What is a frost jacket and how would it work for a tall rosemary bush? The rosemary is in a backyard in Vancouver, Canada
Certified GKH Gardening Expert
This would be anything that would provide warmth to unhardy plants to keep them alive. Unfortunately, Rosemary is only hardy in zone 9 and up... Which is the very southern portion of the U.S. You are well outside of the growing zone for these plants, and there would be no hope of getting these to survive in your area. The only thing I could suggest is growing them inside under horticultural lighting.
This article will give you more information about growing rosemary: https://www.gardeningknowhow.com/edible/herbs/rosemary/growing-rosemary-plants-rosemary-plant-care.htm