Answer: It is never advisable to plant trees or shrubs over or adjacent to a leach field. The roots will grow through and clog the drain holes in the pipes and grow inside the pipes, blocking the system and damaging the pipes. This can cause a situation where the entire field has to be replaced, potentially a very expensive problem.
This is why turf grass is the most common planting, followed by other undemanding groundcovers. You could possibly grow some perennials such as creeping phlox, assorted sedums, purple coneflower, and black eyed Susan for example, plants that prefer full sun and a well drained soil. You would need to be very careful however during the soil preparation phase so as not to disturb the pipes or change the overall surface drainage pattern in the area.
I'm sorry I can't be more encouraging.
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