Hi,
Since you describe the seedhead part as so small, it sounds like your seeds never developed properly. If the seedhead part is not hard(you can bend it easy with you nail,) they are no good.
Remy

Remy,

Do you know what the viable seed of Clematis montana looks like? Could you describe it or attach a pic?

Has anyone successfully grown C. montana from seed?

Thanks, Rae

Rae,
I have at least a half dozen straight species type seeds and I can tell you they do not vary much. All clematis seed gets a hard head if viable. They all look relatively the same. Some have a lighter colored edge to the seed, but it is still hard. The only clematis I know that have small viable seed are the non-climbing types like C. stans, but of course the good seed are still hard.
On the link below, is a pic of a Clematis montana seedhead. It is not the best pic, but I think you can see the thickened seeds at the base.
Remy

Here is a link that might be useful: Clematis montana seedhead