Pyrus calleryana, a very commonly planted ornamental tree species, is documented as an escape from cultivation in the District of Columbia and 152 counties or parishes in 25 states, and is reported as new to California, Michigan, Missouri, New Jersey, and West Virginia. Evidence is presented that the species is rapidly becoming invasive in much of its horticultural range in at least the eastern United States. Some of the escaped individuals (from 14 counties or parishes in 11 states) appear to be of hybrid origin, perhaps between callery pear and P. betulifolia or P. bretschneideri. Callery pear often produces thorny thickets as it escapes into marginal and disturbed areas, and appears to be reproducing readily in the wild.
“An ill weed groweth fast.”—John Heywood, Proverbs (1546)