Climate Matters
Later Fall Frost Extends Allergy Season
Fall warming extends the growing season for ragweed and other fall allergy offenders that affect millions in the U.S. who are allergic to pollen and mold.
Climate Matters•October 8, 2025
Later First Freeze in 179 U.S. Cities
KEY FACTS
Fall warming can delay the first freeze of the season, disrupting important ecological cues in forests, farms, and gardens.
Climate Central analyzed how the first fall freeze date has changed since 1970 in 204 U.S. cities.
The first freeze now arrives later in 88% (179) of cities analyzed — 12 days later, on average, than in the early 1970s.
A later freeze can worsen fall allergies, lengthen pest seasons, and reduce summer fruit yields.
This Climate Matters analysis is based on open-access data from the National Oceanic and Atmospheric Administration (NOAA). See Methodology for details.
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Warmer fall, later freeze
The fall season has warmed in every county across the contiguous U.S. since 1970.
A warmer fall season means that risky heat, demand for air conditioning, wildfire smoke, and allergies linger later into the year — affecting public health and energy bills.
Fall warming can also delay the first freeze of the season and disrupt important ecological cues in forests, farms, and gardens — with potential effects on fall foliage as well as hibernation, bird migration, and fruit ripening.
First freeze arriving later in 179 U.S. cities
Climate Central analyzed how the date of the first fall freeze (the first day with minimum temperatures at or below 32°F) has changed from 1970 to 2024 in 204 U.S. cities. See Methodology below for details.
The first freeze is arriving later in 179 cities, or 88% of the 204 analyzed.
Across these 179 cities, the first freeze is arriving 12 days later, on average, than in the early 1970s.
The first freeze is now arriving at least two weeks later in 70 cities — especially cities across the Northeast, Ohio Valley, and Upper Midwest.
The cities that have seen the longest delay in their first freeze are: Reno, Nev. (41 days later); Bend, Ore. (38 days later); Santa Maria, Calif. (35 days later); Toledo, Ohio (29 days later); and Boise, Idaho (29 days later).
Later freeze can affect health, agriculture, fall color, and more
A later fall freeze can lengthen and worsen fall allergy season. In most U.S. areas, ragweed pollen typically peaks in September and lasts through October. But warmer fall temperatures and a later fall freeze extend the ragweed growing season and the allergies that come along with it.
Conditions that trigger fall color — including cool nights — are shifting with climate change. Cool nights are one of several factors that promote colorful leaves. But fall nights have warmed in 212 U.S. cities since 1970, potentially impacting the ecological and economic value linked to fall foliage.
Timing of the first freeze affects planting, pest management, and harvest schedules. Warming driven by carbon pollution results in more freeze-free days each year and therefore longer annual growing seasons. The growing season is now more than two weeks longer on average across the contiguous U.S. compared to the beginning of the 20th century. Although some plants and animals may benefit, longer growing seasons can also boost pests and lead to more heat-driven drought and water stress.
A later freeze allows disease-carrying insects such as mosquitoes to linger later. Fall warming can also disrupt the timing of ecologically important events like bird migrations, hibernation, and fruit ripening.
Delayed freezing can affect fruit and nut crops — a nearly $27 billion industry in the U.S. A certain amount of sustained chilling (starting September 1) is key for high quality non-citrus fruit and nut yields. But 181 U.S. locations have seen this chill period decrease since 1980.
LOCAL STORY ANGLES
When does the last spring freeze and first fall freeze typically occur in your area?
Check out NOAA’s national maps showing the historical date of first freeze and last freeze to understand when to generally expect freezing. Monitor NOAA’s National Weather Service for frost and freeze watches and warnings in your area.
The Freeze Date Tool, from the Midwestern Regional Climate Center and the USDA Midwest Climate Hub, provides information about the climatology of freezing temperature dates across the north-central and northeastern U.S.
The North Carolina State Climate Office has a Freeze Date Explorer tool to view both the long-term average first and last freeze dates, along with the observed annual occurrence of freeze dates since 1950. Other State Climate Offices may have similar tools.
CONTACT EXPERTS
Sara Kuebbing, PhD
Director of Research, Yale Applied Science Synthesis Program
The Forest School at the Yale School of the Environment & the Yale Center for Natural Carbon Capture
Relevant expertise: Conservation biology, invasion biology, forest ecology and management, natural climate solutions
Contact: sara.kuebbing@yale.edu
FIND EXPERTS
Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts.
Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.
Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices.
Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area.
METHODOLOGY
Local daily temperature data from 1970 to 2024 were obtained for U.S. weather stations from the Applied Climate Information System, which is developed, maintained, and operated by NOAA’s Regional Climate Centers.
The annual first freeze date for each location was determined based on the first day of each year (starting August 1st) with a minimum temperature of 32°F or below.
Locations without a consistent freezing season (defined as fewer than three months of minimum temperatures at 32°F or below) were excluded from the analysis.
The long-term change in the first freeze date from 1970-2024 was calculated using linear regression.
This analysis included 206 weather stations. For reported data summaries based on linear trends, however, only 204 stations are included due to data completeness measures that were not met by two stations: Hazard, Ky. and Wheeling, W.Va.