It is getting late to place mums grown in containers into the ground,since there isn't time for the roots to establish, but you can try to plant them while our soils are still relatively warm. Water and mulch aroung the roots. Here is information from the University of Vermont:
OVERWINTERING GARDEN MUMS Dr. Leonard Perry, Extension Professorm University of Vermont
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.
Geraniums would not be hardy outside in Douglas County over the winter. You would need to bring them inside in a heated garage or sunroom to over-winter geraniums. Below are some ideas for overwintering geraniums:
by Cindy Haynes, Department of Horticulture, Iowa State University
With the rainbow of flower and leaf colors, it is hard to watch beautiful and costly geraniums die from a hard frost. There are several ways to keep those geraniums through the winter for a head start on blooms next spring and a savings to your garden budget.
Keep them growing in containers
Geraniums grow easily indoors in containers with proper care and environmental conditions. Before the first frost, cut back plants to half of their original size and inspect them for signs of insects or disease. Then, dig up healthy plants and transplant into containers. Use a potting mix made for containerized plants instead of garden soil. Garden soil is often heavy, compacted, and drains poorly in containers. Place containerized plants in a cool location with plenty of bright, direct sunlight. Water plants well after potting and as needed when the soil begins to dry. Shoot tips may need pinching once or twice during the winter to promote branching and prevent weak growth. Before planting outside in May, fertilize lightly. Plants kept in containers over the winter are typically larger than most geraniums sold in the spring. This allows you to have a head start on growth and blooms for next year's garden.
Taking cuttings from outdoor plants
Geraniums root readily from cuttings. This is also a great way to multiply the number of plants for next year's garden. To take a cutting, remove a 3- to 4-inch section of the plant's stem tip with a sharp knife. Pinch off the leaves from the lower half of the cutting and dip the cut end into a rooting hormone. Rooting hormones are sold in powder or liquid form at your local garden center or discount store. Stick the cuttings in a moist, porous, well-drained rooting media such as coarse sand, perlite, or vermiculite. Cuttings can be rooted in individual pots or several cuttings can be placed per container. Make sure the container has holes for drainage. Ideally, cuttings root best in a moist, humid environment. This is easy to achieve by securing a clear plastic bag over the cuttings and container. This "mini-greenhouse" should be placed in bright, but indirect light. Check the media occasionally to insure it remains evenly moist. Rooting normally occurs in 6 to 8 weeks. After roots are approximately 1-inch long, transplant cuttings into a 3- to 4-inch container with a standard well-drained potting soil. Place in a sunny window and water as needed. Pinch shoot tips back to force branching and prevent spindly growth. New plants produced from cuttings should be vigorous and about the same size as most geraniums sold in spring.
Geraniums are unusual and unlike many annual flowers, they have the ability to survive for most of the winter without soil. If properly stored, they can resist extended dry periods due to their thick, succulent-like stems. To overwinter geraniums in dormant storage, dig up the entire plant before frost and gently shake the soil from the roots. Place the plants inside open paper bags or hang them upside-down from the rafters in a cool, dark location for the winter. Ideally the temperature should be between 45-50 F. Two or three times during the winter, take the plants out the bags or down from the rafters and soak the roots in water for 1 or 2 hours. At this time, inspect the stems. While many of the leaves will die and fall off, the stems should remain firm and solid. Discard any geraniums with shriveled stems, since those plants will most likely die. Pot up healthy dormant geraniums in containers in late March or early April. Water plants thoroughly and cut back the dead stem tips. Place potted plants in a sunny window to initiate new growth. It often takes several weeks for plants to initiate growth after dormant storage.
No matter how geraniums have been overwintered, they should be healthy, free-flowering plants for spring. After being indoors all winter, your geraniums may be as anxious as you are for spring planting. Plant them after the danger of frost has passed and enjoy their colorful blooms all summer. You can invest your savings in new geranium varieties to overwinter next year.