Mountain mahogany is fine and wispy. I have a row along and they don't really screen much.

Pacific Wax Myrtle?

How narrow? How tall? How fast? Narrow is going to be difficult without some patient judicious cutting back. Coffeeberry can dense up. R. c. 'Mound San Bruno' is a taller, dense selection. Rhus integrifolia makes a fabulous dense dense screen you can trim to narrow proportions but it isn't going to be fast. You could do a scrub oak hedge, they can be clipped and sheared to dense narrowness because they evolved where the deer are always nibbling. Again, not fast. Prunus ilicifolia ssp. ilicifolia can be hedged, and is a faster grower.

Keep in mind south facing wall will generate considerable reflected heat, which not every plant can handle when they are not yet established--extra care until they can shade the wall themselves.

Most coffee berry get pretty wide. How about cactus or cactus-like? Why must it be native? That is going to really restrict your choices as most native plants are not tall and narrow. Natural selection doesn't really favor that growth habit in So CA.

@hoovb: Narrow as in 4', tall as in 12' or more, and moderately fast (so, faster than a manzanita). I never thought of lemonade berry nor using scrub oak as a hedge!

Now the Hollyleaf sounds like a really good idea. I live next door to a neighbor who has seven outdoor cats that she does not provide litter for (or flea control--lucky me), so they often like to hop the wall and use my yard. The leaves might be just rough enough to deter them.

@Sara I'm doing my best to use native plants for any ornamental plantings in my yard. The wildlife has responded to smaller bushes and plants so wonderfully that I'd almost feel guilty planting an alien species for such a large area. However, I've noticed that my native choices for this application are limited for the very reasons you mentioned. Maybe I can convince my husband that a wider hedge is in our best interest...

@JXBrown A native gardener also mentioned Pacific Wax Myrtle to me over the weekend so I'll definitely check that out. Its structure is perfect for this application. Thank you!

Lavatera assurgentiflora is also a good native and very fast growing shrub for use as a hedge.

If you want a tall, narrow non-native shrub that feeds hummingbirds, Callistemon 'Slim'. Fast if you keep them watered. My two footers planted in June are almost seven feet now. They are from areas of seasonal flooding so you can just about drown them with rain water in winter, when we are lucky enough to get some rain.

Zero outdoor cats in my neighborhood; the coyotes get them. An indoor cat that slips outdoors does not survive. :(

@hoovb Now that's what I've been seeing all over this town! It's a great size and unusual, which I like.

I've heard rumors of coyotes in my neighborhood but whenever someone posts pictures of them on NextDoor or Ring, it's usually just a large dog...

Here's one of mine. It is about 3' wide and 10' tall. I think it will get taller, and will get more dense as it matures. Still a youngling.

That's a great plant. Never seen one for sale around here.

Example of Prunus illicifolia clipped into a hedge here. It can be hedged, and with selective pruning instead of shearing, (if you have the time), it can look less formal.

Selectree rates both the Prunus and Callistemon root damage potential as "low".

Callistemon 'slim' is a great narrow hedge but some people have a serious aversion to bottlebrush down here because the much larger species was widely planted. Another good option is Acacia craespedocarpa. It's more vase like in form than C. Slim.

C. Slim should be easy to find in LA. Every wholesale grower in the region has at least hundreds of them and most have thousands.

I've seen some people do an espaliered apple tree as a wall. There are several varieties that can produce apples in Los Angeles despite the lack of chill hours.

(Post #5 here for more information: [https://www.houzz.com/discussions/high-chill-fruit-varieties-in-low-chill-regions-success-stories-dsvw-vd~4333792[(https://www.houzz.com/discussions/high-chill-fruit-varieties-in-low-chill-regions-success-stories-dsvw-vd~4333792) )

It takes a lot of maintenance and pruning to keep an espaliered tree to shape though. But they do make a beautiful and intriguing hedge. The espalier apple tree can be as low as 4 feet and still be loaded with fruit.

And in your climate fruit trees do need to be kept well watered, so not low maintenance.

Curious what did you end up planting. I am in the same situation, thinking to use Cercocarpus betuloides blancheae (Mountain Mahagony) at first, but now I am leaning towards using an espalier of Garrya elliptica (Silk tassel) instead. I do not need a very dense screen since there is a wooden fence already, so I think for me either would work.