retaining walls and roots

Hello, I've done the Google searches but I didn't really find any information that fits what I am trying to do. I have excavated 3 feet of soil for a sunken "courtyard" and I plan on using the Anchor Diamond interlocking blocks as the retaining wall. The edge of the wall comes within 8 feet of several Mulberry trees and while digging I had to hack through a small number of roots, the largest being about thumb sized. These trees are currently about 15 feet tall (small). If you draw a circle with the edge 8 feet from the trunk, the wall will occupy maybe 1/4 of the circle. My concern is that the roots will want to mess with the wall. My plan for the wall is basically standard. 3 foot wall with about 12 inches of gravel backfill with the gravel seperated from the dirt with landscape fabric. There should be plenty of drainage and the soil is very dry (central NM). I am thinking that the gravel and landscape fabric will not hold back root growth. My thought is to place corregated steel roofing panels directly behind the wall blocks to hopefully guide the roots downward should they happen to reach the wall. Is this nuts? Is there a better way? Am I being too paranoid about the roots in the first place? Thanks! -Bruce
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growth.
I'm not familiar with Mulberry trees but normally plant roots follow moisture. If the gravel is well-drained they will probably avoid growing out into it. They can of course, grow under it, which may or may not be a problem.
You might want to repost this question on a gardening newsgroup such as rec.gardens.
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