The main requirements for planting on top of a septic drain field are that the plants should be drought tolerant and have roots that are not too extensive. You are right about trees; they should be planted well away from a drain field. Shrubs are OK around the perimeter, and native grasses with deep root systems would also be OK around the perimeter but not on top. According to St. Olaf College's "Natural Lands" Web site, big bluestem can reach heights of 10 feet. It also says that two-thirds of the biomass of indian grass is in its root system, with roots that frequently go as deep as 7 feet. So, no, these two native grasses would not be appropriate on top of the drain field. Take a look at Landscaping Septic Systems. It contains a list of native plants and grasses that would be appropriate.

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