I think you plant some shrub in small bag,it are drought tolerant.after sometime,make small hole in the bottom of every the small bag(the shrub root can into soil of the steep, narrow slope).lay the shrub on the steep, narrow slope follow the pic what you like.such as:

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Here is a link that might be useful: share other some ideas

Awhile back there was a poster that was marketing some kind of hillside velco fabric specifically designed to retain mulch on a slope. I have never seen the material used before, but it may be an option.

Mulch retention also depends on how much water flows over the slope. Try to avoid draining upland areas over the slope, so the only water to be drained is rainwater incident on the surface slope surface area only. Areas where rainwater hits the surface and is then concencentrated into a channel may have sufficient velocity/force to dislodge mulch. But those few areas can be identified and done over in gravel or rip-rap to reduce water velocity.

Another way to reduce water velocity on a slope is with trees,shrubs, and groundcover. Something as simple planting will go a long way to avoiding slope erosion and mulch migration.

Sorry, no professional advice. My experience indicates that burying one timber and two on top with 3/4" rebar driven another 2' into the soil below (every 4' or so)would be stable enough, providing you add a few inches of gravel above the soil line and against the back of the timbers for drainage. Leave some spaces between the timbers for water to escape and alternate the vertical seams. Note rhodium's comment above and avoid any additional water draining over the slope from above.

I concur with the good advice given by rhodium and stevega. But if you really have a slope of 4' rise in 4' run; 1 to 1, you are headed for trouble. The landscape timbers will be stable as proposed. But the slope is steeper than the angle of repose (http://en.wikipedia.org/wiki/Angle_of_repose) for all common mulch types I know, and will not stay in place.

I agree with rhodium about using a groundcover and shrubs. I posted these photos of a slope treatment a few months ago, maybe October. I walked by recently, and now the weeds are gone and the netting is completely and deeply covered by the groundcover.
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I was assuming that the wall would be placed near the bottom of the slope. If it were a foot and a half high, the effective slope would lessen considerably from a 1 to 1 to .6 to 1. Would big single ground mulch stay on that?

Just try a pilot study to see if the mulch stays without any engineering controls.

I have slope about as steep as the one in Catkims picture. I use oak and maple leaves for mulch (raked from the yard), as I have a woodland themed garden. I dumped the whole leaves over the top and they stayed in place for awhile, except at the very steepest part of my slope.

I'm also planted red-twigs on the slope and the planting holes also help to keep the mulch on the slope.

All great advice.

I do not want to mulch the entire slope from top to bottom as at the bottom is where people park. So, if I go halfway and put in the timber edging, I'll still have lawn at the bottom to cut, which is fine, because I will be on flat ground while mowing that portion. The top portion of the slope in question is the perrenial thorn in my side!

Thanks all.

I think a lot depends on the type of rains you get. I have a steep slope, some of it 1:1 and I find that wood chip mulch stays in place quite well. We don't get many heavy down pours and I use quite rough wood chips.

If you add the timber, and the hill is only 2 ft high above the timber, the slope will be fine for mulch.

Add a wood mowing strip along the bottom of the wall so you saave yourself the job of trimming the grass.

If you build the retaining wall at the bottom of the slope, you won't have any grass to trim and the wall can act as a bench from the driveway.

Well, the time is coming up for me to install the said edging.

I am now trying to decide between which types of timbers to use.

What is the better option? The 4"x4"x8' squared off timbers or the the rounded landscape timbers which are 3"x'4"x8'? I would think the squared off ones would be sturdier, and the rounded ones, while they are more aesthetically pleasing to the eyes, seem like they would not be as sturdy as the squared off ones in this application

Obviously both will be pressure treated. The ground at the top of the slope is flat and is our yard. At the very top of the slope, is a fence, which has flower beds along it on the interior of the yard. On the exterior of the fence is the slope, which is where I want to remove the grass and put mulch. I do have some blue rug junipers along the whole length of the slope now, but the weeds come back with a vengance in this area. Which is why I want to "edge" and mulch the top portion. Allow the junipers to grow in and maybe put in some pachysandra or daylillies to control erosion until the junipers fill in.

No matter which timber I choose, I feel that from the base of where I want the edging at, I should dig down 12" inches and back 12". Fill the bottom 6" with drainage gravel and install 6" of timber below the base of the edging. Will also be using rebar, about every 6' to secure it down. Also, those spikes to secure the timbers to one another. Above that base, I am looking at having about 8" of height to my edging. Will also be installing deadmen timbers every eight feet on the base course. Then, back filling the wall with gravel.

Does this sound like this edging will be structurally sound and if it does move, it will be minimal?

Any help would be greatly appreciated. I hope I gave enough information.

I re-evaluated the hill in question. As I do not want to do this all at once due to time and money constraints, I am going to do this project in sections.

Since now is the time for planting, I am going to remove all the grass from the top of the hill, transplant a ton of day lilies and put in more blue rug junipers to fill in more space. After that I will mulch the bare dirt as needed.

Now in the area where the freshly removed grass meets the remaining grass, is where I want to put the edging. Can I temporarily install the black plastic edging that comes in 20-40' rolls until I put in the timbers?

My thought process is this: Plant what I can now as this is the time for planting/dividing/transplanting. Then, when that is done, install the timbers to edge it in.

Now should I put in the plastic edging temporarily to prevent the grass from creeping in? I can re-use it in my yard later on. Or, will this more effort than it is worth?

Thanks again.

Regardless of what others might think - the black plastic edging does serve a purpose. Have used it in the past before perfecting my trench edging technique... which is something else you might consider.

The black plastic is infinitely easier if there are two of you - one to dig and the other to hold the edging down while you fill in around it. Easy to rip up later. And despite those telling you it's like looking at a garden hose outlining your project, it's a fairly effective barrier.

I have used the black plastic edging and I like it as well. I am just not sure if it will stay in place with the slope and the freeze/thaw of winter, let alone hold back mulch.

Maybe the timbers are a better way to go. Put vertical blocking (sunk to a depth of 3 feet) in front of the horizontal timbers every eight feet. That way, I will not need to install deadmen and it will still be secure.

For our steep hill water flow is a problem, not sure if that will be for you but we've planted more things on our hill to soak up as much water as possible vs just a run off which comes right towards our house if there ever was a super rain, we've improved things a lot but it's something I always think about.

This hill goes toward the street, away from the house. And there is never a lot of water runnoff that comes off of it.

Since this "wall" will only be three timbers high, are deadmen needed? Or am I okay putting the vertical pieces in the ground behind the timbers and securing them with carriage bolts. If deadmen are needed, where do I install them? The second course?