I put straw and a black barrel over mine one year. The rosemary was fine. Even had to leave it on there for 3 or 4 days and it was still alive and well

The plants will be fine under straw and, since straw is not airtight, they can breathe underneath it.

Be sure that you are planting the most cold-hardy named Rosemary that you can find. I grow "Arp", which is supposed to be cold hardy in zone 6 (with protection like mulching).

If you keep losing rosemary plants, either your weather is getting abnormally cold, or the soil doesn't drain well enough. Rosemary needs very well-drained soil that doesn't stay soggy wet. If your soil holds too much moisture, amending the soil with a lot of organic matter to improved drainage and forming the amended soil into a raised bed that drains better might help. Or, grow your rosemary in a large pot that can be moved indoors in winter.

I got my rosemary from a nursery who claims theirs never dies. maybe I had put it in the wrong place. this year I placed it where there is a lot of sunshine, and it really grew. I probably didn't need to cover the tansy, but it was hard to find, and since it is next to the rosemary I covered it. if it ever warms up some I can uncover them some. if this dies I can find a nursery online that has the arp kind. thanks to you both.

You're welcome. I see Arp in lots of local places in the Spring, but I don't think I see it in the places that exclusively carry Bonnie Plants. I see it more often at actual nurseries, as opposed to the garden centers of some big box stores, and sometimes at local farm stores who sell plants from someone other than BP. In the DFW area, if you're ever down there in Spring, I know I see Arp at the stores belonging to the Calloway's Nursery line.