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Fall 2010

Urban Jungle

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

October 19, 2010

Norway maple: The wrong tree

Not all fall hues in Washington are local color

In years past, as arborists hurried to replace the city's dying American elms, they often turned to exotic nonnative trees that possessed qualities that were -- at the time -- desirable.

The hardy, fast-growing Norway maple was a favorite choice. Miles of District streets are flanked with them. But their aggressive growing habits and prolific seeds have ruined their reputation: They are now considered an invasive plant.

"I'm constantly pulling these things out of my back yard," says Mark Buscaino, executive director of Casey Trees, a nonprofit organization that works to improve D.C.'s tree canopy.

"Their root mass is so tight that nothing will grow undeneath," says Buscaino. Cutting down the trees won't kill them, he explains: "They store so much starch in the roots that they'll
keep sending up sprouts for years."

That tight root mass can sometimes be the tree's undoing, says Casey Trees urban forestry manager David DiPietro. "If planted
in tight spots, the roots can encircle the trunk." This is called girdling, and it strangles the tree, cutting off the flow of starch from the leaves to the roots. The roots die, cutting off water to
the leaves.

Self-destruction isn't a problem for the tree when it escapes to open spaces. Rock Creek Park biologist Ken Ferebee often sees Norway maples at the edge of the park, where the seeds have blown in from nearby neighborhoods: The saplings "can grow pretty densely," he says. "In some areas, that's about the only thing there."

Sources: Casey Trees, Rock Creek Park, District Department of Transportation

Norway Maple Norway Maple street trees

Map by M.K. Cannistra/The Washington Post