I have a semi-shaded perennial bed on the north side of my house that was beautiful in early July, when the bee balm (Monarda) was blooming its head off. The blooms were an intensely saturated purple, and they attracted all kinds of bumblebees, including those strange-looking bumblebee moths (Hemaris diffinis). Also there are some daylilies and Siberian irises, a rose bush, and two clumps of peonies. (This is a bumblebee moth on a thistle, not a bee balm blossom, and I didn't take this picture.)
The problem is, the bee balm has just about taken over the whole bed, and in early August it looked pretty ratty. I thought about just cutting it to the ground, but when I did that, I saw that it was seriously impeding the growth of the daylilies and the irises. I started scraping at its roots, and I found out that the roots were very thick, matted, and pervasive. No wonder the daylilies looked puny! I dug out almost all the Monarda to move to wilder places on my farm. According to Allen Armitage, Monarda is native to stream sides, under trees. Check. We have that on Brangus Lane. (His book, Herbaceous Perennial Plants, is worth having as a reference book, especially for Southern gardeners, as he gardens and researches perennials in Atlanta.)
While I was at it, digging up this bed, I decided to divide some of the peonies. Some of the trees near that bed have gotten so big that they cast more shade than the peonies like. So I dug around one of the clumps and removed some of its underground buds, or "eyes." They look like this:
There were enough under that clump to plant two new clumps of peonies in a brighter, sunnier place, and yet leave enough at the original clump so that it would grow again in spring. This is a good idea, in case the transplanted clumps don't survive: that way you won't lose that clone entirely. You plant the "eyes" about 2 inches deep, and no deeper than 3 inches, or they won't flower.
August is a good time to divide perennials that flower in early spring. I also divided some of the Siberian iris. These grow in clumps with rather matted roots, so you simply stick a digging fork into the middle of the clump and pry a piece off, to move elsewhere. People in my neighborhood are in the process of dividing perennials and getting rid of some, so it's a good time to swap. I will give some of the Monarda to my neighbor who has a semi-wild garden down by the Caney Fork, and I will get some Echinacea, daylilies, and irises from another neighbor who doesn't like pink flowers (the Echinacea) and has too many daylilies and irises. That way, I can expand my own perennial beds, and it won't cost anything but some time and labor.
I plant perennials rather closer together than is usually recommended, because that way they shade out the weeds more quickly: usually I use a spacing of about 12 inches instead of 15 inches between daylilies for example. In the fall, I also scatter around alfalfa pellets for fertilizer in the perennial beds. This is especially good for the roses. Then I mulch with leaves, pine needles, or straw.