Weinar, there's no way to not destroy a large percentage of the roots from your description of this project. If the digging parameters can't be adjusted, you will simply have to proceed and that damage will have to happen. One thing that can help is to saw any roots encountered rather than letting the high hoe just rip and tear them out. They will still be lost but the resultant wounds will be of a type that the trees can deal with a bit better. The gravel backfill will not be a problem in and of itself.

Following this project, you can help in the trees' recovery by making sure they don't suffer from dryness if the weather becomes droughty. Beyond that, it's all snake oil. I would be remiss if I didn't mention the to-me snake oily product "Superthrive". I still can't quite believe it but I have heard a number of testimonials by people whose intellect I otherwise respect, claiming that this material did indeed help with situations not unlike yours. But man, what a whacky deal that is.

Good luck.....+oM

I'm guessing that you need to replace the retaining wall because the arborvitae cracked or destroyed it? I see this happen often around here, cracked retaining walls or ones that are knocked down because someone planted a Maple within a few feet of it. I think even if you had it done professionally with large stones, there is still a very good chance it can happen again. I have even seen large slabs of reinforced concrete leaning outwards at almost a 1:00 angle because of a large tree behind it.

I'm not sure how aggressive arborvitae roots are though, but to be 50+ feet tall they can't be very small. I don't think large trees and retaining walls are meant to be next to each other.

Good points Rick. I don't think arborvitae roots are in the same league as maples though. But it's a difference of degree, not kind.

+oM

Weinerfalk, it just has to be better for the trees to have this done while they are dormant.

Also I am concerned about their stability. "Right up to the trunk" sounds like 50% stability loss. Can these fellas fall on your house?

i predict loss of the large trees.. losing 50% of their roots structure.. not to mention the already noted stability issues ...

get rid of them.. do the wall project properly.. and then replant ...

i would guess.. that removal will add barely anything to the cost of the wall job... but should they fail in a few years.. you are going to take a substantial hit in the pocketbook.. to have peeps come out and remove them ...

in other words.. if they have a backhoe there.. for the wall project.. they can have them out in minutes.. rather than coming back.. and having to surgically remove large trees w/o damaging the wall ... [a crew of 5 guys with shovels.. working for 3 days digging them out]

be done with them ...

ken

I would take them out because of the weakened root structure. The chances of all them surviving this process are not in their favor thus leaving large holes in the row.

Dave