Springtime in the city smells like flowers and cum and vomit. And I'm not talking about your neighborhood dive bar airing out after a long, dank winter—I'm talking about the trees.

These nauseating aromas are infamous throughout American metropoli. Every year, local blogs in New York and Washington, DC are flooded with the same question: "Ummm, what is that terrible smell?" What, indeed. It took me several years to find their source, and as I discovered, two arboreal species were to blame.

Let's do the jizz tree first. Or Pyrus calleryana, commonly known as the Callery pear. This pretty, floral tree somewhat resembles the cherry blossom. Its five-petaled flowers are dainty and white. If they could talk, I imagine they'd say, "come and have a picnic under us!" Wrong.

Stay away from these pretty Callery pear flowers.

The Callery pear smells like straight-up semen. Over at the University of California, Santa Barbara, they want you to know that it smells like human male semen, to be sure. So many people have called it the "semen tree" that it warranted an entry in Urban Dictionary, aptly defined as the odor of "used sex rags."

But why? According to Eloy Rodriguez, a plant biology professor at Cornell University, the Callery pear emits chemical compounds called "volatile amines." Specifically, trimethylamine and dimethylamine. These compounds are derivatives of ammonia, which coincidentally, is also present in semen. Certain amines are associated with fishy smells or the stench of decayed flesh. And, when the weather's warm and its flower are blooming, the tree wafts them out freely.

Our revulsion at the Callery pear, however, is the height of human narcissism. Their pungent perfumes aren't meant for us, said John Murgel, a horticulturist at the Denver Botanic Gardens. "We normally associate sweet smells with trying to attract bees, but a lot of plants all over the world use really terrible smells in order to attract beetles and flies as pollinators," he noted.

Knowing this, let's continue to the tree that smells like vomit: Ginkgo biloba. This special species is the bane of our nation's capital. Few neighborhoods in Washington, DC are immune to its hideous scent. When I lived there, I often wondered if their vomitus odor was intensified by the real vomit of people it had made sick.

Ginkgo biloba leaves can be bright yellow or green.

Unlike the Callery pear, Ginkgo trees possess butyric acid, or the molecule present in fermentation. It's what makes poop smell like shit, and parmesan cheese smell like (delicious) feet. Only female Ginkgos emit this unpleasant essence, and it comes from their soft, buttery fruit. (Strangely, these nut-like edibles are tasty appetizers in Japanese cuisine.)

Ginkgo trees are apparently very ancient, having "existed virtually unchanged for over 200 million years," according to a Slate investigation. There's some speculation that the tree's prodigious smell served to attract herbivorous dinosaurs, who would subsequently disperse their seeds. They're so unbelievably hardy that six of them survived the atomic blasts of Hiroshima, Japan in 1945.

Ginkgo biloba fruit on the ground. Will inevitably be squashed under some poor soul's shoe.

Today, urban planners sensibly plant them all over our cities. A 2005-2006 census of street trees in New York placed the Ginkgo among the top 10 most popular species.

To contain their smell, some cities aggressively spray their Ginkgos with a chemical called "Shield-3EC 24(C)." This substance also goes by "Shield potato sprout inhibitor," and effectively controls the fruiting of Ginkgo trees, thus preventing their stench. I have no idea how safe it is, but the District Department of Transportation deploys it under Environmental Protection Agency oversight. It's sort of sad when you really think about it. Poor trees.

Luckily for us, springtime is slowly getting a lot less stinky. Callery trees in New York have declined by 50 percent over the last decade. And property owners can simply opt for male, instead of female, Ginkgos.