Mitchell and Webb, British comedians, offer another candidate: The linden tree.
ETA: Link to Column: I want to get my nipples pierced. Are there health risks? Plus: sperm trees of Los Angeles - The Straight Dope
The first thing I thought of was the stinkhorn, a fungus that ironically looks a little like a penis. http://urbanmushrooms.com/index.php?id=48
I encounter these sometimes in my neighborhood in Southern California, but I don’t remember the season. It’s easy to blame the smell on a tree since the stinkhorn is small and likely hidden. However you can smell them from one or two houses away.
The Evil Prince Zorte, thanks for that. I had these growing in my bushes. Yes, they smell like semen. They would pop up at night and be sprouting in the morning. Can you imagine your front door smelling like sperm?
I finally seem to have dug them all out. I started gathering them and throwing them in the trash to prevent spreading.
Yes, they start out looking like an egg, then sprout a plug from the top and grow out. The stem is very soft and fragile. They are slimy and stinky.
And I would never consider eating them, but then I don’t particularly eat mushrooms anyway.
The only line from John Irving’s World According to Garp that always stuck with me:* “When he came, she felt his shot wet and hot in her hand. Under the sheet it smelled like a greenhouse in summer…”*
So the relation in smell sensation from vegetation (yo diggity) is neither far fetched nor unique. Ever since I read Garp in high school, I can’t enter a greenhouse without giggling.