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Spring 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

April 17, 2012

White oak pollen: job done

They are clogging your gutters, carpeting your decks and landing in your salad at that outdoor cafe. White oak catkins are being shed by their trees after concluding their pollen release for the season, much to the relief of people tortured by oak allergies.

Warm spells followed by cool temperatures and low humidity created ideal conditions a couple of weeks ago for the three-inch-long male flowers to release their golden-green dust, a process that each flower accomplishes in two or three days.

Breezes deposit only a wee fraction of the airborne pollen onto the freshly emergent and inconspicuous female flowers that now appear on the stems of new oak shoots.

A pollen grain fortunate enough to land on a female flower's pink stigmas will sprout a pollen tube that grows toward the ovary to fertilize an egg, which could develop into a ripe acorn that will drop in early September.

When open air space among the oak branches is no longer needed for fertilization, young oak leaves rapidly expand, and the spent catkins float to the ground to gather in drifts, turning rusty brown as they decay.

Before July, next year's catkins will begin forming inside buds on the oak's twigs. By autumn, the packed catkins will be mature, poised to expand in the spring of 2013 to discharge yet another round of blond powder.

SOURCES: Forest Science, U.S. Forest Service, "The Ecology and Silviculture of Oaks," staff reports

White oak catkins

White oak catkins (male flower stalks) with details of a male flower and its open pollen sacks, lower left, and a female flower, lower right.