Edging steps and so eroding on slope

LindzH

Advice please would be appreciated. 
We levelled an area and put in some steps, not quite finished, in my hill top garden which is exposed to the elements and backs onto a disused quarry.  The garden used to be covered in bracken but after 10+ years of it being covered and cut back I am now ready to continue working my way to the end.  The soil is clay and has numerous rocks that appear when digging some small others huge and can't be moved. 
The steps don't have an edging/sides other than the soil that was already there and some add on top so have a mound of unsieved soil,  I've started getting green alkanet over the last 2 years, so don't want to disturb that and the bracken. 
Any advice on how to ensure the sides don't erode by planting or another method and what to plant on the mound site root support and ground covering. 
Thanks in advance

jonathan.cole

I don't think any planting is going to hold the soil at nearly 90 degrees like that. You have a few choices:

- build a retaining wall. You might have enough stone on site already for a dry stone Waller to use, but they do need quite a bit. Or you could build a sleeper wall or similar.

- reduce the angle to a more manageable slope and plant up so that the plant roots hold things together. You will need perennials or evergreens so the roots stay over winter. Some evergreen would be useful to keep the soil protected during the worst weather.

Busy-Lizzie

Hypericum calycinum and Vinca are good for growing on slopes, tough and easy. However I think the sides of those steps need some support, like stiff metal mesh, but you would have to work out the best way to fix it. If the sides are supported you could grow rockery plants, like aubretia, to fall over the sides.

K67

As others have said the right hand side is too high and will collapse in time.
You will have to lower it or as suggested a retaining wall.

Loxley

You'll have to dig into the sides and build a sort of drystone wall, it can be in keeping with the lovely natural looking steps. You can get plenty of planting pockets between the rocks.

Balgay.Hill

I would remove the steps, place the side rocks and soil into the trench at a shallower angle, and re-seat your steps.  They don't need to be so deep in a trench.

LindzH

Thanks everyone - yes need to make the angle less than 90 degrees on both sides, the photo doesn't show that the left is worse than the right, some of the stone steps are located where a enormous rock appeared when digging, so my partner decided to incorporate it and I can't really move them and start again.  
Was thinking of trying some coir/coconut matting and planting through that after reducing the angle. The creeping St John's Wort sounds good thanks Busy Lizzie. I wondered about Mahonia Repens? - so another creeping plant. It's the smaller varieties that I have little knowledge about, so evergreens but with some interest.