Generally speaking a septic tank or a septic drain field would not pose a threat to plants if the system is working properly, that is, no oversaturation and surface standing water. However, I would not recommend planting one in the drain field area as the roots from a shrub or tree would tend to invade the septic lines, and could create problems in time. Also, even a good septic field can tend to be very waterlogged at certain times of the year. This would be a problem for most types of shrubs and especially ones like agave and aloe that prefer dryer conditions.
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