This is one of the best, if not the best clematis to cover a large area.

http://www.gardenvines.com/shop/non-vining-clematis-5/clematis-jouiniana-praecox-118.html

Can I please see a picture of your climbing hydrangea? I'm getting ready to toss mine & need a reason not to.

Are you a member of Gardenbuddies by any chance? There is a thread going on by Clematisguru that has a recta that is growing 7' long that stays in bloom from June-Sept zone 5. The intregrifolia types don't climb & can be used for ground cover. There are more prolific bloomers than others, but for the most part they bloom from June til Aug/Sept.

I have Mrs. Robert Brydon, a clematis that is compared in relation to the link Janet gave you, & it didn't bloom nearly as long as the intregrifolias. New in my garden, yes, but so were the intregrifolias & they far out performed.

I've read that people will use the regular climbers as ground cover with success, although I've never seen it in practice. I think it really depends on what you are planting around it.

I use two clems as ground covers. The first is Paul Farges (Summer Snow), white blossoms continually from June through fall, very vigorous. Arabella is blue and she too covers a nice piece of ground. I have eight planted that cover an area 15x12. If any other clems trail on the ground, I just let them go. I find Huldine does this and looks lovely but the bloom period isn't as long.