Cascading plants ... not rocks.

You might be able to use an occasional tree form "shrub," but we wouldn't know based on the single photo.

I should add, if you see the water run off, I’m trying to figure out what I can do to prevent that as well. Any ideas?

Plants that cover the area will deflect/absorb a lot of run off. No type of mulch or rocks can do the equivalent.

Consider planting Ilex "Maryland" & Miscanthus Grass: I am assuming you have 4-5' of planting space. The dwarf holly will cascade over the wall.

http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=a235

http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=250978

The original picture shows an area that doesn't seem wide enough for Miscanthus, much less Miscanthus with something else in front of it. Beware of mock ups; they can be deceptive.

What is the actual depth of the bed (front to back).

It comes in many sizes height-wise.....very little differentiation with mature width. Yaku Jima will fill in at least 3-4'! Given that a standard fence panel is appropriately 8' in length, it is not difficult to extrapolate that the depth of this area - from fence to block wall - is something less than 4'. Probably in the 30-36" range, if that. And this is indeed too tight a spacing for any miscanthus, let alone including a holly that can have a 10' spread!!

Mock ups are only visual approximations. They are not realistic, they are not a design plan and they can be very deceptive in appearance, especially to those who are not intimately familiar with the plants in question.

It wasn't you I was referring to. However, as "experienced" as you claim to be, you should be able to more accurately judge that distance than Joe Homeowner and be more familiar with the spacing requirements of the plants you call out.

So feel free to disagree all you like - it doesn't change the reality.

Is there room to plant some small trees or shrubs? That would look good and then you could fill in with a layer of rock around them.

Derek, could you give us the dimensions of the bed? And perhaps tell us the difference in height between the top and bottom ie what is the slope?

I will measure it once I get there tomorrow! Thank you all for your suggestions. I am really not very good with these things so your suggestions are all awesome!

I would look for plants described as good for erosion control.

Before recommending any plants we need to know the geographical location and aspect.

If there's very much washing away of the mulch and soil there something will have to be used to keep new plantings secured. Maybe jute erosion netting or Hold-a-Hill concrete blocks.

If you like the idea of miscanthus, there is the cultivar 'Adagio', which is smaller. I have had one for two years. Right now it is about 3 feet wide and 4 feet tall.

I call them messcanthus, because of what happens when they start coming apart after the growing season is over. Picture broken off leaves etc. blown all over the site every time it gets windy at that time - unless you climb up there every year before it starts and cut the plants down, gather up and cart off all the debris so generated.

Also exclamatory plants like Miscanthus in flower (and sitting up high) seems like a bad idea for a situation where things already seem uncomfortably vertical. Use instead

groundcovers or low, prostrate or possibly cascading shrubs or subshrubs

It looks like they needed 2 more blocks of retaining wall to properly control soil erosion and water runoff.