When do I trim back the mums in my garden? I would like to keep them about one foot tall. How often and how much of the plant can I safely remove at one time to keep them at the desired height? After blooming in the fall, how much of the plant do I cut back? Also, when and how should I divide the plants? How many divisions can I make of one plant?
Mums planted the year before should have new tip growth pinched back every 2 to 3 weeks beginning when they are about 6 inches tall and until about the first of August. This makes them full and bushy, and loaded with blooms. It prevents the tall leggy mum that opens up in the center and flops down. Repeated trimming holds down their height.
This year, however, our hot weather has accelerated many of our plants. So if you sheared or pinched your new growth in early August, you probably sacrificed any fall blooms before the first hard frost. If you don’t have the time to trim back tip growth every 2 weeks, a rule of thumb is to trim the plant by half around Memorial Day and trim again by only a third around the Fourth of July.
In late fall, the first hard frost generally kills the vegetative growth. Once the plant is dormant, cut off the stems about 4 to 6 inches above the ground, clean up old leaves and debris, and mulch the area. In spring, when new growth appears, you can make more plants by dividing the new offsets from the original crown. You are only limited in the number of new plants by how many offsets the crown has produced. Remove them carefully with as much of their root system as possible. Transplant to your area of choice or place in pots for later planting.