Eyelids feel like sandpaper? Sneezing nonstop? Welcome to “cedar fever,” an annual malady that plagues San Antonio and Central Texas every winter.

Ashe juniper trees, commonly known as “mountain cedars,” cause the seasonal affliction. As part of their reproductive cycle, male trees produce small pollen cones, while the female trees produce small seed cones that look like blueberries, explained Estelle Levetin, PhD and a professor of biological science at the University of Tulsa who has studied Ashe juniper for decades. Upon maturity, the male trees release billions of pollen granules into the wind –usually on a cold, breezy day. That yellow pollen dust, filled with myriad allergenic compounds, can travel up to 200 miles.

The bushy evergreens are native from southern Missouri to northern Mexico, but are especially dense in Central Texas and the Hill Country.

Cedar fever season typically starts after Thanksgiving and continues through February. But climate change may cause those who suffer its effects to adjust to a longer, more intense season. Warming temperatures appear to be extending the trees’ reproductive cycle and boosting pollen counts, according to the Fourth Annual Climate Change Assessment.

“We are early into the season,” said Dr. Eliseo Villalobos, an internist and Fellow of the American Academy of Allergy Asthma and Immunology (AAAAI) at the Allergy Institute of San Antonio. Villalobos said that conditions this season seem pretty typical, but that the highest allergy counts are coming earlier in the season.

Usually it peaks mid-January, but the counts last year peaked around late December … which it seems to be doing again this year,” he said.

The AAAAI pollen counter has listed San Antonio in the red zone with pollen counts rated “very high” every day since Dec. 23.  

Female Ashe juniper trees produce blueberry-like berries.

Allergists point out that cedar pollen is especially bothersome to human beings.

A single tree can produce up to 500 billion pollen grains, said Daniel Katz, a postdoctoral research fellow at the Dell Medical School at the University of Texas at Austin. He added that the pollen itself contains many allergenic compounds and is especially susceptible to bursting, which releases the allergens into the air.

Texas State Botanist Chris Best speculated that the super-abundance of Ashe Juniper trees in Central Texas is a major factor in the severity of local cedar fever.

“Just look at the pollen counts during a bad season — many thousands, even tens of thousands of particles per cubic meter,” he said. “Compare that to other pollen or mold spores that reach peaks of hundreds to low thousands per cubic meter.”

Cedar fever is so bothersome and pervasive that Katz and other scientists at the University of Texas at Austin recently launched Pollen Trackersa citizen science project that will gather data on allergens in the atmosphere. Tracking mountain cedar pollen will be the focus of Pollen Trackers’ first campaign.

The program asks volunteers to monitor and report data on local cedar trees to alert the community when they release their pollen into the atmosphere.

“We don’t know yet how much pollen people are exposed to in their daily lives,” said Katz. “If we can predict airborne pollen concentrations, we can warn people about pollen hotspots in advance and help them maximize the benefits from their allergy medication.”

Katz described Ashe juniper’s “phenomenal amount of pollen production,” combined with its massive tree population and its particular pollen structure as “the perfect storm of allergies. … That’s why why cedar fever is one of the worst allergies in the U.S.,” he said.

So does he recommend getting out the chainsaws and clearing all the cedar, as some Hill Country landowners attempt to do?

“I wouldn’t go that far,” he said, noting that pollen-producing male trees would be better candidates for cutting down than clearing cedar indiscriminately.

The tree species is an important part of the regional ecosystem. Ashe juniper provides food, shelter, and nesting materials for wildlife, specifically the endangered golden-cheeked warbler. Ashe juniper trees also provides a unique, rich bed for particular native plants that thrive in its mulch.

“Trees do a lot of benefit for us. … Allergenic pollen is just one of the things we need to weigh,” he said.

San Antonio Report co-founder Monika Maeckle writes about pollinators, native plants, and the ecosystems that sustain them at the Texas Butterfly Ranch website. She is also the founder and director of...