My Sedum 'Autumn Joy’ are talking to me. They are splayed flat, saying, “Divide me!” Many perennials behave this way. Every 3 to 5 years, they begin to develop a hole or fall away from the center, signaling that they need to be divided to be reinvigorated.
In the case of Autumn Joy, this would be a terrible time to dig them up. They are in spectacular bloom right now and would be very unwieldy to handle. I will make a note of which plants to divide in spring, when they are compact and will have plenty of time to recover before they bloom again at the end of the summer. Mums and ornamental grasses belong in the same category.
Lady’s Mantle (Alchemilla mollis) and peonies (Paeonia) are good examples of perennials that are better divided now. And this is the ideal time to divide Daylilies (Hemerocallis species). Daylilies respond especially well to dividing and will reward you with multiple vigorous plants. (Peonies may bloom poorly or not at all the year after division, but they will thrive after that.) All of these plants have passed their peak for the season.
On a day that is mild—neither too hot nor too cold—I use a transplanting spade to dig up the chosen plant(s). I dig a ring several inches outside the edges of the plant, digging and lifting until I can feel the root ball is loose. Then I lift with spade or hands and tease the plant out of the soil. This is where my trusty soil knife is indispensable (see go-to gear). It has a serrated edge that helps cut through the root ball. I always try to leave at least 3 buds or stems with each new plant. Sometimes the plant will fall apart easily, forming its own divisions. It is not necessary to treat most perennials gently; they will tolerate fairly rough handling.
Then I dig a hole slightly broader than the root ball of the new plantlet and mix in some compost or manure. I spread out the roots, or in the case of daylilies, tubers, and cover with soil to the same level as the original plant. And water generously. It pays to listen to forecasts and do dividing and transplanting when rainy weather is expected. Nothing gets a plant off to a better start than a good rain to soak it in.
Having said all this, I have learned that most perennials and shrubs can be divided and transplanted any time during the growing season. The key is keeping them moist till they are established. The larger or older the plant, the longer it will take to settle in.
What fun it is to take one plant and make it three—or more. This is a great opportunity to develop large drifts of a particular plant or to share with gardening friends.