However, some cultivars, such as 'Bradford', are particularly susceptible to storm damage and are regularly disfigured or killed by strong winds, winter weather, or limb loss due to their naturally rapid growth rate and acute vertical branching.

[citation needed] A famous example of a Callery pear is the one that was found to have survived the destruction of the Twin Towers on 9/11 and was restored to full health subsequently.

The following varieties are currently accepted:[1] The Bradford pear and related cultivars of Pyrus calleryana are regarded as invasive species in many areas of the Eastern and Midwestern regions in North America, outcompeting many native plants and trees.

[2] In the northeastern United States, wild Callery pears sometimes form extensive, nearly homogeneous stands in old fields, along roadsides, and in similar disturbed areas.

The species was first noticed spreading outside of human cultivation in the 1990s, and by the latter half of the 2000s, Callery pear trees were widespread and could be found in habitats ranging from wetlands to forests.

However, if different cultivars of Callery pears are grown in proximity (within insect-pollination distance, about 300 ft or 100 m),[2] they often produce fertile seeds that can sprout and establish wherever they are dispersed.

[17][obsolete source] While these wild plants are sometimes called "Bradford pear" (for the 'Bradford' cultivar), they are actually wild-growing descendants of multiple genotypes of Pyrus calleryana, and hence more correctly referred to by the common (or scientific) name of the species itself.

[2] Currently, the spread of the invasive trees is limited by their intolerance to extreme cold, but they are creeping northward as climate change causes warming temperatures, and have been found as far north as Madison, Wisconsin.

[18] The Bradford pear in particular has become further regarded as a nuisance tree for its initially neat, dense upward growth, which made it desirable in cramped urban spaces.

Because of this, and the resulting relatively short life span (typically less than 25 years), many groups have discouraged further planting of 'Bradford' and other similarly structurally deficient Callery pear cultivars (such as 'Cleveland Select') in favor of increasing use of locally native ornamental tree species.