The winter conditions sound a bit risky for rosemary. Although it can take a light freeze it won't be happy for a prolonged period at or below 32. If you could get it above 32 it would be better. Here, where rosemary overwinters without any problems, our coldest month (Feb) has an average low of around 38. Can't help on the mix as I don't grow Rosemary in a container.

OK, thank you. There have been times the sun porch goes lower but I can move it inside for that. Will give it a go!

GG - do you think Rosemary would be OK at a consistent 30 ie below freezing for several months? I was thinking it would cope with that temp briefly but would need a higher average. Especially in a pot. At 30 the soil would be frozen if the temperature persisted for any length of time, wouldn't it?

I guess this might be open to some interpretation :-) The OP states the sunporch stays above 30F so I would assume that there are days during winter when it is warmer than that, maybe quite a bit warmer. Plus, being attached to the house, no doubt the sunporch receives some radiant heat from the house itself.

I don't have a sunporch, but below freezing nighttime temperatures outside are common enough in my area and can persist for a week or more at a time. The soil never really freezes.......even when in the low 20's for 3-4 days, at best it will be just the top inch or so that is frozen.

I guess what I am doing is comparing the 'no lower than 30F' comment to my typical outside winter conditions - quite similar - and the ability of containerized plants to tolerate those sort of temperatures. Also there should be some consideration given to the size of the container - the larger it is, the less risk.

btw, ground soil requires lower temperatures than just water does to freeze. Temps at 30-32F will have little to no effect.

I have to be careful in late winter/early spring, a sunny day will get the porch too hot, killed some bonsai a year ago.