Freeze-free days are piling up across the US, and allergy season is following
A new analysis of 198 cities finds 87% have a longer freeze-free growing season than in 1970 — up to 100 extra days in Reno — pushing pollen exposure further into the calendar.
Allergy sufferers reaching for tissues earlier in the year are not imagining it. A Climate Central analysis released March 4 finds that the freeze-free growing season — the stretch of the year with no nights cold enough to kill off plant growth — has lengthened in the overwhelming majority of US cities studied since 1970, extending the window in which plants can bud, grow, and release pollen [s1].
What the data shows
Climate Central examined 198 US cities and found that 173 of them, or 87%, now have a longer freeze-free season than they did in 1970, with an average gain of 21 days across all the cities analyzed [s1]. The increase is not evenly distributed. Reno, Nevada tops the list with 100 additional freeze-free days since 1970, followed by Medford, Oregon (68 days), Las Cruces, New Mexico (65 days), Boise, Idaho (55 days), and Tupelo, Mississippi and Myrtle Beach, South Carolina, tied at 51 days each [s1].
Broken down by region, the Northwest gained the most, 31 additional days, bringing its average freeze-free season to 187 days [s1]. The Southwest gained 22 days, reaching 190 days [s1]. The Southeast added 19 days for a 136-day season, the Northeast added 17 for 185 days, and the Upper Midwest also added 17 days, reaching the longest average season in the analysis at 207 days [s1].
Why a longer freeze-free season means more pollen
A longer freeze-free window gives plants more calendar time in which to grow and, critically, to bloom and release pollen — the mechanism connecting the freeze-free data to allergy symptoms. Climate Central's report references a separate 2021 study finding that human-caused warming had already driven a documented lengthening of North American pollen seasons by an average of 20 days between 1990 and 2018 [s1]. The freeze-free trend and the pollen-season trend describe the same underlying shift in different terms — one measuring the temperature conditions that permit plant growth, the other measuring the pollen output that results from it.
Who this affects
Seasonal allergies are already common: the report cites CDC data indicating roughly one in five children in the US has a seasonal allergy [s1]. A longer pollen season translates for these households into a longer stretch of symptomatic days, and potentially earlier and later exposure at the shoulders of what used to be an allergy-free winter.
What the data doesn't establish
The freeze-free season is a proxy — a measure of the climate conditions favorable to plant growth — not a direct pollen count, and Climate Central's report does not claim to have measured actual airborne pollen concentrations in 2026 for the cities listed. The report also does not draw a line from any single city's freeze-free extension to a corresponding rise in local emergency visits or prescriptions; a longer growing season is one of several factors, alongside precipitation, plant species mix, and urban vegetation, that shape how bad a given allergy season actually is for the people living through it. What the underlying trend does establish, with 55 years of data behind it, is the direction: the calendar window in which the country's most common chronic condition-triggering exposure occurs has been getting wider, and 87% of the cities studied show it.
Sources
- 2026 Allergy Season Report — Climate Central , March 4, 2026
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