Need suggestions on overwintering Boxwood

I have a B microphylla and would like Zone 4 residents for some suggestions. I aquired these from a nursery at year end closeout and would like to overwinter in the nursery pot. Suggestions are appreciated.

Hi David,

It's zone 4 here in the Twin Cities and I've been wintering bonsai since the early 90's. One of my favorites is the boxwood which are kept in a cool room(30 to 45 degrees) with the trees that need dormancy. I imagine you will need to provide some type of protection against freezing and boxwood definitely should not be treated as a tropical.

Best Regards,
Norma

2 photos of boxwoods (larger are "Kingsville" var.) in my garden were taken in 2008 and 2011. I've had them growing outdoors here in the Allegheny foothills of Western New York for over 20 years. I dig them out nearly every spring to keep their root mass in check. and periodically style and pot them. I have a 1/2 dozen very small (a bit bigger than mame) Buxus microphylla in addition to those visible in the photos. My local W N Y area has been classed as low as zone 3 but in recent years I'd say it's more like zone 4-5 and another "mild" winter has been predicted. Some of the boxwood have only experienced very small die back on new growth a few times in their entire existence here. Leaves do turn color in the late fall but return to bright green in the spring. I get the same color reaction from my junipers. I've put tons of gravel and coarse sand in my yard after digging out the soil (about 2' deep). Most of it is covered with rocks and flat stones. I built a stone wall for a raised bed area and grow several boxwoods there as well. I cover my trees with lots of snow as soon as there's enough to use. I suspect that the rocks heat up enough to provide a warmer micro-climate effect in my yard ( a small city lot )
I do get some die back on J. maple branches. It seems to happen in the spring. I gave up long ago on Trident maples after loosing several. I'm wintering about 30+ Ginkgo seedlings out there that were started from seed I'd collected in 2010.

I've made it through two winters here in Chicago by building a box from closed cell building foam (isocyanurate form Depot or Lowes) big enough for all my trees. You can duct tape the panels together but I use long construction screws. Then just add a greenhouse thermostat (google "Ranco") and a reptile heater element (like a std lightbulb but solid ceramic and almost unbreakable, try ZooMed). Get a Lacrosse wireless thermometer w/ outdoor sensor if you want to be sure everything is working, from the comfort of your living room. Total cost should be around $100

I keep my boxwood bonsai in an attached garage, 25-40 degrees, no problems even with freezing occasionally. They stay a nice green all winter.

Thanks for the suggestions, I appreciate you seasoned pro's advice.

New to the forum, here. You may consider asking the nursery, if they are local. My location is a bit warmer than yours, but I overwinter my boxwoods in bark mulch.