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Spring 2012

Urban Jungle

The changing natural world at our doorsteps | Illustration and text by Patterson Clark      

March 20, 2012

Callery pear

Pretty tree going rogue

At the margins of the city, commuters can't help but notice tight groves of white-blooming trees in sunny waste areas, old fields and roadsides.

A first glance, the trees might appear to be cherries, which bloom at about the same time and have similar five-petaled flowers. However, these are more likely to be thorny and aggressive escapees of Callery pear, the seeds of which were probably flown in by starlings engorged with the tree's pea-sized fruits.

The Chinese native hasn't always been so invasive. In 1962, the USDA made commercially available a thornless variety of Callery pear, naming it Bradford pear. The tree, hailed for its bright-red autumn foliage and profuse spring blossoms, was widely planted as an ornamental in the United States. It was unable to fertilize itself and therefore couldn't yield fruit. But as the trees matured, they began splitting apart in windy weather and heavy snowfall.

That disappointment inspired horticulturists to develop new varieties of Callery pear that were able to grow up and weather storms. However, this injection of genetic diversity allowed cultivars of Callery pear to cross-breed and fertilize one another, yielding fruits that would transform a once tame and thornless tree into a problem weed that stabs anyone who gets too close.

SOURCES: USDA, Biological Invasions, BioScience, Virginia Tech

Callery Pear, Pyrus calleryana