season, Longer Allergy Season in 172 U.S. Cities

Climate Central's analysis of 198 U.S. cities shows that 87% have experienced longer freeze-free growing seasons since 1970, increasing by an average of 20 days. This trend, driven by carbon pollution and warming, extends the period plants release pollen, worsening seasonal allergies for millions, including approximately 19% of U.S. children. The report highlights that CO2 levels also boost pollen production, potentially increasing it by 200% by the end of the century, with disproportionate health impacts on Black and Puerto Rican children who face higher asthma risks.

• Since 1970, the freeze-free growing season has lengthened in 87% (172) of the 198 U.S. cities analyzed, with an average increase of 20 days.
• Regional increases in the freeze-free season were most pronounced in the Northwest (+24 days) and Southwest (+20 days), while the longest average seasons (180 to 231 days) occur in the South, Southeast, Ohio Valley, and Northeast.
• Specific cities saw dramatic increases in growing season length; Reno, Nevada, experienced the largest increase with 96 additional days, and its season is now 70% longer than it was between 1970-1979.
• Carbon pollution increases pollen production in plants, specifically ragweed and grasses, with a 2022 study suggesting a possible 200% increase in pollen production by the end of the century.
• Extended pollen seasons exacerbate respiratory issues for children, with approximately 19% of U.S. children suffering from seasonal allergies and 6.5% affected by asthma. Black and Puerto Rican children are two to three times more likely to have asthma than white children.

APA

Rorie, A. (2025). Longer Growing Season, Longer Allergy Season in 172 U.S. Cities. Climate Central. https://www.climatecentral.org/climate-matters/2025-allergy-season

Chicago

Rorie, Andrew. Longer Growing Season, Longer Allergy Season in 172 U.S. Cities. Climate Central, 2025. https://www.climatecentral.org/climate-matters/2025-allergy-season.

Wikipedia

{{cite report |last1=Rorie |first1=Andrew |title=Longer Growing Season, Longer Allergy Season in 172 U.S. Cities |publisher=Climate Central |date=5 March 2025 |url=https://www.climatecentral.org/climate-matters/2025-allergy-season |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{rorie2025longer, author = {Rorie, Andrew}, title = {{Longer Growing Season, Longer Allergy Season in 172 U.S. Cities}}, institution = {Climate Central}, year = {2025}, month = mar, url = {https://www.climatecentral.org/climate-matters/2025-allergy-season}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }