Experts and BHG readers answer.
Well the time to keep them from getting leggy is in the early summer. Next year, as your plants begin to grow, pinch them back when they get about 6 inches tall. This is usually around May or June. By pinching out the terminal bud on each stem, you'll force the plant to become much more bushy and compact. All you need to do is use your thumb and index finger to pinch off the top inch of each stem. This will help keep your plants much more stocky. BUT, you may also want to offer the plants some support at the same time, just in case they still grow too tall. At the same time you pinch, install a peony support ring over the top of the plant. The mums will grow up and through the peony ring and stay upright. Plus, once the plants grow through the ring, you really won't see it. You can buy peony rings at most garden centers or from Gardener's Supply, www.gardeners.com