I would agree with Mohamed Sanaulla in that it is not crucial because JavaFX (currently) is based on a scripting language. However, if you do start to pick up JavaFX, one thing should become very clear: JavaFX becomes a much more power experience when you can leverage the functionality of custom Java classes in your applications.
So while it is definitely not essential to know Java in an attempt to learn JavaFX, it would absolutely benefit you to have even a basic working knowledge. Your experience will have more and more of an impact as the level of your root Java knowledge grows.
Hope that helps!