I've recently acquired a nice-sized Black eyed Susan (Rudbeckia hirta) plant. But unfortunately I've only got one, when I would like to have several. How would I go about multiplying this plant?
The easiest way is to use division. Every year, the plant crown diameter will increase. After a few years, you can dig the crown out, split it into several evenly sized pieces (all over 8" in diameter, for lowest maintenance - you can do smaller, but they dry out more easily). This is best done in fall, after the tops are cut back, or in spring, right after the frost is out.
You can also save seed. It is a very similar process to saving sunflower seeds, but on a much smaller scale. When the flower stems are dry, take the flowerheads and rub them, over a tray to catch the seeds. You can clean out some of the debris, and put them in a paper bag. Store them in a dry, cool place with consistent temperatures until planting. The seeds look like this:
They can be sprinkled over a well prepped, very fine natural soil bed and raked in very lightly, barely enough to cover them. They should begin to emerge in a couple weeks, and you will need to keep them weeded until they are moved to their permanent locations. They won't always bloom in their first year. Using seed is longer, but gives you rather a lot more plants than division.