Female ginkgos are at peak fruit drop now, dropping their malodorous fruit (smells like vomit - caused by the release of butyric acid, which also gives rancid butter its horrible smell). This photo was taken on the sidewalk outside 1044 Shrader (cross street Carl), in Cole Valley. There aren't many places in San Francisco where you can find the female of the species, which is why I created a page in Trees of San Francisco listing all of the SF locations where I knew of female ginkgos. Particularly if you have a pre-adolescent boy in the house, now is the time to experience one of nature's most unusual and interesting smells!
No one really knows why ginkgos adapted to have smelly fruit, but the best guess is that it was attractive to an animal, which helped the plant disperse its seeds. You hear stories of dogs, for example, eating ginkgo seeds. But since ginkgos have been around for hundreds of millions of years, the interesting question is, are the things that adapted to disperse it still around? Or are they extinct?
Update: a reader just emailed to tell me about three female ginkgos in in the courtyard of the Ping Yuen housing project at 655 Pacific (between Kearny and Grant) just inside the fence next to the sidewalk. Great that there are some in Chinatown!