Hi - I am in zone
6, Rockland County, NY and I have had perennial mums for many years. We've been in this house 20 years now and some are from the first year here. Back then, a well known master gardener, Ralph Snodsmith (wrote several gardening books), who lived locally, told me to put the pots in the ground for winter. Make sure the pots are securely in the ground, because they will heave up when we get thaws and frosts back and forth during winter. Then in spring
, you take the plants out of the pots and plant them in the ground. The reason behind this, is that there isn't enough time for the roots to get established in the ground the first year, and the pots will help insulate them. This has worked for me every time I get new mums.
Also, I have found over the years that one can make loads of new plants from cuttings. In order for them not to get too tall and leggy, one should cut them back 3 times over summer. The first time, when they start growing
. Then sometime in June, then again the last time around July 4th. This makes a much fuller plant, and the stems don't get too tall and fall over. I always take my cuttings and put them in a pot of soil, and keep them in shade for a few weeks until they start rooting. Then they go out in full sun for the rest of summer. LOTS of new plants that way!