Spring is in the air. You can tell, because the air stinks.

That foul fragrance is the contribution of the Callery pear tree to Colorado. Those who have not inhaled its near-toxic fumes simply don't understand; it's something you have to experience to believe. But if the Callery continues on its prolific track of pervasiveness, it won't be long before its rotten stench permeates this town.

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For those who'd like to avoid experiencing the Callery's springtime redolence, this could be your lucky year. During less oscillating spring conditions, the Callery will usually smell for two to three weeks when its flowers bloom. But according to John Murgel, a horticulturist at the Denver Botanic Gardens, "Some flowering trees aren't blooming at all this year because of the late frost." Also playing a role in this unpredictable behavior is the fact that Callery trees are ornamental, which means they tend to be planted individually in a wide range of locations. "If a tree is in a warmer, exposed site," says Murgel, "it could already be flowering."

Flowering, and stinking. "Any time plants emit fragrances, it's typically to attract pollinators, and that's what the pear is doing as well," Murgel explains. "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."

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The Callery isn't the only tree that smells bad this time of year. Denver's also home to the hawthorn, which "has a rather odd smell that some people find repulsive and others actually seem to like," says Denver City Forester Robert Davis. The infamous Gingko, which many people describe as smelling like vomit, also resides in Denver; however, Murgel claims that it's the female Gingko, typically found on the East coast, that smells -- and not the male version of the tree, which is found in Denver.

The Callery pear, aka Pyrus calleryana, was originally native to southern Asia. It was transplanted to the United States in the early 1900s amid a fire-blight outbreak that wiped out over 86 percent of the country's European pear trees, known as Pyrus communis. Unlike its European cousin, the Callery pear was immune to blight and most other diseases that threatened popular trees at the time, making it ideal for decorative purposes in heavily populated metropolises. By the early '60s the Callery pear was one of the most widely planted trees in the nation, thriving mostly in the South, where the latitude was nearly identical to that of China and other parts of southern Asia. By the turn of the twenty-first century, the Callery was America's favorite and most pervasive ornamental pear tree.

What everybody seemed to ignore during this time of mass tree-planting mania, however, was that come springtime, the Callery pear emits one of the most foul odors known to man. Although it's difficult to describe, the most accurate description of the Callery's budding flowers would be something like a pungent whiff of freshly excreted semen. Sure, you could euphemize and say it smells like a wet, dirty mop dipped in floury fish guts, but isn't that much more disgusting than likening it to a natural bodily fluid?

These days the Callery pear is experiencing a new level of fame. thanks to social media and a drastically more liberal society, especially in terms of sex. Calling it like it is just isn't a big deal anymore. There are message boards across the Internet that have devoted entire threads to the tree and articles detailing the reasons behind its odor. Of note is a Pittsburgh University Reddit page dedicated to eradicating all the Callerys on campus, and even an Urban Dictionary entree titled "Semen Tree."

Unfortunately for the citizens of Denver and fans of good-smelling things in general, the Callery and its awful stench aren't likely to go away any time soon. While the species is one tough sonofabitch, the tree still needs heat to survive -- and as global warming kicks into high gear, areas in the north where the Callery has previously struggled to adapt are becoming places where the tree can thrive. As a result, Denver has become a major way-station for the Callery.

But that could be a lucky break. As Davis puts it, "Denver has a very small palette of species to work with," and the Callery just happens to be one. "They may stink a few weeks a year," he adds, "but they kick butt as an ornamental for the rest of it."