Ask Mr. Smarty Plants

It is too late in the season to divide and transplant perennials now.  Many perennials can be divided in the spring or fall but others have a preference. Rudbeckia preferes to be divided in the spring.  Because Black eyed Susan's seed heads add such great fall and winter interest in the garden in a place where they are not covered up with snow, you will want to wait until spring anyhow.

They are quite easy to divide and you will be blessed with Rudbeckia for the rest of your days.  When it is time to divide them, digging up the whole plant, splitting it and re-planting is the best method as it rejuvenates the plant and gives you the opportunity to pick out any weeds or grass that have managed to get established in the center.  Alternatively, just dig out the plants around the outside, tranpslant them and leave the center.  Many perennials, however, eventually get so crowded at the center that it dies, leaving a circle of younger plants.

Once you have the clump dug out it will be easy to pry the plants apart from one another with your fingers or a hand tool.

A great reference book (with instructions on pictures) for  all sorts of propogation is Ken Druse's "Making more Plants: The Science, Art, and Joy of Propagation".