Answer: How about a wildflower "meadow" over your leach field? These plants would have shallow root structures and you can have something blooming from spring through fall. Wild Seed offers a Mogollon Rim mix. (PO Box 27751, Tempe, AZ 85285, 480-276-3526) I suggest you check with your contractor if it's feasible to plant anything larger over the leach field.
The texture of the bark on those cedars is really interesting, but they don't get terribly tall. You won't hurt them by watering, but they have adapted to the fairly harsh, dry conditions of the Rim area, so it isn't really necessary, unless there is a specimen that you want to encourage. Good luck!
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