In zone 4, I just cut them back to the ground when they died in the fall and mulched with leaves and or straw. We were in a very dry area so I would water well before the irrigation water was turned off for the winter. The watering prob isn't needed or desired in a wetter area.
a gardening book I've got recommends leaving the stems on mums through the winter and not cutting them down until spring. said it would help keep whatever mulch around the roots better, holding instead of the wind blowing it away.
I'm in zone 5. I'd recommend you just try a different kind of mum. I've found some I can grow here and some I can't. they are so cheap in the fall it's easy to try different ones. and you only need one since all my books recommend you divide them every spring into 4 plants.
my biggest problem here is not in keeping them over winter but in them getting frost bitten before or during their bloom, alot just bloom too late for this area. one of my favorites is a bright purple that starts blooming in late july and is done by the time regular mums start blooming. at my old place I had taken a few plants and with dividing them every spring had ended up with 3-20 foot hedge rows of them and they were just gorgeous in late summer.
also, mums need good drainage especially in the winter. you don't want the ground to get really wet and then freeze. on a slope or raised bed is best.
as a for instance in hardiness: I bought two new colors last year at walmart that I don't have, some beautiful pinks with bright, big yellow centers. all of one color didn't survive the winter, all of the other did and they were planted side by side.
Well....do not forget to also cut them back before July 1st so they won't get too tall and then also bloom too early!! I am in central Ohio and my grandmother always told me to NOT cut them back after July 1st! And it does work out very well for me here!!! Coz I do not want TALL mums & I like the looks of them kind of rounded and short. And mulch very heavy to help them make it thru the winter! I have a brand new one I am very anxious to see bloom so I am gonna take real good care of this one!! Hope this helps you out!!
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