He could create a series of terraces to mediate the slope and then plant good, hardy, NATIVE, and drought tolerant plants which will be low-maintenance. A good way to create the terraces is by building gabion baskets. Highway authorities use them all the time and they can create an interesting aesthetic if done properly.

Can you give us some information about his location and climate?

Thanks for the replies.

I suggested terracing the area, but I think money may be a limiting factor. They recently put in a pool and the observation deck you see in the photo, and I don't think there are many pennies left in the piggy bank for terracing the hill, not in the near future anyway.

Climate is Zone 9, Mediterranean-like: hot (100+) and dry in the summer, down to about 30 on the coldest winter nights. The soil itself is fairly hard and rocky.

Besides planting material, I think he was considering mulch, but I'm afraid mulch will just get washed down the hillside sooner than later.

Slopes are opportunities for drama in the garden. Yes, they may present a challenge, but the benefit of some interesting topography far outweighs those minor challenges.

Is this in California? North or south? Distance from the ocean? Access to irrigation?

He has this viewing deck -- what is he viewing? He can view the slope from there, correct? There are many kinds of plants best viewed from above. Is there any point in suggesting any sort of tree, or should all the plants be below eye-level on the deck?

Is your brother a rose kind of guy, or is that your own taste?

Catkim, it's Northern California, nowhere near the ocean but adding irrigation is a definite possibility.

His house is located at one of the highest points in the area, and overlooks the valley with the city skyline far in the distance. This sightline from the deck should be protected, but smallish trees that can take full sun would work near the bottom.

And the rose idea was merely my own thought -- something rambling but introducing some color to the area. But I'm no expert.

I think the problem is more than aesthetic. The soil needs something to increase organic content and to slow down water runoff when it rains or your brother is going to have an erosion problem. I hope the footings of that viewing platform hit bedrock.

This link might be helpful:

http://www.landscape-design-advice.com/landscaping-steep-slopes.html

I don't necessarily agree with everything they have written, but it is not a bad start. From what I have read and know from experience, fast growing groundcover is the best way to begin if you can't terrace the slope. Don't think ornamental to begin with. Just cover the slope with something green and low. Even weeds will do the trick. Then plant taller, drought-tolerant and NATIVE plants from there.

This may be radical but I would leave it alone and see what grows there or not without any interference. I am sure the brother would agree as there is already a deck in place to overlook that area.