Answer: To be honest, December is very late in the season to try to establish a plant and expect it to overwinter successfully. You might have better luck if you can plant them now (say at a friend's house) and take cuttings or divisions next spring.
Fall planted mums can be difficult to establish in any case. Select a spot with good drainage and full sun. They should be planted into well prepared rich soil amended with ample organic matter to ensure a moist yet well-drained planting area. Water them in well, and keep an eye on the soil moisture until the ground freezes. (Check with your finger.) Follow up with several inches of mulch around ( but not touching) the plants.
Early next spring when tiny new shoots begin to appear at the crown of each plant, divide it. This renews the old plant and also makes additional vigorous new plants. Another method of starting new ones is to take tip cuttings. With good care, these small plants will grow to a good blooming size in the course of the summer.
You might also try to overwinter them in their pots by allowing them to go dormant. Keep them in a very cool but not freezing place such as a garage and keep the soil barely moist. If they are too wet they may rot. Next spring, place them outdoors in a sheltered spot to acclimate, then plant them in their permanent position. Good luck with your mums!
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