It is true there are southern species and the one northern species S. purpurea... and, wonderfully there are many. many hybrids. Some are definitely more winter hardy than others. I have southern species like, S. rubra, flava, oreophila, alata, minor, psittacina etc., I also have purpurea and a bunch of hybrids. I am two zones warmer than you, I am growing in an artificial bog which keeps temps from great fluctuations and I heavily winter protect all my plants. Some don't need it some definitely do but they all benefit and have gotten large with extensive root systems.

I mulch w/ a couple inches of pine needles then put a few plastic pots upside down, cover with a large piece of burlap then mulch a foot or more with leaves. Sometimes I'll put some woodchips on top of that.

Yours are in pots sooooo.... I would sink the pots in the ground in late Oct. early Nov. and mulch away. Either that or put them somewhere that is cold enough for a winter dormancy but not cold enough to kill 'em w/ temps being regulated to some degree, garage? basement? that kind of thing.

Enjoy them and don't worry about it for now...... you have 4 or more months to figure it out.

Good luck!