I know several people growing Seiboldii in gardens as far north as Michigan and it survives for them in their gardens quite nicely. Not much is known about the absolute hardiness of clematis and COTW is collecting information on which clematis will grow in what zones as part of their international survey. If you want to act conservatively, I suggest you allow it to stay outside until the top growth is killed back by frosts and the temperatures outside remain cold and then move it into your unheated garage. I am sure it would remain safe there over the winter. I personally don't have an inside space with adequate lighting to try and keep a clematis alive and growing over the winter.

Interesting. I don't have an inside space that has lighting either. My garage has no windows but I didn't know much light was really needed. My clems don't grow at all during our frozen winters and they don't have anything alive come spring. It all comes from the roots. I just might try planting it outside and give it some good protection. That is encouraging to hear they grow in gardens in Michigan. They should stand a chance here. Thanks.