54 | April 2019
Growing Pains
Pollen allergies are caused by a variety of plants, which
emit pollen at different points throughout the year depend-
ing on the species, local conditions, and other factors. Many
trees start to send out pollen in the spring; grasses often
peak during the summer; and weeds, such as ragweed,
emit pollen into the fall. In this complicated mix, the length of
the growing season—which lasts from the last freeze of the
spring to the first freeze of the fall—can serve as a useful
proxy. By examining how the length of the growing season
changes, we can understand in general terms how plants
are gaining time to flower and affect allergic Americans.
To understand how the growing season has changed
across the United States, Climate Central assessed tem-
perature data for 201 cities. Of those cities, 83% saw their
freeze-free seasons lengthen since 1970. In the average
city, the amount of time between the last and first freeze of
the year grew by just over two weeks.
In 34 cities, including El Paso, Minneapolis, and Phila-
delphia, the season between first and last freeze grew by
at least four weeks. In Bend, Oregon; Medford, Oregon;
and Las Cruces, New Mexico, it lengthened by at least two
months—among the biggest increases in the country. In
general, relative to early twentieth-century averages, the
freeze-free season has lengthened the most in the west-
Allergic America
Over the past few decades, the prevalence of allergies
among Americans has skyrocketed. In 1970, about one
in ten Americans suffered from hay fever, which is caused
by airborne allergens, such as pollen and mold spores; by
2000, three in ten did.
Asthma—which often occurs alongside pollen aller-
gies—has become more common, too. The proportion of
Americans who suffer from asthma rose from 3.1 percent
to 8.4 percent of the population between 1980 and 2010.
Rates are even higher among African-Americans, low-
income households, and children. Today, some 6.2 million
Americans under the age of 18 suffer from the chronic
disease.
These are expensive, dangerous problems. Overall,
allergies cost the United States more than $18 billion per
year. Asthma brought people to U.S. emergency rooms 1.7
million times in 2015; the next year, it killed about 3,500
Americans. And even the more manageable effects of
pollen allergies remain inconvenient and uncomfortable.
Researchers do not fully understand the causes of the
upward trend in allergies. But one thing is clear: as humans
warm the atmosphere, the freeze-free season generally be-
gins earlier and lasts longer each year, extending the time
during which plants can grow and produce pollen.
