Rosemary is cold hardy only to 10° - 20° F.
It gets way colder than that here every winter.
that’s a problem
I am having GREAT success keeping three potted Rosemary thriving in the house this winter. Two are in a south-facing window and one in an East-facing window, but under a grow light.
I learned that the secret is to keep them on a tray of water for extra humidity and to mist them every week. So far I have had none of the usual problems; leaf browning and dropping or mildew issues. They are supple, bright green and growing new leaves & branches.
I’m going to start boosting their fertilizer right now to see if I can get them to bloom for me this winter before they go out to the Greenhouse in late March, weather permitting.