The source of the specific, unpleasant odor often described as rotting fish or decaying matter that appears briefly each spring is the Callery Pear, formally known as Pyrus calleryana. This ornamental tree, especially its widely planted cultivar, the ‘Bradford’ Pear, releases a powerful scent during its short blooming period. The tree is visually striking with its profusion of white flowers, but the odor is a distinct trade-off experienced in urban and suburban environments. This phenomenon is a direct result of the tree’s biological strategy for attracting specific types of pollinators.
Identifying the Culprit: The Callery Pear
The Callery Pear is easily recognizable, typically being one of the first trees to bloom in the early spring, often before its own leaves fully emerge. Its dense, showy clusters of five-petaled white flowers cover the canopy, creating a dramatic, though temporary, visual display. The flowers are small, usually about three-quarters of an inch in diameter, and the strong odor is concentrated during the one to two weeks they are in bloom. The tree commonly develops a distinctive, tight, pyramidal, or egg-shaped crown, which contributes to its uniform appearance in landscapes.
Its leaves are glossy, dark green, and oval-shaped with wavy margins, sometimes turning an attractive reddish-purple or bronze color late in the fall. The ‘Bradford’ cultivar is particularly known for a structural flaw in its rapid growth. This flaw features narrow, upright branch angles that make the tree highly susceptible to splitting and breakage during strong winds or ice storms.
The Chemistry Behind the Odor
The notorious odor is caused by the release of volatile organic compounds called amines from the blossoms. These nitrogen-containing molecules are responsible for the smells associated with decomposition. The primary compounds detected include putrescine and cadaverine, which are diamines that give decaying animal matter its characteristic stench. The smell is also attributed to other related compounds, such as trimethylamine and dimethylamine, which are strongly associated with the smell of stale or rotting fish.
The tree produces these foul-smelling chemicals to ensure successful reproduction. Unlike plants that attract bees and butterflies with sweet fragrances, the Callery Pear is pollinated by insects drawn to the scent of decay, such as flies and beetles. By mimicking the scent of putrefying flesh, the flowers effectively lure these specific insects to transfer pollen.
Why These Trees Are So Common
The Callery Pear was first introduced to the United States from Asia in the early 1900s, initially for research on fire blight resistance in common pear crops. It quickly gained popularity as an ornamental tree due to a combination of favorable horticultural traits. The ‘Bradford’ cultivar, released in the 1960s, was highly valued by landscapers for its rapid growth rate, tolerance for urban pollution, and ability to thrive in poor or compacted soils. Despite its structural weaknesses and malodorous flowers, the tree’s widespread use continued for decades.
The problem of its invasiveness became apparent as different cultivars cross-pollinated, producing viable seeds. Birds readily disperse these seeds into natural areas. These wild seedlings now colonize open fields and roadsides, forming dense, thorny thickets that outcompete native plant species.