The Bradford Pear (Pyrus calleryana ‘Bradford’) was once celebrated across the United States for its rapid growth, tolerance for urban conditions, and spectacular burst of white flowers in early spring. This ornamental appeal, beginning in the mid-20th century, masked a biological flaw that has since caused widespread ecological damage. The tree is now widely considered an invasive species across the eastern and midwestern regions of North America.
The Biology of Invasive Seeding
The Bradford Pear cultivar was originally introduced with the intention of being sterile, meaning it was incapable of producing viable seeds. The original ‘Bradford’ tree is self-incompatible, so it cannot pollinate itself to produce fruit. This initial sterility was a miscalculation that failed to account for the introduction of other Callery pear varieties into the landscape.
The problem began when nurseries introduced other ornamental cultivars of Pyrus calleryana, such as ‘Cleveland Select’ or ‘Aristocrat’. When the self-incompatible ‘Bradford’ clone cross-pollinates with these genetically distinct varieties, or the fertile rootstock onto which they are grafted, the resulting fruit contains viable seeds. Birds readily consume and disperse these small, hard fruits, spreading the seeds into natural areas far from the planted ornamental trees.
The seedlings that sprout in the wild revert to the traits of the wild Callery pear parent species, rather than remaining the thornless ‘Bradford’ variety. These feral trees are genetically diverse and highly successful. They often develop stout, sharp thorns up to four inches long, which makes them difficult to remove and discourages browsing animals.
Ecological Disruption and Spread
Once viable seeds are dropped in fields, along roadsides, or within forest edges, Callery pear seedlings establish themselves with remarkable speed. They are highly adaptable to poor soils and disturbed areas, giving them an advantage over native plants. This aggressive growth allows them to form dense, nearly homogeneous thickets that crowd out native flora.
These impenetrable stands aggressively compete with native trees and shrubs for finite resources like water, soil nutrients, and sunlight. By monopolizing the understory, the Callery pear thickets reduce native biodiversity, causing a cascading effect on local wildlife. Specialized native insects lose their food source, disrupting the base of the local food web and impacting bird populations.
The Callery pear’s early leaf-out in the spring and late leaf-drop in the fall allows it to shade out native species, preventing their successful growth and reproduction. The tree offers little nutritional value to native wildlife compared to the fruit of native trees. This displacement of native plants and resulting habitat degradation are the main concerns associated with the invasive spread.
Removal Strategies and Native Alternatives
Removing an existing Bradford Pear tree requires more than simply cutting it down, due to the tree’s vigorous re-sprouting ability from the stump and roots. For smaller trees, a homeowner can cut the trunk close to the ground and immediately treat the cut stump with a systemic herbicide containing glyphosate or triclopyr. The herbicide must be applied quickly to the vascular tissue around the perimeter of the stump for maximum effectiveness.
For larger, mature trees, it is safer to hire a certified arborist, especially if the tree is near power lines or structures. Simply mowing over new sprouts is not sufficient, as the roots will continue to send up suckers for at least two years until energy reserves are exhausted. Complete stump removal through grinding or digging is an alternative to chemical treatment.
Once the invasive tree is removed, replacing it with a native alternative helps restore ecological balance and supports local wildlife. These alternatives offer similar ornamental appeal, multi-season interest, and support local ecosystems without the invasiveness. Recommended native flowering trees include:
- Serviceberry (Amelanchier spp.), which produces edible fruit for birds.
- Eastern Redbud (Cercis canadensis), known for its showy pink-purple flowers.
- Flowering Dogwood (Cornus florida).
- White Fringe Tree (Chionanthus virginicus).