Nature doesn't cut back garden mums when their blossoms fade in fall. Neither should you.
Gardeners who live in the South, where mums will continue to grow throughout the winter, need to cut their plants back to encourage continued bloom and prevent legginess. But not here in the North. Research by one of the world's leading breeders of chrysanthemums indicates that mums grown in northern gardens may survive the winter when mulched, but not cut back.
For one, not cutting back leaves the plants better able to hold the mulch placed around them. Mulching is a standard technique used to protect plants against fluctuating temperatures. It also helps keep moisture in the soil.
A good snow cover will protect plants, but as there's no guarantee that it will snow or how much we'll get, I recommend using evergreen boughs or applying a thick mulch of straw or bark. Don't use dead leaves as they tend to pack tightly. Apply only after the ground begins to freeze, never before.
The idea is to keep the plants uniformly cold, not to protect them from the cold. Delaying mulching gives the plants time to harden before winter arrives. Of course, the longer the plants are in the ground before the first freeze, the better their chance for survival.
However, in research trials at the University of Vermont Horticultural Research Center in S. Burlington, of the 80 varieties trialled over a period of four years, none was found to be reliably hardy for the Burlington area, one of the milder areas of the state. Lack of a good snow cover affected the plants' survival rate. Many of these same varieties would probably do well in areas that receive heavier snowfalls.
Next spring, if your plants have survived, uncover them as soon as they start to grow again. Divide the plants when new shoots reach four inches high.
After digging up the plants and discarding the old center portion of the root mass, separate the young offshoots. Then plant them 18 to 24 inches apart.
Water thoroughly and apply a 5-10-5 or 10-20-10 liquid or granular fertilizer. Slow release fertilizers, organic fertilizers, and even generous applications of compost can also be used. Fertilize two to three times during the growing season if using the non-organic fertilizers. If using organic fertilizers, and leaves turn light green or yellow, this indicates the plants need more fertilizer.
Within weeks, you will need to start pinching off new growth to produce full, multi-bloomed plants for next fall. Continue pinching whenever new shoots are three to five inches long, stopping around mid-July.
Dr. Leonard Perry, Extension Professor
University of Vermont