Question
when is best time to divide and replant and how do you divide

Answer
Tom,
You don't say what type of hibiscus you have - if it's a hardy hibiscus, the type that die to the ground every winter, you can divide in spring or fall. You'd dig the entire clump up and use a root-pruning saw, or an old knife, to cut a piece off about the size of a luncheon plate. (6 to 8" in diameter) Then Replant the pieces and water well. Do this either now, or in early spring before the plant breaks dormancy.

If you have a tropical hibiscus usually these are single stemmed plants that are shrubs - you don't divide these, but propagate by taking cuttings off of this year's growth. See a good article about taking cuttings here: http://www.finegardening.com/how-to/articles/propagate-your-shrubs-from-softwood...

I hope this helps,
C.L.