Ground cover like vinca

What zone, what kind of soil, how much sun, how much rain?

You can absolutely put down weed barrier, they make "staples" to hold it in the ground. I'm assuming you want low maintenance? A light layer of mulch and ground cover(s), you could put a sturdy edging behind the wall that would create a slight lip to help with erosion until plants take hold. You could also add some "specimen" shrubs, gardenias would smell fantastic and be green all year, Rose of Sharon would be very pretty. Your zone would be helpful for plant recommendations.

Agapanthus wouldn't be hardy in much of the US. Like zone 6A where I live. We need to know where the OP lives before much advice can be offered.

I'm in CT. This area receives sun most of the day. My concern with weed barrier on the corner area is:

1) There is already soil separation fabric as you can tell at the top of the wall. This is separating the soil from the clean rock behind the wall.

2) Putting a weed barrier on top of the soil, then having soil separation fabric below that feels like it would create a sandwich that may hold too much water...but I could be wrong.

3) The corner area is hard to tell but its a short steep incline -- putting mulch down on top of fabric may make it easily wash down the slope. Thoughts?

I guess my only concern is just putting down mulch that may wash away -- but I suppose the erosion is only currently occuring because that entire area is cleared out and is just dirt? Once there is grass/mulch to slow down/absorb the rain it will be fine?

I'd plant grass. Mowing is usually much easier than weeding, unless you think weeding is enjoyable.

Do you think this would help control water from the heavy downpours that erodes the soil and washes it to the back/over the wall?

I would do the wildflower seed mix, you will have to mow it once a year in early spring. Plant a few mature plants spaced far apart, like 10ft+, then seed all around them. The idea is instead of weeding, mowing, mulching and doing all this maintainence, you will have desireable plants growing like a meadow and there will be no room for weeds. The plants will also control erosion. Ive shared this photo before but here it is again for an example:

You plant mature plants to give it structure and make it look like its intentional for the first year or 2, then let the seeds and nature do the rest.

I have a wall and 10ft deep strip of mulched garden similar to yours right in front of my house. With mulch and sparse planting it needs A LOT of weeding, especially when there was landscape fabric. I ripped out the fabric because new weeds grow right through it and the mulch was sliding right off. Now I am planting perennials by the dozen to fill it all in with desireable plants, but it wouldve been easier to directly seed it.

In Connecticut, you will have to weed a wildflower meadow. It isn't a naturally occurring space, but as artificial as any garden. In this part of the world, you mow, weed, or let it revert to jungle. Usually, people object to the jungle, and mowing is by far easier than weeding.

The grass will control the runoff. The mulched areas won't. It sounds like the area behind the retaining wall wasn't graded correctly to guide the water around the wall. In a downpour like we got over the weekend, nothing is going to stop water going over the wall, and a fairly heavy planting will be needed to control erosion.

I think people often overlook prostrate shrubs when looking for ground cover. In California where I live it could look something like this:

some ideas:

Thanks for all the suggestions everyone. My main concern is the runoff that occurs in that corner area. I went out there today and all the new dirt we put down has been eroded with just one major rainstorm from last week (dirt poured down over the front of the wall).

I had a thought today -- what If I put down a 2" strip of landscape edging at the top of the hill to slow down and or redirect water from pouring down the top of the slope? I'll put the edging at the top, then fill in with black mulch.

Any ideas if this would work? I'm afraid that even after I put in some grass in the area behind where the mulch bed will go, there will still be runoff because of the steepness of the slope...so trying to think of ideas that will curb all that runoff.

Do you need to redirect the water for a reason other than erosion control?

I live in CT. You need strong roots to prevent erosion and something with very little maintenance. I would not put in landscape fabric. I think a creeping juniper would look great year round. It is also relatively maintenance free and cold hardy. I suggest nana juniper.

emmarene(9) no other reason, just need to keep as much of it off of the slope behind the wall as possible...

You should show us a good picture of what you allude to as the real problem area. (I take it that it's at the end of the wall ...??) We can't see if the wall end properly returns to the earth, or if it is left hanging so that soil washes out from behind the wall. Too, a swale of sorts needs to be graded away from the BACK of the wall in order that water is directed to flow around the wall instead of over the top. For the large space behind the wall, groundcover, as others have mentioned, would be the solution. Mulch alone -- with no plants -- will neither be the most attractive or lowest maintenance.

I you use mulch, use undyed brown. The black looks really artificial and stands out too much. You don’t want to make a feature of mulch.

There is something called erosion control fabric. I can not say if it could help you because there is only one picture.

Here is my problem area:

You need something there all year round, not just in the warmer months.

Pachysandra will cover the hill quickly, smothers out everything else and is evergreen. Once established, it will hold the soil in place.

For height, bold color, and texture you could put in acanthus as it is the only thing I have found that pachysandra cannot smother.

Creeping juniper would be a great solution. Too much sun for ferns. I’m in CT now, at our lake house, and mountain laurel is a good plant for hillsides, especially if you have acidic soil.

Wildflower seed mixes will just create a big patch of weeds.

I'm more leaning towards a clean look -- something like green velvet boxwood hedges all the way around at the top of the wall. Will the root system be strong enough to control the erosion if it's also all mulched?

Daylilies are an other option in CT. They do a marvelous job of holding soil. We have a steep bank of them that has filled in beautifully over the years. When we first put them in, we mulched in between and weeded once or twice a summer. As they have expanded (and we have divided and filled in holes), they are now essentially work-free, except deadheading and cutting off the stems after the blooms are spent. The leaves can be left to form the next year's mulch.

Boxwoods are wonderful, and I have used them in other gardens. They are a more formal plant, and I don’t recall having seen them used on a hillside for erosion control.

The creeping junipers could be a great solution for you, but some of them are slow growers.

Try asking a good local garden center, not big box place, what they would recommend.

You have many great ideas suggested here, find what catches your interest and go with it. My preference would be for a mass planting of native grasses (not needing mowing), ferns, cornus canadensis, or creeping juniper. Within a few years the ground will be stabilized by the roots to decrease erosion and you will have year-round interest. In my experience, landscaping fabric covered in rocks keeps weeds from coming through from underneath but after a few years of debris/soil collecting on top, weeds start growing on top of the fabric.