Many gardeners like the perennial sedum, because it thrives in many climates. Although it has close to 400 varieties, just a few are typically planted in the garden. Many gardeners call sedum "Stonecrop," or by the name of one variety, Autumn Joy.
You may want to divide your sedum for many reasons. You want to stop its expansion in the garden and reserve the space for other plants. Your plant may be ageing and not be flowering as much as two or three years ago. You may have found a bald spot in the centre of the planting. You want to give a friend who has admired it a start for his own yard. Or you want more plants like this in more places in your own yard, and this is a way to get plants for free. Whatever the reason, you've decided to divide this plant and want to know the best time for making the divisions.
Spring or Fall
Perennials do very well when they are planted when the soil is warmer than the air for a few hours every day. This happens in spring and fall. When you divide and replant your plant at the time when soil is warmer for a few hours, the combination of warmer soil and colder air encourages root growth. These circumstances also slow top growth so the plant's energy can mostly go into getting well established in its new site. It will stress the plant less if you divide it when it is not flowering or before it starts putting energy into flowering. If you've ever moved, you know all that adjusting takes more energy. It's also true with plants, accommodating to a different micro-environment and re-establishing a root base takes more energy than just staying in place. Although not recommended, if your plant is too crowded or it's your only option, you can safely divide the plant when it is flowering, so long as you are careful to keep the roots cool and moist while they are getting reestablished. Make sure to divide sedum by hand and check carefully that you are only planting healthy plants, as any disease from the parent plant can be transferred with the pup plants and infect other areas of the garden.