The source of the smell. Wikipedia This spring, New Yorkers will have their noses assaulted by an unpleasant odor coming from the army of white-blossom covered trees that line many of the city's streets.

These flowers, though lovely in appearance, smell like a mixture of rotting fish and semen, according to a variety of web reports, and personal accounts from those in our own newsroom.

A tall, deciduous tree called the Bradford Pear (scientific name Pyrus calleryana) is to blame for the raunchy-smelling flowers.

The trees were planted all throughout New York in the 1960s because they are hard to kill — they grow fast and can thrive in tough conditions. People also think they are pretty. In the eastern United States, they are considered an invasive species because of their prevalence.

A path of pear trees. Flickr/Avia Venefica

One downside is that these hardy "street trees" really stink.

Urban Dictionary has labeled Bradford Pears the "semen tree," with an equally inappropriate, though colorful, description of their smell, which you can read over there.

Getting to the source

Everything we smell — from bananas to pine needles — comes from molecules, usually made of volatile chemicals, meaning chemicals that evaporate easily. The molecules evaporate from the food or flower and travel into your nose, where they bind with receptors in our nose.

The compounds that make the Bradford Pear tree's flowers smell are likely due to a type of chemical called amines, Dr. Eloy Rodriguez, a professor of plant biology at Cornell University, told Business Insider.

We come in contact with the smell of amines every day in the form of body odors, like under the arm pits.

Pretty, but smelly. Flickr/Janice Waltzer

The fishy odor produced by the Bradford Pear is likely a combination of two amines called trimethylamine and dimethylamine, according to Richard Banick, a botanical manager at Bell Flavors and Fragrances. Although perfumers know what chemicals produce the fishy smell (trimethylamine is often used an indicator of how fresh a fish is) they can't be certain what causes the odor of the Bradford Pear, said Banick.

Plants that produce volatile amines, some that smell like rotting meat, use the gas either to attract flies who will then spread the flower's pollen, or to ward off insects that might want to steal the nectar, said Rodriguez.

Rodgriquez suspects that the volatile compounds in the Bradford tree are there as attractants, and not necessarily to repel pollinators.

Later, the trees produce little green-yellow fruits that you cannot eat.