How far down do you want it to trail? Clematis do grow from one root ball but they can definitely grow in such a way that looks like they take over a certain spot. Shorter varieties could be used, if this trailing down effect does work.

Otherwise, I can think of wave petunias, some verbenas, purple queen, purslane, moss rose, and many more annuals that fit the description.

Some varieties of spiraea could do it. They don't exactly trail like you normally think, but their branches bend over, and could bend over your retaining wall.

I was thinking more of perennials, to keep cost down annually, but I will look into some of the other options. Added a photo of the wall I'm referring to.

I have seen several references of Clematis being used as ground covers or draping. I did find an article on recommended ones for ground cover, but the url would not work here. You might google this blog listed below. You might also send an email to "Brushwood Nursery" asking them which clematis they recommend as a groundcover, and for your zone, as they mention this in the website. I know for sure that Sweet Autumn Clematis can be used as a rambler/ground cover, but I know there are many more that can work well too.

http://randomgardening.blogspot.com/

What zone are you actually in since one of your posts says 10 and one says 4, but your profile says 5? Also what is your soil like? Is it clay or sandy or somewhere in between and what is its pH, more acidic or alkaline? That will make a difference as to recommendations beyond clematis. You might want to ask for suggestions on the perennial forum as well.

I wouldn't plant sweet autumn (or other huge clematis like Mrs. Robert Brydon or Betty Corning) in that space since it is a huge monster of a plant. I would try one of the shorter clematis to drape over your wall without overwhelming the other plants.

Below is a link to the search form on Clematis on the Web. If you put in 1-1.5 meters as the height and then 1.5-2 meters you will get a list of clematis for each. Not all will be commercially available in the US, but some of them probably will be. Many of the Evison-Poulsen such as the ones in the Boulevard collection are shorter/more compact and often Evipo clematis are available in the US.

Here is a link that might be useful: Clematis on the Web search

I'm in zone 4-5. Since I live right on Lake Michigan, and jutting out into it a bit, we kind of have our own little ecosystem here. I didn't realize you could set your zone until second post. I will definitely look into all the suggestions, thank you very much!!

Vines grow up - that's just kind of the way it goes :-) It's called tropism and with vines its upwards towards the sun and above any competition. You have a choice of planting them on the lower level and training upwards or choosing more of a groundcover plant.

Or you could consider a herbaceous, non-vining clematis like xdurandii or integrifolia. These will sprawl and cascade some but even with these types the growth ends will curl upwards.

Ooh, I like the idea of growing a clem in a lower bed and up over the wall instead of down it. You might have better luck with that.

Yes it works very well!! I love them like this. That's a sweet autumn clematis I believe spilling over a container garden!

I had a Warsaw Nike spilling down into my sunken garden and it was stunning. Once they get large enough the sheer weight of the stems and flowers forces them down. The hard winter killed off WN but I replaced it with Sweet Summer Love.