Q: I planted a lot of perennials last spring. How soon can I divide them safely? — Sandy O’Meara, Boise, Idaho
A: There is no need to divide a perennial unless it is dying out at the center and flowering less, or has outgrown its space. Of course, you may simply wish to separate a plant into two or more pieces to help fill out a bed (or to give away). Although there is no universal timetable, it’s generally best to divide late summer–blooming perennials in spring (once the plant has sprouted an inch or two out of the ground) and early spring bloomers in fall. If the plant is growing vigorously, you can safely dig and divide it. Give most perennials a few years to mature in the ground before you even think about dividing them. Plants that tend to form a single stalk, such as butterfly weed, Asclepias tuberosa, and gas plant, Dictamnus albus, are more easily propagated from cuttings.