What can I do to make them fill out? They're spindly and have sparse leaves although they continue to bloom. Also, the leaves are not a nice deep green, more of a yellowish green color.
In my experience, heavy pruning (as I have done any time of the year) produces tons of new growth, bushier growing habit, profuse flowering. Is it getting enough sun? My leggier hibiscus, it turned out, was spending all it's energy growing toward the sun, not on leaves and flowers.