The Bradford Pear (Pyrus calleryana ‘Bradford’) is an ornamental tree common in North American landscapes. It is recognized for its dense clusters of white flowers that appear early in the spring, often accompanied by an unpleasant scent. The tree’s rapid growth and uniform, tear-drop shape initially made it highly popular in urban and suburban developments across the United States. Tracing its journey from its original home is necessary to understand the tree’s current status.
The Native Range of Pyrus Calleryana
The parent species, Pyrus calleryana, is indigenous to East Asia, primarily central and southern China, Taiwan, and parts of Vietnam. In its native habitat, the tree grows in challenging conditions, showcasing remarkable resilience. It is found in forests and on scrubby hillsides, demonstrating tolerance for drought and different soil types, including heavy clay and sandy ground.
This wild species naturally exhibits high resistance to Fire Blight, a bacterial disease devastating to commercial pear orchards. Wild Pyrus calleryana trees possess long, sharp thorns and produce small, inedible, hard fruits. These characteristics made the species an early point of interest for agricultural scientists seeking hardier plants.
The USDA’s Quest for a Solution
The introduction of Pyrus calleryana to the United States was motivated by a crisis in the domestic fruit industry in the early 20th century. Fire Blight was decimating commercial pear (Pyrus communis) crops across the country. Scientists at the U.S. Department of Agriculture (USDA) sought a disease-resistant rootstock that could be grafted with cultivated pear varieties.
The USDA dispatched plant explorers to Asia to find suitable specimens, most notably Frank N. Meyer, who collected seeds of Pyrus calleryana in China around 1916. Meyer’s collection was intended strictly for agricultural experimentation, aiming to breed a disease-resistant base for edible pears. A specific batch of seeds, acquired in Nanjing, China, in 1919, proved significant. These seeds were planted at USDA testing facilities, including the Plant Introduction Station in Glenn Dale, Maryland, forming the foundation for decades of research.
The Rise of the ‘Bradford’ Cultivar
While the Callery pear was initially intended as disease-resistant rootstock, USDA horticulturists recognized its ornamental potential in the 1950s. They noted the species’ dense, early spring flowering, glossy foliage, and ability to withstand urban stresses like pollution and compacted soil. A specific selection from the 1919 seed collection at the Glenn Dale station caught the eye of researchers like John L. Creech and Frederick Charles Bradford.
The result was a single cultivar selected for improved aesthetic qualities, particularly its thornless nature and symmetrical, pyramidal growth habit. This clone was named ‘Bradford’ in honor of Frederick Charles Bradford, a station chief who supported the work. The ‘Bradford’ cultivar was commercially released in 1963 or 1964 and quickly became immensely popular with landscapers and developers across the country. Its desirable traits led to its mass planting, fueled by the belief that, as a single clone, it was sterile.
How the Ornamental Became an Invasive Species
The assumption of sterility proved to be the critical oversight in the introduction of the ‘Bradford’ pear. While a ‘Bradford’ tree cannot pollinate itself and is genetically self-incompatible, it readily cross-pollinates with any other genetically distinct Pyrus calleryana tree. The widespread popularity of the ‘Bradford’ led to the introduction of other cultivars, such as ‘Cleveland Select’ and ‘Aristocrat,’ which were bred to correct the ‘Bradford’s’ structurally weak, narrow branch angles.
When these genetically different cultivars bloomed near one another, their pollen mixed, resulting in viable, fertile fruit. Furthermore, all ‘Bradford’ trees are grafted onto wild Pyrus calleryana rootstock. This rootstock can sprout and produce flowers that also cross-pollinate with the ornamental canopy. Birds and other animals consume the small, hard fruit and disperse the fertile seeds widely in their droppings. The resulting wild seedlings often revert to the aggressive, thorny form of the original species, establishing dense thickets that outcompete native vegetation.