The Washington Post

Pollen expert: Tree pollen decline “seems to have already begun”

Tree pollen under a microscope (U.S. Army Centralized Allergen Extract Laboratory)

“In response to the earlier peak this year, our trees and even area grasses did indeed show up several weeks earlier,” said Susan Kosisky, microbiologist at the U.S. Army Centralized Allergen Extract Laboratory. “The good news is that many of the oak species in the area are already dropping their catkins onto curbsides and rooftops which means they have stopped flowering for the season.”

She continued: “The more pollen we see in March (and we saw 3 to 4 times the normal average levels), the less we have to worry about in April, especially for the oak species which comprise some 67% of our total annual tree pollen load. Many of our area trees are already leafing. Once the oaks start leafing and the catkins drop off we begin to see a huge decline in tree pollen levels which seems to have already begun.”

Today’s tree pollen count of 245 grains/cubic meter still classifies as HIGH, but well off the 2,124 count of March 29, a record high (since 1998). Even Wednesday’s and Thursday’s counts were still way up there at 1,098 and 1213. The drop today is likely attributable to two straight chilly nights along with periods of considerable cloud cover during the day.

“Cooler overnight and morning temperatures help keep tree pollen levels down,” Kosisky said.

On the effect of clouds, Kosisky told WJLA: “Changes in temperature, humidity and even the amount of sunshine can also impact the amount of pollen that is released by our area tree species. As soon as the cooler air and clouds moved into the area, the pollen count did indeed drop. Warmer, dry days with plenty of sunshine and spring breezes facilitate the drying, cracking, release and dispersal of pollen from the anther sacs (pollen producing pouch).”

Although tree counts may be on the way down in D.C., Kosisky cautions: “If you are going out of town and heading north, keep in mind that as we move out of the city, pollen levels will tend to be higher from trees which start flowering a little later.”

Peak of grass pollen season still to come

The worst of tree pollen season may well have passed, but Kosisky says grass pollen shows signs of being on the rise:

“Grasses have grown quite high in fields and along area roadsides. Some early pollinating weeds to include sorrel/dock and plantain will also be contributing to the count with increasing levels in the near future.”

Jason is the Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science, and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.

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