Yes you can grow clematis to cascade over a retaining wall. Clematis that bloom on new wood would be nice as you can cut the dead foliage back in the fall so your retaining wall looks neat and tidy all winter. Pink Fantasy is a type 3 clematis which blooms on new wood. I have never grown this variety but the pictures look nice.

Sure I see no issues with doing so. If the vines reach the bottom of the wall, you could always loop them back up to the top. You could also fashion a support that would allow you to make swags out of the vines as they develop.

I don't know your exact situation but I use chicken wire to give clem a base for climbing. Sounds tacky but the plant will cover the wire the first year and chicken wire can go anywhere. Run it down the length of the wall (if its a low wall) or down the wall itself(if it is a tall wall). Also great to put rings of wire around a post or pole. You should be able to find a method of attaching the wire to the wall. (HD has plastic ties that attach to walls without nailing into the wall) Once the plant starts to take hold of the wire, it will help keep it in place. Just wrap it over the top of the wall if you have to. Easy and cheap and really works.

If you are growing a type I or II clematis, chicken wire might be a good support. However if you are growing a type III, I think it would be a nightmare trying to get the old vines off the chicken wire each year. A single strand of wire that you loop and attach to the wall with either screws (if it is a wooden or similar wall) or lead anchors and screws in the lead anchors (if it is a masonry or brick wall) would make vine removal infinitely easier.

Thanks for the fantastic info! Now that I know that I can do it, a couple more questions come to mind. Will Clematis climb DOWN (instead of up)? And should I put up a structure to support the vines or will the roots be strong enough to support the weight of the hanging vines?

nckvilledudes, you're a godsend! But bear with me, what do you mean by "swag"?

Down and then back up again in soft loops, like a swag curtain valence.

Yardmom described it perfectly but in case you are a visual person like I am, here's a rather crude picture.

{{gwi:621825}}

bwca4 - i wanted to suggest another plant that you could even do in combination with clematis, that would cover the wall pretty quickly for the summer. have you ever grown sweet potato vines? they come in several colors - dark purple, red, chartreuse or variegated, and they grow very quickly and vigorously. they love to cascade, and get very lush and full. i always use them in my window boxes and they will grow down almost to the ground(from the second floor) if i don't trim them back once or twice. i also put a couple in containers around the patio, and have to trim them back there too, they get so big.

they could definately provide some instant gratification while the clematis is getting established on your wall. another nice thing is that they are just annuals, and they don't drop any seeds, so if you decide you are done with them you really can be done.

i just wanted to suggest them in case you aren't familiar with them, we've had a lot of fun growing them.