Alfalfa and sweet clover will put down pretty deep roots as well as a few other weeds and legumes. But it sounds like what you really need is a new retaining wall. Farmers around the world that have done terracing right have kept soil from eroding off of mountainsides for centuries.

Other cultures that haven't done it right have lost much of their growing habitat to erosion and have incurred much damage.

After the new wall went in, I'd try a few cover crops down the hill to try and keep that part from sliding further.

Does the legal description of your property include the cliff and some of the land at the bottom of the cliff? Assuming it does, a final answer to the issue is to build a retaining wall and back fill it.

I've seen road crews building giant, side-by-side cylinders of concrete down to bedrock. The cylindrical shape spreads the forces out so the wall of cylinders does not topple. The cylinders I've seen were probably 6 feet across and 30 feet high above the ground.

Alfalfa was my first thought but I'm not sure how well it would work. Alfalfa roots routinely go 30 feet deep and I've heard of alfalfa roots being seen in much deeper coal mines.

Where do you live? Your geography and geology makes a difference on how you stabilize that slope.

To answer your question specifically, you need plants with deep fibrous root systems. One problem with getting those plants to grow is that it takes a long time for those root systems to develop, and in the meantime the soil keeps moving down hill.

So what we do, is install an erosion control blanket over top of the seeds of the deep fibrous rooted plants. The blanket holds the soil in place until the plants get their roots established. Eventually the blanket decomposes and the plants should have grown good roots by that time.

I would recommend deep fibrous rooted grasses such as:

Indian Grass (Sorghastrum nutons)
Big Blue Stem (Andropogon geradii)
Side Oats Grama (Bouteloua curtipendula)
Switch Grass (Panicum virgatum)
Little Blue stem (Andropogon scoparius)
Buffalo Grass (Buchloe dactyloides)

Not sure exactly your climate, most of those grasses will grow fine East of the Rocky Mountains.

If you can grow those grasses on the slope and grow about a 10 - 20' buffer behind the slope, that can help keep soil in place.

But you may also have to look at what is happening at the bottom of the slope, is there a head cut or toe cut happening the bottom that causing the top to slide?

As we have received scant little information to describe the scope of the problem, guesses in the replies are the best you can expect.

Slope stabilization is sometimes possible with the plants listed above, but there is no guarantee. Likely an excellent retaining wall is your best bet, and it is likely (again, a guess as we don't know the height of the "cliff" or where you live to estimate rainfall or know building codes) that the height will require an engineer. Railroad ties are at best a temporary measure and doing another wall made of them will result in the same fate.

Dan

A retaining wall is much more expensive than blanket and seed and is more likely to fail when not properly designed.

I live in MA, just south of Boston...zone 6. My house is also on the side of this cliff and the back has a large retaining wall made of boulders. The top of this wall, where they put a cement slab around the top, has totally caved and is falling over the cliff. My garden is on the side of the house but is not on a drop like in the back. I figured if I did the railroad ties again it would only be a temporary solution to the problem. So I'm assuming (from the posts here) that the only real way for me to solve this problem is to have someone come out, who knows what they are doing (a mason?) and build a wall. I had mentioned that at this time I do not have a lot of cash to put into this right now...so for this year I guess I am looking for a bandaid as the solution. Then next spring I will hopefully be in a place where I can get it done right.

A mason could build a wall but he could not be expected to design a wall. You have a problem that seems to be threatening your home - maybe not this year but some time in the future. I would be on the lookout for a landscape architect with some experience in cliff retainment (is that a word?). A local highway designer might be able to look at it quickly and tell you what you need. He would know the soils and the situation.

The bandaids suggested above, the various grasses, might turn into permanent solutions for you. They can be surprisingly effective. Your county extension agent might be able to help, too. I believe a retaining wall could work if it is designed properly and you have the right to build one. My worst fear is that your property description says something to the effect that you own all the property out to the edge of the cliff. Nevermind that the cliff is retreating toward the cliffs on the Pacific side.

Check the link to read this trade magazine about erosion control. It might give you some ideas. I'm thinking engineer or landscape architect or, at least, a really reputable contractor for your problem. Plants and blankets might work for the short term and can't do much if any harm but I would seriously look into a structural solution if it turns out to be a structural threat. If your problem is beyond your property line that can be difficult but sometimes you can get a permit to work on someone else's land to save yours (and sometimes you even are lucky enough to have a reasonable neighbor) but that varies from place to place and problem to problem. Maybe you can afford a consult now and the work some other time? That way you would at least know what you're in for.

Here is a link that might be useful: Erosion Control magazine