I am so NOT one of these great pros who have contributed on this thread, but since I live in a somewhat similar zone (I'm East Bay/Oakland hills so similar to much of the SoBay), I thought it might help the OP if they could see what a couple of the plants suggested look like on a retaining wall. There is one plant I didn't see, but I think might do well in this situation - aptenia "Red Apple" evergreen groundcover. It's rightly listed as invasive, but I have found that although it will spread "out", it vastly prefers going "down" due to the weight - once those stems get long, they're heavy and it pulls the plant to trail down. I have been surprised to find it does not root as readily along the stem as one would think (based on its invasive designation). Easy to keep under control with minimal pruning, very easy to remove. I have aptenia in two spots and in the location that isn't shown, it goes happily down a wall as far as it's got room, but only needs to be pruned back maybe twice a year on the flat side. It is much prettier than ivy and needs almost no summer water (like 1 gall per month in summer). Note that the cute red flowers attracts bees; some people find this a disadvantage. Me, I'm all for anything that keeps bees alive and happy, so this doesn't bother us. My wall is smaller, but the lantana shown (both white and purple) are in a very narrow planter box. Due to its lightweight stems, lantana is as prone to go "outwards" as it is "down". It is easy to prune - stems are brittle so they just break off. Because even trailing lantana mounds, it tends to die underneath. I have found this to be mildly annoying as a clean-up task every year: dusty and messy, LOL. Anyway, retaining wall 2001: Retaining wall plants 2004 (they don't look any different now, since I keep them pruned to stay off the driveway). LH side is the aptenia - a single stem can easily get to 5' or more. The purple lantana is in the middle, with the white lantana to its right. Both of these would easily reach the middle of the driveway if I didn't keep them cut back. I have to agree with the negative on clematis. It's a 1 month flower wonder in the Bay Area, and butt-ugly six months out of the year. I planted one and will never bother again. Hardenbergia 'Happy Wanderer' is gorgeous - two full months of purple racemes. But I did find the evergreen leaves a rather dull dark green the rest of the year. This is a big, heavy, aggressive vine. I had to take mine out because it started to travel over the fence and the neighbor hated it. It was a pretty little thing at 2 years old.... ...but only a year later, see that "hump" in the back? The hardenbergia was almost half as big in her yard as it was in mine, smothering her Bird of Paradise and euonymus shrubs: If you don't mind regular watering, there's nothing more fun than passiflower - after a few years you'll have Gulf Fritillary butterflies dancing in the sun for 6-7 months. It needs a grid of some sort, though, it needs something to hold onto to go sideways. I have both the coral and the standard purple passifloras. The GF prefers the purple very slightly over the coral, but I think the coral is much more gorgeous. This is a big vine (estimates up to 30'; I keep mine around 15') but it is very tough. I almost killed it when the planter box it's in didn't get watered while we were away on a trip. A heat spell hit and the passiflora shriveled down to a single stem of tiny leaves. It's recovered completely and is back to its big bully role once again. There's a lot of dead undergrowth to the passifloras over time, but not as bad as the lantana. There is one other evergreen vine I use but I wouldn't recommend it - Solanum jasminoides. Carefree and needs little water, but it gets super thick/dense very quickly and really requires heavy-duty cutting back once a year or it goes out of control. In the right location, however, it definitely fills a need. This is what S. jasminoides looks like less than 7 months after being cut back to a 1" height. Fair amount of dead-leaf litter underneath. This one lives entirely on runoff going underneath the house. I never water it at all: The flowers are beautiful, though, and it flowers almost continuously: I have the star jasmine groundcover in my furthest back yard area. It twines with big thick stems like ivy does, and with its mounding growth is, like ivy, a roof rat palace. Doesn't take much water, smells wonderful, prefers bright shade but I've had it in partial sun conditions where it did very well. Anyway, hope the visuals help.

You have been given so many really great suggestions that I don't think I could think of any that are better or more helpful. I did however want to address the "ugly" wall. It is possible to colorize your wall with what is generally known as "white washing" but by adding a cement additive you can tint it any color you would like. It's very simple really. You would need a bag of white portland cement, a bag of white hydrated lime and the colorant powder of your choice. Mix the portland and lime 3 to 1 and add water so it's a milk consistency then add the colorant until you get the color you like. Measure the colorant as you add it so subsequent batches will be mixed to the same color depth. Brush it on with a paint brush. The painting goes faster than you would think by sort of swishing the brush in all directions to fill all the little crevices, You have about 30 minutes or so to work the cement mixture but if it starts to stiffen just add more water and mix well again and finish up. I did this on my very old cement stucco house this past summer and it is the brightest shiny white you could ever imagine. It looks beautiful with freshly painted trim. Anyway this will work on anything with cement like texture (bricks too) and because of the added portland cement it will not wash away like a simple lime only solution will eventually do. You could even do a really bright color like red or deep blue if you wanted to and consider it an art wall if you wanted. Just something to consider in addition to plants. Any left over materials can be mixed with sand added to make stepping stones or cement planting containers etc. Good luck!

This is in response to the suggestion of taking the plaster walls out in order to insulate. I offer the thought/question for discussion as much as expert responses but with a strong bias toward leaving the plaster intact because drywall is inferior. (Just compare the cost of new drywall to new plaster if nothing else.) At a time when my ex-husband was teaching college classes in thermo-dynamics and consulting for a contractor who built super-insulated, passive solar houses that could function without furnaces in cold climates (by way of expert references,) what I retained from some conversations was that the best insulation is dead air. e.g. when the pink stuff isn't fluffy, the trapped air is pushed out and insulation value lost. So I have thick plaster over wood lath interior walls and thick stucco outer walls in a 1913 house in a very cold climate (Central Minnesota, planting zone 3.) Whatever insulation was ever in the walls is most likely packed down in the bottom of the stud bays. The outside walls are cold to the touch in the winter and cool the rest of the time. On the other hand, it takes at least 24 hours for the house to lose its heat/cold - change interior air temp - to match the outside temp, even with some windows open. With the thermostat set at 65 during the day and going to 55 at 9 p.m. (very comfortable for me,) the radiators heat up in the morning and stay cold the rest of the day as long as the outside temps are +0. In short, I vote to keep the plaster and don't worry about wall insulation in a mild climate.

If in shade all the time, move the rhody to another location - they will bloom better with some sun - and plant Sarcococca confusa (or S. ruscifolia). While it is not a heavily flowering plant as you seem inclined to prefer from your thoughts, it IS evergreen, which is always an appreciated attribute at a front entry (no bare emptiness in winter!). And sweet box is a plant that thrives in any shade, even dense shade, with minimal watering required once established. And the best feature is that when it does bloom (mid winter, Christmas time into early January and on through to March) with a profusion of tiny little white flowers, it exudes an amazingly fragrant and far reaching aroma - a mix of jasmine, vanilla and honey - sweet and spicy at the same time. There are few things more welcoming than a wonderful fragrance to greet holiday visitors at your front door and this is a handsome, undemanding plant to boot.