Answer: Most climbing plants will cascade down if they're planted atop a wall. Evergreen honeysuckle is a good candidate; grapevines would work, all you'd need to do is provide wires along the wall and they can follow the wires. Once the stems mature and become woody, they'll remain horizontal on their own. Grapevines are deciduous but their bare stems would add interest to the stark wall during the winter months. Vinca minor is evergreen and will cascade down the wall. You might also try trailing Rosemary, trailing strawberries, sapanaria, cerastium tomentosum, and aubrieta. Not all of these plants have the evergreen trait you're looking for but if you plant a variety of things, you'll have seasonal interest all year around.
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