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

Click to download custom graphics

Click to download custom graphics

Click to download custom graphics

Click to download custom graphics

KEY CONCEPTS

• Climate change contributes to earlier, longer, worse allergy seasons for millions in the U.S. that suffer from seasonal allergies to pollen — including about one in every five children.

• Warming trends lead to more freeze-free days each year — giving plants more time to grow and release allergy-inducing pollen.

• The freeze-free growing season lengthened in 87% (172) of 198 U.S. cities analyzed since 1970. 

• These 172 cities saw their freeze-free season lengthen by 20 days on average. 

• Cities across the Northwest (+24 days) and Southwest (+20 days) have experienced the largest average change. 

This Climate Matters analysis is based on open access data from the National Oceanic and Atmospheric Administration (NOAA). See Methodology for details.

Download local data

Warming climate, longer pollen season, worse allergies

The first spring leaves and flowers are unfolding early in parts of the south including in Atlanta, Georgia and Charlotte, North Carolina. 

That’s bad news for people with seasonal allergies — about one-in-four adults and one-in-five children in the U.S.

A warmer, earlier spring means a longer growing season — giving plants more time to grow and release allergy-inducing pollen earlier in spring and later into fall. 

Earlier springs, longer pollen seasons, and worse seasonal allergies are all linked to our warming climate.

• Climate Central’s report Seasonal Allergies: Pollen and Mold reviews weather and climate trends that affect pollen seasons and the related health risks. 

Carbon pollution (primarily from burning coal, oil, and methane gas) traps heat in the atmosphere and contributes to spring warming trends across the U.S. 

A 2021 study found that human-caused warming was a primary driver of North American pollen seasons lengthening (by 20 days on average) from 1990 to 2018.

Longer freeze-free growing season in 172 U.S. cities

Climate Central assessed temperature trends in 198 U.S. cities to see how the length of the freeze-free growing season has changed from 1970 through 2024 (see Methodology). 

• The freeze-free season lengthened in 87% (172) of the 198 cities analyzed since 1970. 

• These 172 cities saw their freeze-free season lengthen by 20 days on average. 

• In 70 cities, the time between the annual last and first freeze grew by at least three weeks.

• The freeze-free growing season increased the most in: Reno, Nev. (96 more days); Las Cruces, N.M. (66 more days); Medford, Ore. (63 more days); and Tupelo, Miss. and Myrtle Beach, S.C. (both 52 more days). 

Longest freeze-free seasons: South, Southeast, Ohio Valley, Northeast

Averaging city-level results across U.S. climate regions, all regions have seen their freeze-free growing seasons lengthen by at least 13 days on average since 1970 — led by the Northwest (+24 days) and Southwest (+20 days). 

Cities across the South, Southeast, Ohio Valley, and Northeast experience the longest average freeze-free seasons: an average of between 180 and 231 days per year. 

Freeze-free season now at least 10% longer in 70 U.S. cities

The lengthening freeze-free growing season can also be expressed in terms of relative change in season length between the first (1970-1979) and last (2015-2024) decade analyzed. 

The current freeze-free growing season is now at least 10% longer in 70 cities (35% of the 198 total cities analyzed). 

The relative change in season length was largest in:

Carbon pollution also boosts pollen production

Climate change makes pollen seasons not only longer, but also more intense due to heat-trapping pollution. 

Higher levels of planet-warming CO2 in the air can boost pollen production in plants, particularly in grasses and ragweed. 

With continued high rates of CO2 pollution, the U.S. could face up to a 200% increase in pollen production by the end of this century, according to a 2022 study.

Effects of longer, worse pollen season on children

Longer and more intense pollen seasons can have serious consequences for people with respiratory illnesses like asthma — particularly for children. 

• Climate Central’s fact sheet, Climate Change & Children’s Health: Seasonal Allergies, details how the climate trends that are worsening allergy season affect children’s health and well-being.

Around 19% of children in the U.S. suffer from seasonal allergies, with symptoms that include sneezing, coughing, itchy or watery eyes, and runny noses. 

Pollen is also a trigger for asthma, which affects 6.5% of children in the U.S. According to the Asthma and Allergy Foundation of America, allergic asthma is the most common type of asthma, and is most prevalent in early childhood.

These burdens are unequally shared. Black and Puerto Rican children are between two and three times more likely to have asthma than white children, and are therefore more at risk for allergic asthma. 

Protecting children’s health

Parents and caregivers can protect children’s health, now and in the future, by minimizing the harmful effects of seasonal allergies in a changing climate. A few key ways:

• Respond to symptoms, not the season. Children with allergies may experience symptoms earlier or later than usual as growing seasons expand and shift. 

• Minimize exposure. Check local air quality reports and allergen forecasts before heading outside. When pollen concentrations are high, consider limiting time outdoors for children with allergies or asthma.

• Make the indoors safer and more comfortable. Close windows and doors to minimize aeroallergens inside. Use high-efficiency particulate air (HEPA) filters in living spaces and bedrooms to clean the air. 

• Commit to rapid, sustained cuts to carbon pollution from burning fossil fuels. Because higher levels of CO2 increase pollen production, reducing carbon pollution will bring more and sooner benefits to allergy sufferers of all ages.

LOCAL STORY ANGLES

Find local pollen and mold counts.

There are pollen and mold spore monitoring stations across the U.S. Local allergen counts and forecasts can be found through resources such as the National Allergy Bureau. State or tribal agencies for environmental protection or public health may also have relevant air quality reports.

See where your city ranks.

The severity of the allergy season varies across the country. The Asthma and Allergy Foundation of America (AAFA) releases an annual report on the Allergy Capitals in the U.S., ranking cities based on pollen scores, over-the-counter medicine use, and the availability of board-certified allergists. Check out how your city ranked on AAFA’s 2024 Allergy Capitals Report; the 2025 list will be released in March 2025. The AAFA’s annual Asthma Capitals report also ranks U.S. cities where it’s most challenging to live with asthma. 

CONTACT EXPERTS

Andrew Rorie, MD 
Assistant Professor
University of Nebraska Medical Center Division of Allergy and Immunology
Relevant expertise: Allergies, asthma, effects of climate change on allergen exposure
Contact: arorie@unmc.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

Daily minimum temperature data from 1970-2024 were obtained from the Applied Climate Information System, which is developed, maintained, and operated by NOAA’s Regional Climate Centers. 

The length of the annual freeze-free season was determined based on the annual count of consecutive days with minimum temperatures above 32°F, i.e., the time between the last day with minimum temperatures of 32°F or lower (January 1 through June 30) and the first day with minimum temperatures of 32°F or lower (July 1 through December 31). Reported long-term (1970-2024) trends in freeze-free season length are based on linear regression.

Years with freeze-free seasons of less than two weeks (e.g., beginning June 30, ending July 3) were dropped from analysis. This condition affected several years in Bend, Ore. and Butte, Mont.

The relative change in growing season length was calculated as the percent change between the average season lengths in the  first (1970-1979) and last (2015-2024) decades.

Of 247 total stations assessed, 44 had an average growing season length of less than three months or a year-round lack of sustained freeze and were therefore excluded from this analysis. Summary statistics were reported for 198 of the 203 remaining stations due to data completeness issues for five stations: Bend, Ore.; Hazard, Ky.; Jefferson City, Mo; Twin Falls, Idaho; and Wheeling, W.Va.

Regional summaries for major U.S. Climate Regions exclude the West because this analysis includes only one station (Reno, Nev.) in that climate region. The eight other regions included between eight (Southwest) and 46 (Ohio Valley) stations.