I live in Los Angeles. Notwithstanding rumors of LA's ugliness, I find it to be a sometimes-quite-charming city, which often has as much to do with the number of trees around as anything else. Trouble is, every Spring somewhere between 10-20% of those trees bloom, and smell positively awful. And not just any kind of awful -- they smell, no joke, like sperm, and are colloquially known as the Sperm Trees of Los Angeles.
I decided to get to the bottom of this. What are these nasty tree-beasts? According to Wikipedia, they are the hilariously-named Tree of Heaven, (or Ailanthus altissima), an invasive species from China, brought to California by Chinese miners and railroad workers around the turn of the last century. They has since earned the monikers "stinktree" (for obvious reasons) and "ghetto palm" (for their tenacious ability to grow in the harshest of urban conditions, like gravelly parking lots and under freeway overpasses). It's also "one of the most tolerant species to air pollution," which may further explain its prevalence in Southern California.
It's like War of the Worlds, plant-style: the Tree of Heaven grows aggressively, out-competing and quashing native species, and tends to grow up against the sides of buildings, where its invasive root structure can burrow underneath, destroying sewer pipes and housing foundations. Cutting the saplings yields an unpleasant, sticky substance and releases a cloud of malodorous stank. So there you have it -- the green trees of LA, once thought to be this city's environmental saving grace, are actually trying to kill us.