Should the clematis vines be cut near the soil and then mulched or do I just deadhead and leave the vines on the trellis and mulch their roots? I have been leaving them on the trellis but they seem to get thinner and less "bloomy" each year. If I cut them back, will they send new shoots up in the spring? The vines are at least 12 years old.
They do survive in your area without much help, but you can give it a little boost. Cut it back to half size after dormancy, and fertilize in the spring. Here is an article for more information: https://www.gardeningknowhow.com/ornamental/vines/clematis/growing-clematis.htm