Ah, spring. The sun is out, the streets are humming, the days are getting longer, and the air smells like… like… um… say, can anyone else smell that? It’s not just me, right?

Right?? It’s not just me. All over America, spring is getting smellier every year, and the culprit is the Bradford pear, a tree that gained popularity in the mid-20th century for its ornamental properties.

But it has since proven to be kind of a nightmare. As well as its distinctive smell (described as smelling like rotting fish, sweat, or semen), it has a nasty habit of shedding branches during storms. Its hardiness makes it awfully difficult to get rid of, to the extent that it’s now considered invasive in most of North America.

Oh, and you can’t eat its fruit, either, because it’s full of cyanide. The tree isn’t just a problem here in the US, either. In my home country of Australia, we have a folk song called “Give Me a Home Among the Gum Trees,” which has been repurposed in an entirely predictable manner to celebrate the apotheosis of the Bradford pear.

So if you’re wondering where these trees come from, why they’re everywhere, why they stink, AND what on earth they have to do with 9/11… read on. Ah yes, spring has arrived…and it smells terrible. Image: Popular Science Why does it smell like that?

We should start with the thing that makes most people aware of the Bradford pear: its smell. People tend to step coyly around the tree’s unique odor: it’s been described variously as “funky”; “fishy”, and “ammonia-like.” But we’re all adults here, so let’s be honest: Bradford pears smell uncannily like semen. So why is this?

Like most scents emitted by plants, the Bradford pear’s perfume exists to attract pollinators. Unfortunately, the tree’s pollinators are flies, rather than bees, so rather than a heady sweet scent, the Bradford pear pumps out smells that attract flies. These scents of death and decay get their odors from nitrogen-based compounds called amines.

Amines are a vast and diverse family of organic compounds, and their common feature is that they’re all derivatives of ammonia, which is the simplest simplest compound of nitrogen and hydrogen. The diagram below shows ammonia’s structure: the blue nitrogen atom is bonded to three grey hydrogen atoms, forming a pyramidal structure. In amines, at least one of those hydrogen atoms is replaced by a carbon atom.

In the simplest case—methylamine, a name that’ll be familiar to Breaking Bad fans—that one carbon atom simply has three hydrogen atoms bonded to it, as shown in the diagram below. Note that the blue nitrogen atom now has only two hydrogen atoms bonded to it, with its third bond going to the black carbon atom. Methylamine is a simple molecule, but more complex amines may include one or more carbon chains, which may branch off further, connect to other groups, and so on.

It’s no surprise, then, that there are a lot of different amines, and they turn up everywhere–from psychotropic substances like ketamine and amphetamine to dyes and fertilizers. But for all their diversity, they have several things in common. They are generally bases; they’re volatile, with low boiling and melting points; and they all kinda stink.

Amines are also ubiquitous in living organisms, where they form long chains called amino acids. When living things die, those amino acids slowly break down into smaller amine molecules with cheery names like putrescine and cadaverine. It’s these molecules that release a smell that creatures like flies and beetles associate with death.

This is all very well, but what does it have to do with the smell of semen? Well, semen also contains amines. It does so because of their alkaline properties, which helps counteract the acidic environment of the vagina and keep sperm alive long enough to swim for glory.

And it seems that the mix of amines is very similar to that released by… Bradford pears. Why are there so many of these damn trees? There’s at least one other glaring question that remains unanswered here: if these trees’ scent falls somewhere between swingers’ party and graveyard, why on Earth are there so many of them?

Well, say what you like about Bradford pears, but they are pretty. They were originally bred in the early 20th century after a fungal disease called fire blight tore through pear crops across America. The Department of Agriculture went searching for a species resistant to fire blight, and found what they were looking for in the form of the Callery pear, a species native to Asia.

That tree was attractive to look at, resistant to disease, and able to thrive in a variety of environments. Unfortunately, it also had nasty thorns all over its branches, and while it brushed off fire blight, it fared less well with several North American insects. Scientists started trying to breed new varieties without these shortcomings, and the result was the Bradford pear, a thorn-less and insect-resistant tree that arrived in the 1950s. Initially, people loved them, and the