Kyle Daniel, Commercial Nursery and Landscape Outreach Specialist, Department of Horticulture and Landscape Architecture, Purdue University
As you are driving near disturbed areas or along the highways and interstates in the next couple of weeks, you may notice some familiar trees with prolific white flowers. Chances are you are most likely looking at a progressively increasing invasive plant that is also one of the most commonly cultivated ornamental plants in trade, the callery pear. The callery pear, Pyrus calleryana, also known by the cultivated names of Bradford Pear, Aristocrat Pear, Cleveland Select, and others, was introduced to the United States in the early 20th century as a potential tool for combating fire blight in the common pear. Acres and acres of fields were planted with callery pear, with origins from eastern Asia, to evaluate trees as a potential tool. It was in these fields of germplasm screen plots that a single plant stood out amongst others as a plant that became one of the most planted urban/suburban ornamental trees in history.
This ornamental pear had many attributes that made it an outstanding choice for the landscape. The eventual demise was due to the bad crotch angles that caused the tree to break after 10-12 years in the landscape. When the ‘second-generation’ callery pears became a common replacement for the Bradford pear, fruit began to form in trees that never before produced fruit. For many years Bradford pear was sterile, with no escapes, as it was widely planted and was never found in natural areas. Callery pear is self-infertile, meaning that it requires genetically different trees to produce fruit. A cultivar is produced asexually from the same parent tree; so therefore, all the Bradford cultivar contained the same genetics. Other cultivars that have become prevalent were found in different trees and locations, which led to genetically different trees in landscapes within close proximity to the Bradford cultivar. This cross-pollination has led to the production of fruit, which birds (especially robins) feast upon in late fall and early winter.
The qualities that make callery pear a highly effective landscape plant (i.e. fast growing, tolerant of variable soil types, etc.) are, unfortunately, the qualities that also make it an invasive plant. If possible, mowing at low heights can help in control. Chemical controls, especially growth regulator herbicides, must be used very carefully around more desirable species. Stump cut treatments with products containing glyphosate can also be an effective means of control.