Answer: I would definitely not plant a vegetable garden on top of a septic field. Though a properly functioning field processes harmful pathogens in the decomposition and soil filtering processes, septic fields can fail, resulting in leachate rising to the surface, or at the very least to the level of plant roots. Septic fields are designed to provide good drainage off the site; anything you do to change this can cause troubles. Also, septic systems do need periodic maintenance, and you don't want to plant anything that will get in the way of this.
It is generally recommended that you plant only grass on a septic field--at the very most I would plant annual flowers. I would stay away from anything perennial, because they will tend to have deeper roots that can damage the underlying structures.
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