Ever wonder why during the following year in the garden the garden mums you purchased at your local nursery and planted in fall grow more leggy in your garden the following year rather than dense and compact as they were when you first bought them? There's a reason. Left to grow naturally in the garden, many varieties of garden mums become leggy by summer and will even start to produce flowers, which you don't want to let them do. Why? Because the flowers of garden mums will quickly fade and die in the heat of summer. This is because they prefer cooler temperatures and will hold their flowers for much, much longer during the cooler temperatures in fall.
Why pinch back mums?
Mums sprout in early spring and then start to grow in a bush-like fashion, sometimes. However, depending on weather and the environment, if left un-pruned, as mentioned, some varieties will grow quite tall and leggy and start blooming way too early. When this happens the weight of the flowers and leaves on the stems will often cause the branches to fall over. To prevent this from happening and to maintain a compact shape with more branching, you can pinch mums back starting in early spring and continuing into mid-summer arounf the 4th of July.
NOTE: You don't have to pinch or prune mums at all. If you choose not to prune you might only get a short bloom period in summer and it might be necessary to use plant supports to keep leggy stems from flopping over throughout the season.
When and how to pinch back mums
If you decide to prune your mums, you can start pinching stems in spring when the plants have reached a height of about 6 inches. To pinch them back, simply grab a stem between your thumb and index finger about 2 to 3 inches above the base of the plant and just above a leaf and pinch it off. If you're like me, and keep your fingernails clipped short, you can use a pair of sharp puners or snips to cut the stems. New lateral stems will then emerge from just beneath where you initially pinched plants. When these new stems have reached 6 inches in length pinch them back by about 2 to 3 inches just above a leaf. Continue this process for each stem of the plant. You can repeat this process until mid-July at which time you can cease pruning and allow the plant to grow and produce buds that will bloom in fall. Pinching back any later than mid-July can reduce the number of flower buds
This pinching process may seem drastic, but the results will be bushy, well-shaped fall-flowering plants.
NOTE: If it's already early summer and you have done no pruning, and your mums are tall and leggy, you can simply cut the plants about one-third to half way back. This should help to create a more dense plant that will bloom in fall.
How To Care For Mums After The Fall Bloom
After your mums have finished blooming in the fall, and the foliage has gone completely dormant, you can cut the dead stems back to just above the ground. That being said, northern gardeners can leave the dead stems there to help protect the roots from severe cold weather during winter. No matter what region you garden in, it's a good idea to apply a 2-inch layer of pine straw or shredded wood mulch around the roots of your mums. If you place mulch directly on top of the plant make sure to remove it in late winter so that the new growth can emerge in spring without being damaged.