"At no point will there be more more than 6" on top of any roots..."

6" of soil is likely to eventually kill your tree. I'd use 2"-3" as a maximum depth, and even that might pose a potential for damage. Think of someone saying that they are going to put a pillow over your face. They don't want to suffocate you, so they are only going to use a 6" thick pillow.

"Planter would be native soil with some peat moss..."

Peat moss is water retentive, when wet, and water repelling, when dry. It tends to exaggerate moisture extremes, which is not a good thing. If you are going to use mostly native soil, why not just stick with what you already have there with a light coating of mulch?

Changing the grade of a tree is never a good idea. Six inches is totally unacceptable. Trees don't like having their roots disturbed by a lot of planting activity, either.

i need a pic to see what you are talking about ... your words fail me ...

i would have no problem.. placing retaining wall brick on the slope side ... to make it more aesthetically pleasing.. but i would do no more than use the mulch that was there ... it would be a trick of the eye.. not really changing the grade on the tree ... jsut giving a cleaner presentation .. presuming of course.. you can place the bricks properly.. w/o doing significant damage to the root system ... [and by that i mean chopping thru 9 large massive roots ...]

lets simplify this for you ... the tree put its roots where IT!!! wanted them .. and roots need air as much as water ... if you add 6 inches or soil ...... then the root are NOT where the tree wanted them.. and you risk the tree itself ... as you are changing the air/water variables ....

BTW .. it will be hard to garden under a mature prunus .... your herb dreams ought to find another place ... as just about anything else ... what you can do around a young tree ... is different than trying to force new things under an old tree ...

need a picture to really understand the whole deal ... the above is ALL SPECULATION ...

ken

Thanks for the replies.

The plan was for a 1 stone border on the high side and level out. It's not a steep slope. We were thinking field stone, so I could put it around the roots with no chopping.

As for plantings, we have some wild grasses (clumps), chives, garlic, and onions (gifts from squirrels), and ground cover that pop up there and do quite well.

I'll see if I can dig up a pic with the trees.

Pic of the old driveway / tree. this is uphill from the tree, so you can't see the slope.

Another pic of the second tree

i have spent decades trying to kill chive.. onion and wild garlic.. you might want to think long and hard.. about intentionally planting more around your garden ...

place your rocks to your hearts whim.. do NOT raise the soil level ...

if you want to kill the grass in a bigger area.. go ahead.. and cover it with mulch.. and plant some QUALITY plants in the new area..

but please .... DO NOT increase soil level.. nor dig holes.. too close to those trees ...

and i wonder if the driveway job itself.. messed with the trees.. and only time will tell about that ... no matter what you do.. somewhere down the line.. the trees MIGHT fail ... and you will never know what the cause was ...

ken

We went with stamped asphalt to replace an existing concrete slab. No deeper digging off the original, which was 30 years old. not sure how old the trees are.

We did asphalt because of the trees and to allow any root expansion to flex the driveway as opposed to be cracked. Anything else would have required more excavation.

The real problem this is solving is a washout of mulch from the base of the tree on the sloped side and mowing between the exposed roots. I haven't chopped on them with the mower, but it's always close and a pain. The mulch ring you see is about as clean as it ever is.