For decades, the fields of Pyrus calleryana trees grown from seeds imported from China for research purposes were basically forgotten. In the 1950’s, with the post-war boom in housing and beginnings of the suburbs, horticulturists needed street trees that were basically weeds – that would grow in a variety of conditions and be free of pests.
Lo and behold, at the USDA Beltsville Maryland site a particular callery pear – loaded with flowers and with no thorns – was selected and thousands of grafted individuals were created and planted under the name Bradford pear. The Bradford pear is self-sterile and produced no viable seeds. But pears from the research fields with different characteristics – taller, narrower, etc., were also added to the trees planted and now pollination was possible. Birds and water moved these fertile seeds and new trees with dangerous characteristics proliferated.