Varieties of rosemary will vary as to their cold tolerance. The trailing types seem to be more sensitive. In general, I am finding recommendations of the very lowest temperature as -3F, or other sources say 10F, 14F, or 23F. Some say bring it indoors when temperatures dip below 30F. These are wide ranges. To be on the safe side, if the plant will experience temperatures below 33F, then you should bring it in. 40s will probably be OK, especially next to a building. It might be a good idea, though, to gradually introduce the plants to the outdoors — such as 1 hour one day, 2 hours the next, etc., to help them get acclimated to the new conditions.
Varieties of rosemary will vary as to their cold tolerance. The trailing types seem to be more sensitive. In general, I am finding recommendations of the very lowest temperature as -3F, or other sources say 10F, 14F, or 23F. Some say bring it indoors when temperatures dip below 30F. These are wide ranges. To be on the safe side, if the plant will experience temperatures below 33F, then you should bring it in. 40s will probably be OK, especially next to a building. It might be a good idea, though, to gradually introduce the plants to the outdoors — such as 1 hour one day, 2 hours the next, etc., to help them get acclimated to the new conditions.