Sounds like a good plan. Rototilling a steep slope is never a good idea. The minute you loosen all the soil, you'll get a frog-strangler which will wash all the soil away.

Dead lawn grass isn't much in the way of mulch, and it won't suppress weed growth. It might be worth applying a deeper layer of real mulch, but you still may have erosion problems, with the mulch being washed down the slope.

Is there any way you can terrace the slope?

We did that here except I planted cactus, native shrubs and desert perennials. I am adding grasses now which have the same requirements. Many grasses don't require rich soil and mine are doing quite well. I added sand to the compacted soil to loosen it and it makes it much easier to water, it soaks right in quickly.

The dead grass keeps the soil from being exposed for quite some time, keeping it from washing down and we didn't have weeds for a year and a half or so. Since you aren't loosening it except where you are planting that will just be a nice wheat color "ground cover" while your grass establishes.

One thing to be careful of that I learned the hard way is to avoid buying large root bound grass plants. They might seem like the best choice for instant gratification but the smaller ones fill in much faster and better and you want those roots spreading nicely to hold the soil.

We got a pickup load of river rock and covered the area a year later. It serves to hold in the soil and looks very nice, especially on a slope. It was very economical buying it from a place outside of the city which sells by the ton.

thanks all. how fast can i cover the grass after spraying with round up? I'd like to do it right away, but I've heard it needs exposure to sun and air to be effective.

You sort of have to just wait, but you know when its dead just by looking close to the roots of some of the grass. I think it took mine a couple of weeks at least but it looked like it had been sprayed and was starting to die the very next day. I had to let mine sit like that all winter because I did it in late summer three years ago. No weeds came up anywhere the next spring.

I suggested the rock mulch because bark types or cypress will run downhill and float down. I put a wide border of larger river rocks all around the edges to define it. Saves money in the long run over mulch that breaks down and it keeps the soil moist a long time and is easier to water when necessary by preventing water run off.

Your problem, you have probably already figured this out, is getting water to any roots of what is planted. If it was me, I would have a big bucket to hold dirt, dig your hole, take out half the dirt and replace it with sand, mixing it with the existing soil. That way, the water will soak in fast rather than just running off the slope. I've done a lot of that and it works very well. I would also consider grasses that like it on the dry side because its so hard to water a slope and some would just love it, like the Muhly grasses for instance. You have a situation that will grow fabulous plants requiring perfect drainage that would not make it anywhere else. The possibilities are endless. Check out High Country Gardens on line. For me a well draining slope is paradise when it comes to plant/grass selection and you can let it work in your favor.

Just one more thing. I don't know what type of grass you are killing. If it is bermuda make absolutely sure the roots are dead before covering or digging. It takes a while on these roots and the last thing you want is bermuda roots dug down deep or coming up through mulch put on too soon. That is the kind of grass I killed. I do not know about other types and how fast or slow they are killed. Weeds that don't have extensive roots die off pretty fast.