Ralph Kreider, secretary & editor of the Illinois Nut Tree Association (parent organization here), stopped by our house this morning to take a branch from the female ginkgo tree in our front yard. (Well, he just pulled up in the drive and helped himself, but I was feeling neighborly and he was venerable, shall we say, and we had a good long discussion about ginkgos and oaks and I wound up thanking him for stopping by and inviting him to come on over anytime to show off the tree.)
It seems that male ginkgos are common in the area, including our south yard (the tree on the left in the picture), but this is the first female of the species Mr. Kreider has ever seen. Ginkgos are unusual in that each tree is a specific gender; the female ginkgo bears nut-like seeds. These make the female unpleasant as a shade tree because during the autumn the fallen seeds produce butyric acid which also gives rancid butter its overpowering stench.
More ginkgo information here.