And a lot of other plants. The Greensboro News & Record explains:
Bradford pear is a great example of how non-native plants can become a problem. It’s pretty, it grows fast, it doesn’t attract pests — the kind of plant that nursery owners and homeowners love.
Those characteristics — especially pest-free — should set off alarm bells, [botanist Lisa] Gould says, because invasive plants tend to share those same qualities.
The Bradford is a hybrid, but when birds eat the fruit and scatter the seeds in their droppings, the resulting plant reverts back to the original species: callery pear. Callery pear has sharp, hard thorns and is extremely difficult to remove.