Answer: There are many different cultivars of mums available on the market. The winter hardiness will vary depending on the specific cultivar. Based on my experience, unless the variety is labeled hardy with a zone as cold as or colder than my own, I would not expect it to overwinter successfully (although occasionally they do.)
There are several other factors that can help or hinder winter survival. Planting in a location that is well drained increases the odds, as does protection from sweeping winter winds. Plants that are set out in the spring and have an entire season to become well rooted also have a better chance.
With mums, wait until late fall to mulch, then apply a generous mulch over the root area but not over top of the plants. Do not cut them down until spring as the branches give some added protection to the crown.
Finally, winter weather can vary from year to year and this can also affect hardiness. Steady snow cover is helpful as it is a good insulator, while freeze thaw cycles and oscillating temperatures are not.
If you have a few plants that seem to be reliably hardy for you then you might consider dividing them in the spring or taking tip cuttings from them in the spring to start more plants.
I hope this helps you trouble shoot.
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