I found a discussion of the Oath Rod and fertility on Dragonmount, but I'm unconvinced that it's a contraceptive. Few Aes Sedai have children (quoted from CoT 10, but I don't know who's speaking or what the context is). I would buy the argument that the blackberry bush is the Two Rivers. I just remember that he was surprised by the number he chose, and Tar Valon would be a place to be surprised by the number of men who can channel. It's also pretty dangerous to go there, since word of Asha'man is getting around, even if Elaida's a fool and doesn't believe in them.