When is the best time to divide and replant very large hostas?
Hostas don't usually need to be divided, even when they get big. If your plant is overgrown, I would recommend trimming the leaves back first. If you still want to divide it, do so in the spring or early fall. Here is a good article on dividing hostas: http://www.hgtv.com/outdoors/flowers-and-plants/transplanting-hostas