- The freeze-free growing season lengthened in 87% of 198 U.S. cities analyzed since 1970.
- Warming trends lead to more freeze-free days each year, which give plants more time to grow and release allergy-inducing pollen.
- Illinois cities studied show an average change of +13 days in the freeze-free season.
People may have a runny nose for longer in the spring in Illinois than they once did, thanks to a longer growing season.
Most of the U.S., too, faces a longer growing season, a recent study found, that looked at temperatures in various cities from 1970 through 2024.
Here's what the analysis says about Illinois.
How much longer is the growing season?
All the Illinois cities listed -- Peoria, Rockford, Chicago, Quad Cities, Champaign, and Quincy -- are located in the Ohio Valley, which has seen an average change of +13 days in the freeze-free season since 1970.
Where are these numbers from?
Climate Central is a nonprofit that aims to "research and report facts about our changing climate and how it affects people's lives.
For their analysis, the researchers looked at daily minimum temperature data from the Applied Climate Information System, which is which is developed, maintained, and operated by NOAA’s Regional Climate Centers.
So, what does this mean?
A warmer and earlier spring means a longer growing season, authors said. That also means plants have 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 also all linked to a warming climate, authors said.
A 2021 study published in the Proceedings of the National Academy of Sciences found that North American pollen seasons became longer by 20 days on average, and more intense, from 1990 to 2018, the study referenced.
Why is this happening?
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., authors said.
The analysis again cited the 2021 study mentioned above that found that human-caused warming was a primary driver of North American pollen seasons lengthening.
Why does the grow season look different throughout the state?
There are several local factors that influence when and how often a given location experiences freezing temperatures, said Kaitlyn Trudeau, a Senior Research Associate with Climate Central.
Latitude, storm tracks, topography, and living in an urban vs. rural environment are some factors, she said, in addition to the year-to-year variability of weather.
"That being said, of the six Illinois stations included in our analysis, the change in the growing season increased with latitude," Trudeau wrote in an email.
Chicago, Peoria, and Rockford are all seeing their last freeze before summer earlier and their first freeze after summer later (each amounting to an increase in the average length of the growing season by three weeks), she said.
In Moline, (where the nonprofit's weather station is located in the Quad Cities) both the date of the last freeze and the first freeze is coming later in the year, but it’s mainly the date of the last freeze in the fall that is driving the expansion of the growing season.
Champaign has not seen much of a change with the season being on average one day shorter than it was five decades ago.
In Quincy, the change in the growing season is mainly driven by the date of the last freeze before summer, which comes about eight days earlier now than in the 1970s.
Tom Ackerman covers breaking news and trending news along with general news for the Springfield State Journal-Register. He can be reached at tackerman@gannett.com.
