The Callery Pear, Pyrus calleryana, was first thought to be imported in the early 1900s to help fight fire blight resistance in the common pear. Its rootstocks were used long before the 1950s when the ornamental and hardiness traits of several cultivars, including the Bradford pear, were recognized by the landscape trade.
Thinking botanically, while some genotypes are self-incompatible, meaning they require cross pollination from another genotype in order to set seed, others can pollinate themselves. Different genotypes growing near each other (e.g., within about 300 ft.) can cross-pollinate and produce fruit with viable seed. Also, cultivars are often grafted onto seed-grown rootstocks with varying genotypes; if the plant produces shoots from the rootstock (which it often does), then these shoots and the graft can pollinate one another. Thus, the Bradford pear cultivar is one of several cultivars (varieties) of Callery pear capable of spreading and being invasive. Here’s where it all goes south and sounds like the Amur honeysuckle situation.
Any plant resulting from a seed produced by Pyrus calleryana ‘Bradford’ is a different genotype of Pyrus calleryana and not a member of any cultivar (unless somebody propagates that seedling and names it as a new cultivar). The plants that spread in natural areas are not cultivars. They are sexually reproducing populations consisting of multiple genotypes that recombine every generation.
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.
The solution? Do not plant Callery pear or any cultivars including the well known Bradford pear no matter what assurances the garden centers tell you. Seedlings and shallow-rooted plants can be pulled when soil is moist. Medium to large trees should be cut down and stumps treated with a systemic glyphosate or triclopyr-based herbicide. Several native trees would make excellent substitutes for Callery pear, including common serviceberry (Amelanchier arborea), Allegheny serviceberry (Amelanchier laevis), cockspur hawthorne (Crataegus crus-galli), green hawthorne (C. viridis) and the native sweet crabapple (Malus coronaria).
Read more in this publication: ODNR Division of Forestry’s PDF Weed of the Month.