“Frost-free” days increase, so does allergy season


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Climate Central’s graph illustrates how the number of frost-free days in Des Moines has increased over time. (Climate Central)
Jenna Ladd | April 11, 2018

Given that spring snow fell across Iowa this weekend, it may be hard to believe that the frost-free season across the U.S. is actually getting longer.

A recent report found that, on average, the last spring freeze is occurring earlier while the first fall freeze is happening later. Researchers define the frost-free season as the total number of days between the last day of 32 degree Fahrenheit or lower weather in the spring and the first day of 32 degree Fahrenheit weather in the fall.

The lengthening of this season means that pollen-producing plants have a longer growing period. One study in particular found that the growing season for ragweed, a common allergen in the U.S., lengthened by two to four weeks between 1995 and 2009. This data was collected from ten sites from the southern U.S. through Canada. Iowa has added nine days to the average length of its frost-free season from 1986-2015 when compared with the average from 1901-1960.

Not only are allergy-causing plants benefiting from longer growing seasons, but an uptick in atmospheric carbon dioxide also increases pollen counts. Last year was the worst allergy season in recent record and experts expect this year to be similar.

Dr. Joseph Shapiro, an allergist and immunologist from California told CBS news, “A recent study showed that pollen counts are likely to double by the year 2040, so in a little more than 20 years we’re going to see a significant increase [in seasonal allergies].”

Climate Central’s recent report provides an interactive graph that allows users to select a U.S. city and see how the frost-free season’s length there may have changed since 1970.