first .... if you have good draining soil ... simply sink pots and all in one hole ... and loosely backfill ... the key is .. GET THEM DORMANT ... AND KEEP THEM DORMANT ... [wish i had a dime for each time i typed that in the last few weeks ...] if zone appropriate... temps are not important ... NO SUN ON POTS... they can easily thaw roots.. which then refreeze ... etc.. more than once.. and the plant will die ... and it wouldnt hurt to keep the plant out of blistering winter winds ... can do any of the following: north side of the house .... once frozen.. tip on side.. this avoids sun on pots.. and tipping in late dec avoids water accumulating in frozen soil ... buy 3 bales of straw.. and place against house.. and circle with bales... reduces wind ... and sun on pot or.. as above... dig hole in veggie garden.. and sink pots and all.. presuming you do NOT have clay soil ... review the KEY ... and you will succeed.. no matter which option you take ... it might be a great time to give them a thorough soaking .... ken

Guys, thanks for the add'l replies. Last winter, I mostly kept them in storage room adjacent to the house, bringing them out for occasional watering on warmer days. That is, except for 1 tree that I brought inside maybe a bit more than a month after the first frost (i.e., allowing some dormancy, but before the coldest part of the winter) for a test case. The one kept inside did fine, and began to grow. The rest took a beating, and their main stems all died. They did resprout from the very base, but it wasn't pretty. I can technically boast of 100% survival, but last winter was a fairly mild one, temperature-wise. For various reasons (mostly, lack of space!) I need to keep these potted; I will try putting them in a bed of mulch and top soil, to see if that helps. They're so small that I'm not worried about snow load breakage yet, but I will watch for that. Sam, just saw your post as I was typing this... that's interesting that they will live up to the DC area! Thanks again.

I wouldnt know the answer to your question Im a zone 4-5 and I have a couple.But I do have a question for you did you purshase these ?or did you grow from seeds?If purshased did you get a good buy?I have been looking for some of these rosie

I'm guessing you are in the UK. In which case it is Carex pendula. A native but a self seeder and can be a nuisance. Not a disaster in that position but it will send babies out around the garden. Any herbicide will need to be specific for grass/sedge, not a broad leaf weed killer.