Though there are many ground covers out there that will quickly cover the ground - some even during a drought with high heat, the truth is that clumping grasses have the deepest roots of just about anything (including most trees). In order to control erosion you need deep roots. Turf grasses have deep roots during the growing seasons of spring and fall but the roots shrink back during the peak of summer and the depth of winter - even then the deepest they go is like 15 inches while any clumper can have 15 foot roots. Heavy rains will wash out under any standard ground cover regardless of how thick it covers the ground.

I would agree with John about the deep roots.

That said, there are spreading shrubs that CAN handle dry, the ubiquitous spreading junipers among them, much though I feel they are over-planted. The one thing that spreading, low-growing shrubbery does do is dissipate the force of heavy rain as it hits the ground, esp if the plantings are started above the top edge, so the 'wash-out', as it were would be a bit less than if the rain-drops just hit a bare, dirt slope.

What about spreading rosemary? It will mound up at about 18-24", but spreads well, rooting where it touches, especially if given some supplemental water.

Again, I am not fond of it, since it seems to over-grow plants in my garden that I DO want more, but vinca (the spreading vine [minor NOT major, which is quite a 'thug'], not the summer bedding annual), will grow and take any amount of neglect. I have been trying to get rid of what I got with gifts of more desirable plants, and STILL have it 4 years later, since the roots are intertwined with the other plants' roots.

If you planted the grasses first - or planted large ones - so they were well established, then any scrambling ground cover wouldn't overwhelm them.

It goes without saying that watering WILL be needed for the first year, at a minimum, for whatever gets planted. And you will still have some weeding to do, of the trees which sprout from seeds, if nothing else.

Thanks for the information. After some more thought, I think I will go with the grasses idea . . . at least initially and then consider ground covers down the road.

Any recommendations on what grass(es) to grow?

There are many beautiful ornamental grasses. Most of my garden is on a sunny slope and I planted it up with perennials, annuals, grasses and shrubs. However, that makes for more maintenance.

My favorite ornamental grass is Miscanthus due to the fountain form factor. I have 'Cosmopolitan' and 'Little Zebra'. 'Morning Light' is beautiful, too. Do you need height? If not height, then there are some clumping fescues.

I also have a ground-cover rosemary and I love it. One plant easily covers 4 ft of ground in 2-3 years. It has the same fragrance as the upright. I think my ground cover blooms more than my upright, but I tend to trim the upright a lot for shape to keep it from getting too woody.

Lavender works well on a slope due to the drainage. There are spring and summer blooming varieties. They need trimming about twice a year.

Cameron

Here is the rosemary (1 plant):

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This was a 3-gallon size Miscanthus 'Cosmopolitan' planted in spring 2007. I should have divided it when I planted it! It will get even taller.

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This is 'Little Zebra' in front and another to the left of the stepping stones. These get about 3 feet tall. One of my absolute favorites.

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Some attractive, medium height grasses are Pink Muhly grass (Muhlenbergia ?) and Mexican Feather Grass (Stipa tenuissima - its name has changed, but I don't remember the new 'correct' name, and it probably can be found under Stipa). Other attractive possibilities are the various "colored" grasses - bluish, variegated or red. I know that purple fountain grass is an annual, but several forms of grass do have red/purple colored leaves for at least part of their life.