Summarize:

Pinching refers to the pinching off of the top of the stems of a plant to encourage bushy growth and blossoming. Pinching can be done alongside the removal of dead blooms in late spring to early summer, as doing it before winter can also cause the mum to be vulnerable to the cold weather. Pinching is not strictly necessary but is a good addition to your deadheading routine, as it can make your plant fuller and healthier while keeping the shape of the plant compact and bushy. Because pinching is best done on tall stalks that you want bushier and more fruitful, take a good look during the late spring and early summer for stalks that you want shorter. It’s best to pinch your mums once the new shoots that emerge in the spring are 3 to 4 inches (7.62- 10.16 cm) high. Also look for any tips that look dead or browned. Once you have identified some stalks that should be pinched, grasp the tip of a shoot right under the first set of leaves, about ½ to 1 inch (1.27 to 2.54 cm) down the stem from the tip. Use your fingernails to pinch off this tip and discard the pinched off tip of the plant. Repeat the process of pinching on all unhealthy or overly-tall stems. Pinching not only shortens the height of a tall plant, it also allows encourages rapid growth of leaves and blooms right below the pinched off area. The optimal time to stop pinching for most mums is in mid July so that the plant has time to blossom and grow before blooming season. If your mum plant is an early cultivator then you should stop around mid-June, and if it is a late cultivator like a “Minnyellow” or a “Minngopher” mum, you can get away stopping pinching around early August. Look up your type of mum to see whether it is an early or a late cultivator.
Pinch mums during late spring and early summer. Identify leggy stalks to be pinched. Grasp the tip under the first set of leaves. Pinch the tip off with your fingernails. Stop pinching in mid June to early August.