Suggestions for 4' tall hill left after removing retaining wall

Jules (5a S.E. VT.)
last month
last modified: last month

This hill is in South-Eastern Vermont, zone 5a, elevation 554', part sun/shade, morning-noonish sun then it's behind the trees and filtered (you're looking due west when facing this hill - the sun sets through the talls pines you see in the background). About 4-5 years ago, we ripped out a woody needled groundcover that I think was yew, that grew on the ground like juniper. It came back the year after, but we kept ripping it out and now it's gone. What's left now is this weird thick mat of fine roots that is hard to dig a hole through, and the only thing succeeding here are these dainty little Northern Maidenhair ferns that grew in one big patch. I dug a few of them up and moved them to other spots to see if they would be transplantable, and they seemed to do fine, so I guess I either need to find more of this seed (or however ferns come) or figure out what else could possibly grow here. We tried to put rhododendrons here but they all tried to die, and as soon as we moved them, they came back to life, so the ground here is toxic or something. There is some galvanized sheet metal buried in this hill (looks like that's how they made the hill stay in place when they built their retaining wall which we ripped out). Could that be leaching toxins? Could the old groundcover have been one of those thing that kills other plants by sending out chemicals? Maybe we just need better soil to plant them into since they did better once we moved them to another area on the property that has a little topsoil on it? Maybe it's too acidic from all the pine/hemlock needles? This hill started out much further out, and we've been digging it out and back for the past 3 years. A little to the left in these photos, that azalea was twice the size it is but we've been hacking it back with a pick-axe and ripping the roots out then digging out the sand and moving it to other low areas in the lawn. We now we have this really steep sheer drop-off of nothing but sand where that azalea was removed/cut back (you touch it and it slides down into a pile at your feet). The whole yard has been in a pine/hemlock grove for centuries. We've been slowly cutting down very large pines and hemlocks, and slowly getting some grass in the lawn (previously no grass, just the pine needle litter you'd expect in a pine grove in the forest - lime helped), but we're thinking this hill is going to be too steep to mow. Ideas? Maybe we could throw some perennials between the ferns if we pick-axe the mat layer out everywhere that is barren (as you can see we did that in several spots and filled them with our lawn sand (we don't have soil at this house, just 0-5" of topsoil in some places then pure glacial sand below it (like playbox sand, pure sand, no gravel), but if we do that, it will just erode down into the newly acquired flat lawn area. Water rolls right off the top of the mat layer, and erosion with the sand is a problem in other areas of the property where this mat layer doesn't exist if we rake the water-repelling layer of pine and hemlock needles up. We need some ideas that can work with our sand (we're open to adding compost/manure/peat/etc to holes we dig when we plant stuff). We just removed two hemlocks from the right side of this area, so now it has more light and less hemlock needles. What it may come down to is another (better) retaining wall, but thought I'd post here and see if anyone has other alternatives since retaining walls are so expensive. Anyone know of acid-loving evergreen ground covers that like acidic sand hillsides and part sun/shade?

The sand that is what's under the mat layer is shown at the bottom (we've been cutting this hill back for more flat lawn area for a few years now).

The mat layer that holds all the sand in place (with some junk grass that's trying to grow in it):

More photos of whole area:

From the back door looking left (towards the shed):

From back door panning right from far left showing the hill:

From back door panning right:

From right, looking behind house to left, showing the hill:

Backing up a bit more to the far right, turning more to right:

From left, looking behind house on the right (back door shown):

From a little more from the front right:

Panning right from right, towards front yard:

The area on the left still under construction (digging stumps out, will have fire pit here halfway between house and shed):

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