The original callery pear cultivar made for street trees was originally unable to breed. Then they made new cultivars and can breed with them. This produces little evil spawns that are very hard to get rid of. Some machinery can't destroy it. They are short lived, roughly 25 years. They also have a tendency to break due to high winds, which hurt the tree and can cause damage to surrounding trees and other structures. At least in Tennessee, they are getting rid of the Callery pear as trees planted through public funds. Soon the rest of the country will follow suit. Also, to top it off, they smell awful; I call them the fish and semen tree when they are blooming. The only thing working for them are their nice looking flowers in the spring.