Janet McConnaughey/Associated Press
A stinky but handsome and widely popular landscape tree has become an aggressive invader. Callery pears create dense thickets that overwhelm native plants and bear four-inch spikes that can flatten tractor tires. Bradford pears and other ornamental Callery pears were cultivated from an import that saved pear orchards from a deadly fungus. And for decades, the decorative trees seemed near perfect, aside from a tendency to fall apart after about 15 years — and their stench. But they cross pollinated with other ornamental varieties. Invasive stands now have been reported in more than 30 states. Fourteen states have formally listed the trees as invasive.