When I have potted up clematis to overwinter or to grow them out in I have used any commercially available potting mix such as Miracle Grow or Scotts.

I used Miracle Gro with moisture crystals. Very helpful for keeping the medium constantly moist.

I have used that media also Alina for growing stuff out over the summer when moisture rentention is an asset but would probably use the type without the moisture absorbing crystals if I were overwintering the plants. The last thing you want is too much moisture around the crown during a period when the plant is essentially dormant.

Miguel,

In my experience, these crystals do not make soil soggy. They just help keeping it constantly moist. I had 30+ potted Clematis planted in this potting soil for over 2 years. I lost 3 of them: VIENNETTA and two other Evison's hybrids. All of them turned out to be very finicky for growing in my zone 6, no matter where they were planted. Potting medium had nothing to do with this. I am going to give VIENNETTA one more try (will plant it close to the house).

Tracy, make sure that the drainage is excellent. Soggy soil can cause crown rot. Also, protect them from freeze/thaw cycles. If you have unheated garage or basement, you can place your pots there.

Alina, I couldn't agree more that the crystals help maintain soil moisture and I can't imagine growing clematis in pots in my zone over the course of the summer without them. However I interpreted the poster asking what to pot the plants up in to overwinter them-not sure if they are going to be placed in a cold garage or into holes in the ground. Either way, dormant plants don't need the water that actively growing plants do. The biggest killer of houseplants over the winter is overwatering them since even they require less water when they are not as actively growing during the winter as they do during the spring/summer when actively growing. If the poster has a greenhouse and is going to be able to keep the plants alive and actively growing, then if she wants to pay the extra price for potting mix with water absorbing granules, then she should go for it. I just think paying the extra price for potting mix with the granules for overwintering dormant clematis is a waste of money and I would hate to risk losing any of the 21 clematis I had purchased, even if the risk might be small.

I believe you expressed concern about the size of the roots on some of the plants that you got from Chalk Hill. Smaller root systems will require even less moisture than larger established plants. Again, I think it is better to err on conservative approaches that go overboard with something that could lead to loss of plants, especially when the buffering capacity of the granules for moisture storage is not needed.

I guess we can agree to disagree! :)

Hey, I do not disagree! Especially, if Tracy is going to plant them in the garden next spring.
I kept my potted Clematis outside. I was too lazy to go outside and water them when it was cold. It can get really dry in winter as well. I am not sure that it would harm dormant Clematis, but I lost some potted Lilies because of the dry soil in pots. I guess, the safest solution would be regular soil in conjunction with moisture monitoring.