If you could create a temporary ”holding” bed elsewhere on the property, preferably with some shade, you could start moving your plants and shrubs there at times best suited for each type. They could spend next season recovering and regenerating roots, and you could evaluate which are good candidates for transplanting again after the wall is rebuilt.

Is there sufficient room to build the new retaining wall immediately in front of the second old wall in order to avoid removing the wall and the bushes in the 2nd/3rd tiers?

I agree with the holding bed suggestion. A lot of large mature bushes and perennials had to be moved for my pool installation. The landscaping company created a large bed of soil to temporarily hold them in a dappled shade area. They dug out the bushes and I dug out as much of the perennials as I could. Covered any visible roots with mulch. I watered faithfully and was rewarded when almost all transplants survived when reinstalled about 3 months later (early Sept.). The transplants took a season to recover. I pruned the bushes about a year before the project started.

Above advice is good. A holding bed for the plants would be great. In your climate, I'd wait till spring to move things. Easier to see what's

what then, without the foliage, and less chance of winter kill.

@designfan I was curious if I could just leave one of the tiers in place, but I have no idea what that would mean for drainage or long-term stability. Wouldn't the railroad tie eventually decompose and leave a giant gap there?

That’s a good point… we have drainage pipe behind our stone walls as well as weep holes which you may need to do as well.

With big shrubs, it's helpful to dig a semi-circle around the outer perimeter of the plant to cut the roots. Do this in the fall. If you don't move them in the spring, do the other semi-circle. This cuts roots, forces them to produce new ones within the circle that you'll dig up.

That is an exquisite planting and I am sorry you have to dig it up.

When I had my gross old railroad tie retaining wall replaced with those one of those blocks-on-pin systems, the landscape contractor potted up 75+ perennials and shrubs (no room for a holding bed), I watered religiously, and had good survival. Good luck!

@sigrid- just to clarify: I would dig 1/2 of the semicircle this fall, and the other side of the circle in spring to prepare for a fall move?

@MelissaL- Good to be reminded that pots are a good option too! I was a bit worried about finding a large enough space for a holding bed. Sucks to have to disrupt it all, but it will be more sustainable (and maintainable) in the long run this way.

The big thing is plants grow where they are happy so becareful where you move them too. it would be helpful to see the whole area from further back so maybe post more pics in a comment here . I wonder if there is not some other way to access the other from another place or mybe plant things that do nor require mowing since IMO grass is the last thing I want it needs constant care, too much water and always some spray or other to keep it healty . So many awesome cround covers that need virtually no upkeep,I have one hill on my property that is covered in vinca no need for anything I rarely even water it and stays green all winter and pretty purple flowers in the spring a perfect plant for a large area. I have actually beed growing it for a client on a sunny hill too. They do like a bit of shade are tough .

I have vinca in another area of our property and it's quite invasive-- which tends to be the trouble with anything we call a groundcover, in my experience. We need a wider grass lane not only to be able to run a large garden tractor through there, but that is actually the only flat spot on our property and we like the idea of having a spot to play bean bag toss or similar when we have friends over. Grass gets a bad rap, but it is actually pretty easy maintenance and it is not a bully to neighbors! Just my opinion of course- to each their own.

I'm not sure what the question is. Can't you place them in pots while they await their new home? The ones you can salvage that is.

You have a lot of good ideas here. I just want to remind you that plants often appreciate being divided and spread out a bit and will reward you by filling out your new garden too! Another option for some of the larger plants that cannot fit in the big spaces is to take cuttings and replant a smaller plants in the new smaller space.