The big willow oak in the Arboretum, April 12, 2015 (Zsaj via Flickr)

Allergy sufferers may have noticed the pollen season began gently in February.  But pollen levels are rapidly closing in on normal levels and a big spike is likely some time in the next two weeks.

“Get a string of warm, sunny days and the trees are just going to explode,” said Susan Kosisky, director of the U.S. Army Centralized Allergen Extract Lab in Silver Spring.

Kosisky explains that peak tree pollen levels usually occur between the middle and end of April.

Close-up of tree pollen under microscope (U.S. Army Allergen Extract Lab) Close-up of tree pollen under microscope (U.S. Army Allergen Extract Lab)

“In the next few weeks the pollen from our heavy pollinators to include oak, cedar/cypress/juniper, pine, mulberry, sycamore, ash and birch will be flying about,” she said. “The greenish-yellow film that builds up on outdoor car surfaces will become readily visible and is a good indicator of when our oaks and the much larger pine pollen begins to reach peak levels.  These two tree species produce enormous amounts of pollen on their own.”

This year’s highest tree pollen count so far occurred on April 13, when the concentration was measured at 783 grains per cubic meter of air (today’s count was 382).  But the maximum levels over the last 16 years  – observed mostly between April 15 and May 1 – have ranged between about 2,000 and 4,000 grains per cubic meter.

“All of the major tree species have yet to really come out,” Kosisky said.


(Susan Kosisky)

Pollen levels tend to be highest on days that are breezy, sunny, and warm  whereas cold and rainy conditions keep the pollen at bay.

The much colder than normal weather in February suppressed and delayed pollen release to just 6-7 percent of normal, Kosisky said. But in March, some of the early pollinating trees (elm, maple, alder and early cedar/cypress/juniper species) played catch-up and pollen rebounded to slightly above normal levels.

(Susan Kosisky)
(Susan Kosisky)

The early April counts were close to normal averaging 318 grains per cubic meter (normal is around 340).

Last year, the average daily count in early April was just 145 – but on May 1, the daily count surged to over 2,000. “So get ready,” Kosisky said.

And it’s not just exploding tree pollen in the pipeline.  “Grass pollen and early flowering weeds to include dock/sorrel are also starting to show up,” Kosisky said.

Grass pollen usually peaks around mid-May.