Well, Thedarksun you are almost right. Most plants are hermaphroditic, having both male and female parts. However there are still a few plants that are not like this. One that's commonly used in magick that most people might recognize is a tree, the ginkgo. The ginkgo tree has separate male and female trees.
To answer the question, if you are doing a ritual to Aphrodite I'd do a little research and use herbs that are sacred to her, a few examples I can find (just doing a quick google of "herbs sacred to Aphrodite") are myrtle, apple, pomegranate, lettuce, myrrh, anemone and red rose. If you can't find 12 that are sacred to her, I'd use feminine herbs to honor her, she's a very feminine goddess. You could also throw in an herb that your love interest is fond of...for example if they love oranges, use orange peel...if they like the smell of cedar trees, use cedar... you get the idea.
Hope this helps. Remember, rituals are all about strengthening the intent and using herbs that represent the situation is what's important. Whatever you use it should "feel right" to you before you begin. With some practice you'll see what I mean. Good luck.