In many areas Sweet Autumn clematis (C. terniflora) is invasive and seeds around. I don't think it has a long enough time to ripen seeds well in my area, but I've read numerous times of it causing problems where it's warmer, so you might want to consider other plants or at least research if it creates problems in your area. If you aren't familiar with Clematis on the Web here is their search engine so you can put in the height you want, and/or pruning type, month with bloom (though take months of bloom with a grain of salt since weather has a lot to do with this) and see what pops up. (Anything you don't care or don't know leave blank.) Not all those plants will be available commercially in the US, but many will be by mail order from reputable nurseries such as Hummingbird Farm, Joy Creek, Brushwood, to name my three favorites.

Sweet Autumn is vigorous enough that I don't think there are any other clematis that won't have issues trying to not be overwhelmed by it. I'd suggest planting a few more plants that all have similar size, and look for plants that have longer bloom time. For instance, Mikelite blooms fairly fully for at least 4 weeks and perhaps longer for me even though it's only been in the ground 2 seasons. Ville de Lyon is another plant with a long bloom period in my garden. Confetti blooms later than any other type 3 plant in my garden, but you can also look at Mrs. Robert Brydon which blooms in August for me. Hopefully, others will have suggestions for longer-blooming or later-blooming plants.

Thanks, I'm worried that SAC is in a league of its own and may even impede our ability to enjoy the deck without a constant battle. I'd commit to stay on top of cutting it back before it goes to seed, but if it will overwhelm anything else that's a problem. If anyone has any other ideas for a late bloomer, I'd love to hear it (SAC blooms into September in lower NY from what I hear).

My clematises are all still babies so I don't have much to report, but I just bought some Golden Tiaras and Polish Spirits. I hear they are both very vigorous growers that bloom July - October or something like that. I put SAC on the pergola on the north side of my house for its shade tolerance, but I haven't tried growing any other types with it.

There's a comment on this thread that "Madame de Bouchard is blooming until mid of November in zone 6 PA." It's not one I grow, but I thought I'd bring it to your attention.

Thanks for flagging that comment, Babs!