At left is a seed of the coco de mer tree. This species produces the largest seeds in the world, but as you can tell, they're not exceptional just because of their size.
But wait! There's more! The stamens of the male trees look just as suggestive as the seeds of the female trees - and they're just as large. The whole thing kind of reminds me of the Babel Fish joke from The Hitchhiker's Guide to the Galaxy, where the fish is such an obvious clue to the existence of God that it actually negates God's existence.
Then there's the jellyfish tree. If ever a plant was designed to feature in a tentacle sex story, it's this one. Sadly, it's an endangered species, but at least in my story the aliens have a way to give it an edge on survival.
Finally, here's a clue that you might want to tuck away in your memory until the story gets published: the garden is an interesting place to visit, and depending on where you go, you might have a life-changing experience there...but what's behind the garden could be even more important.