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A leach field is a series of relatively shallow (a minimum of 6 inches below the surface) underground perforated pipes set in gravel trenches that allow septic tank effluent to drain over a large area. As the effluent seeps into the ground, it is purified by the soil. Plant roots can help remove excess moisture and nutrients thereby making the purification of the remaining effluent more efficient. However, roots that clog or disrupt the pipes will seriously damage the drainage field. The challenge of leach field gardening is to find plants that will meet your landscape needs but not clog the drain pipes.