When Holly whips(carriage style) are made the wood is aged to dry for years. the bark is stripped off, I beleive some steam to straighten slightly, yew wood is also used, sell able to straighten, even knubbier. Willow is sometimes used as well as Bamboo. Not sure if I know of anyone who makes holly whips in the US though. I do have a contact in the UK that makes them. its a complete art in itself.
-Andrew