Why they are a problem from the National Park Service:
Once established Callery pear forms dense thickets that push out other plants including native species that can’t tolerate the deep shade or compete with pear for water, soil and space. A single tree can spread rapidly by seed and vegetative means forming a sizeable patch within several years. Its success as an invader results from its capacity to produce copious amounts of seed that is dispersed by birds and possibly small mammals, seedlings that germinate and grow rapidly in disturbed areas and a general lack of natural controls like insects and diseases, with the exception of fire blight.
From the Ohio Division of Forestry Invasive Plants of Concern
in Ohio Report:
Callery Pear (Pyrus calleryana)
Callery pear is a deciduous tree that is widely planted in the urban landscape. Different cultivated varieties of pear that are sold for landscaping purposes are self-sterile; meaning that they cannot pollinate themselves to produce viable fruit. However, different cultivars of pear can cross-pollinate with each other when they are planted in close proximity in the landscape. This results in the production of viable fruits which are dispersed by fruit-eating birds such as European starlings. Callery pear grows well in full sun to partial shade. It tolerates a variety of soil types and can withstand occasionally wet or droughty soils. Wild pear often escapes into disturbed areas, meadows, roadsides, and open woods.
Callery pear can reach a height of 45 feet. The bark is smooth with horizontal lenticels when young and later develops vertical fissures. Leaves are oval, glossy, leathery and have round-toothed margins. Abundant white flowers develop in early spring before the leaves expand and this is when escaped wild pears become most noticeable in the landscape. Fruits are approximately ½ inch in diameter and brownish with white specks. They ripen in early to mid fall.
Alternatives to the Callery Tree or Bradford Pear: