From the NY Times:
Street trees weren’t always as allergenic as they are today. Back in the 1950s, the most popular species planted in the United States was the native American elm, which sheds little pollen. Millions of these tall, stately trees lined the streets of towns and cities from coast to coast. Sadly, in the 1960s and ’70s, Dutch elm disease killed most of the elms, and many of them were replaced with species that are highly allergenic.
This has caused trouble for Americans with allergies — as many as 30 percent of adults and 40 percent of children — most of whom are sensitive to pollen, as well as for the many millions who have allergy-induced asthma. Although some pollen can be carried great distances by the wind, most atmospheric pollen comes from plants growing nearby. In other words, the pollen that’s making you sneeze as you walk down the street probably came from the tree you just passed. So it makes sense for gardeners, especially public gardeners who plant trees by the dozens, to pay attention to the pollen their trees produce.
Some trees shed huge amounts of highly allergenic pollen; others produce very little, or their pollen is only moderately irritating. Female plants produce no pollen at all. But arborists rarely take this into account. In New York City, street trees are selected only for their hardiness in winter; their resistance to disease, insects and drought; their ability to withstand smog; and their size, shape and color.
The pollen that causes the most severe allergic reactions comes from a few so-called monoecious species of trees, which have both male and female flowers, and from the males of separate-sexed (dioecious) species. Many arborists and landscapers like to plant male trees and shrubs because they’re “litter-free” — that is, they produce no seeds or seedpods. But male trees shed lots of pollen; that’s their job. And once it’s released, it can be blown around for months.
In New York City, about 30 percent of the street trees are Norway maples and London planes, both monoecious kinds that always produce allergenic pollen.
Photo from Solar One