The Bradford Pear tree (Pyrus calleryana ‘Bradford’), a common ornamental species, is widely recognized for its uniform shape and profusion of white flowers that mark the arrival of spring. However, this initial visual appeal is often overshadowed by a notorious, unpleasant odor that emanates from the dense clusters of blossoms. Bradford Pears produce a highly distinctive and disagreeable scent, a direct result of the tree’s biological strategy. This phenomenon has made the species a polarizing presence in urban and suburban landscapes across the country.
The Chemical Cause of the Odor
The foul scent associated with the Bradford Pear’s flowers is caused by the release of specific volatile organic compounds (VOCs). These airborne molecules are primarily nitrogen-containing compounds known as amines. The odor is often described as a combination of rotting fish and other bodily fluids, linked directly to the chemical structure of the released amines.
Chemical analysis suggests the presence of compounds like trimethylamine and dimethylamine, which produce a strong, fishy smell. Other polyamines, such as putrescine and cadaverine, associated with the decomposition of organic matter, may also contribute to the overall scent profile. These amines are molecularly similar to the compounds that create the smell of ammonia and decaying flesh, explaining the highly offensive nature of the tree’s fragrance.
The biological purpose of this pungent chemical release is pollination, a strategy known as sapromyiophily. Unlike trees that rely on sweet fragrances to attract bees and butterflies, the Bradford Pear’s odor is designed to attract specific insects. The smell mimics decaying organic material, successfully drawing in pollinators like flies, crucial for spreading the tree’s pollen.
Identifying the Tree and the Timing of the Smell
The period of the odor is short-lived but intense, coinciding precisely with the tree’s mass flowering in early spring. The blooming season is one of the earliest, occurring before most other trees have leafed out, typically lasting for about two weeks. The tree is covered in dense clusters of white, five-petaled flowers.
The tree has distinctive physical characteristics across the seasons. It typically maintains a dense, narrowly pyramidal or oval shape when young. Its leaves are glossy green in the summer and turn a deep, reddish-purple color in the fall. The bark is generally smooth and gray. The tree is often found planted in rows along streets or in commercial landscapes due to its past popularity as an ornamental species.
Context: Why This Tree Became Popular and Its Issues
The Bradford Pear became a widely used ornamental tree in the United States following its introduction from Asia in the early 1900s, gaining popularity in the 1950s and 60s. It was initially promoted as an ideal street tree because of its rapid growth rate, uniform shape that required minimal pruning, and high tolerance for adverse urban conditions such as poor soil quality, pollution, and drought.
Despite its initial promise, the tree’s popularity has since waned due to two major, interconnected problems. The first is its structural weakness, stemming from the ‘Bradford’ cultivar’s dense, upright branching pattern. The narrow angles where the branches meet the trunk create weak unions highly susceptible to splitting under the weight of snow, ice, or high winds. This structural flaw often leads to catastrophic failure, resulting in a short lifespan for the tree, typically only 15 to 25 years.
The second issue is its ecological impact as an invasive species. While the original ‘Bradford’ cultivar was thought to be sterile, it readily cross-pollinates with other Pyrus calleryana cultivars and related pear species. Birds consume the small, hard fruit produced by these cross-pollinated trees and disperse the fertile seeds into natural areas. The resulting feral trees form dense, thorny thickets that aggressively crowd out native plant species, causing significant disruption to local ecosystems.