Volunteers are reclaiming public parks and carving out nature trails in spaces overrun by innocent-sounding plants like Tree of Heaven, multiflora rose and purple loosestrife. Gardeners call these "invasives," and the volunteer armies aim to clear them out. Most of the weeds are non-natives and may have entered the U.S. as a seed, tucked inadvertently into cargo, or as a pretty garden addition—and have since made a mess. "When I was a kid, you could see straight through any woods. Today, you can't. It's just blocked" with weeds, says Marc Imlay, a retired biologist who volunteers from five to 40 hours a week clearing invasives from parks in the Washington D.C. area.