Around Baltimore, Gary Letteron is known simply as Tree Man. Cruising town in a maroon pickup, he seeks out vacant lots, barren sidewalks—any arboreally challenged patch of the cityscape. Then, owner or nearest resident permitting, this green guerrilla cracks open the pavement with a jackhammer and plants a maple, peach or pear tree. Declares Letteron, 48: "I want to see all streets in Baltimore tree-lined."

He's getting there. The Brooklyn-born Letteron has planted thousands of trees over the past 12 years, beautifying some of the city's most blighted areas. What started is a whim for the ex-keyboardist-cabbie-silver smith evolved into jobs with the Parks & People Foundation and a neighborhood planning council. "People sit back and complain about what the city should do," says Baltimore Mayor Martin O'Malley. "Tree Man sees a need and fills it." A Pied Planter with a party ethic (his motto: "It takes a case of beer to plant a tree") Letteron draws hundreds of volunteers. With one, paralegal Hilary Matzinger, 48, a romance sprouted six years ago. "His intensity attracted me," she says.

The roots of his arboriphilia are obscure. The youngest son of Leon, 78, a data processor, and Janet, 79, a teacher, Letteron grew up in Towson, Md., and in 1988 bought a row house in West Baltimore. "One day I walked out and said, 'This neighborhood needs trees,' " he recalls. But not everyone reveres his work. "The first time I saw someone had vandalized one of my trees, it was major trauma," he says. "But you move on." Doing just that, Tree Man sizes up an abandoned parking lot. "Hmmm," he says. "I feel like putting my Kudzu seeds in."

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