Some time ago on the LiveJournal Gardening list, the question came up: How do you tell if you have a male plant or a female plant in the Goatsbeard (Aruncus) family?
I have the Dwarf Korean Goatsbeard (Aruncus aethusifolius) and knew that I had seen two very different types of flowers. These are all going to seed now, but I pulled one of each to demonstrate the difference.
From what I read on a site discussing the larger goatsbeard (Aruncus dioicus) (sorry, I didn't get the link at the time), the larger, showier flowers (top in photo) are from the male plant and the smaller, denser flowerheads are from the female. This seems to be borne out by my example, as the female flower (below in photo) is forming a thick clump of seeds.
Two plants will produce a plethora of seeds, which in my zone five climate, seem to have a 100% germination rate. They are easy enough to identify as they sprout, and thinning them or weeding them out entirely would not be a terrible chore.
I leave the seedheads on all winter, as they are quite decorative in the snow. Areas with heavy snowfall would not see this however, as these dwarf plants are only about twelve inches tall.