A pear seedling selection named Bradford was cloned by the gazillion to become the ubiquitous street tree of America’s postwar suburban expansion. Then it turned invasive.
Its invasive tendencies became widely noticed by the late 1990s, and by the mid-2000s, it had become a weed in the District and 19 states, from Texas to New York. “While callery pear was introduced with the best of intentions, it now seems that a plague is truly upon us,” botanist Michael Vincent wrote at the time. It has now spread to 29 states.