Miguel, I asked that same question in the past few years for the same reasons. What I seem to remember was that the montanas and the atragenes don't seem to cross with the other types.

Grey-Wilson's subgenus Cheiropsis, section Montanae seems to be a closed loop in terms of crossing from what I've been able to find (the Hull database shows no crosses outside of it, though there are crosses within it, such as C. chrysocoma x C. montana var rubens).

Likewise, the his Atragene subgenus seems to be closed as well, crosses I can find only exist within it (fauriei x sibirica, etc.).

I've not troubled myself with some of the subgenuses since they're not hardy here (like Meclatis), but there are plenty of examples on the web, in the Hull database and in other places of crosses among the large Flammula and Viorna subgenera.

The Tubulosae subgenus crosses with itself, and there are successful crosses with the Flammula subgenus (x jouiniana is C. heracleifolia var. davidiana x C. vitalba, for example).

I have a large list of less obvious successfl crosses I've found evidence of (fusca x lanuginosa, for example). If you'd like the list, email me and I'll send it to you.

I think the person who told me about montanas and atragenes not crossing out of their own subgenus might have been Paul over in Gardenbuddies.

Let us know what crosses you do come up with, though!

Michael thanks loads for the information. So far I have run across nothing like it. Will email you for the complete info. I have made tons of crosses this year but to be quite honest since I have things packed so close together the bees and other pollinators have probably done as much as me. Some plants are just loaded with seed--unlike anything I have seen in the past. Does this mean we are going to have a real winter this year? LOL Any way, Debbie at Silver Star Vinery is going to get the bulk of the seeds since she has far much more patience with raising clematis from seed than I will ever have. She and I have joked quite often, in fact just earlier this week joked via email, about us having children together! LOL Carol Lim is also getting some of my texensis seed since she liked the color of the flowers on the plant I have.

Thanks for the info you supplied and the offer of the additional information!