It may be fall, but where the perennial herb garden is concerned, it's time to think spring. As cooler, damper weather approaches, check your garden for herbs that need to be divided. Some-mints, sorrel, chives, oregano-can be divided now, moving the divisions to a permanent location or into a cold frame. Mark others-Silver King artemisia, catnip, bee balm, tarragon-for spring division. Tag them now, before the first killing frost, so you can find them easily when spring comes.