Answer: Go ahead and plant your annual and perennial flowers over the lines.
Vegetables are debatable, although many people feel that even they are o.k. The septic effluent never contacts the fruit or leafy part of the vegetable, so bacterial contamination is not a significant concern. Heavy metal uptake by the roots is a possibility, yet most experts indicate that a home septic system poses little heavy metal threat, especially with lines as deep as yours (36"). The real concern is if your system fails and the effluent comes to the surface.
It really comes down to what you feel comfortable with. Either way though, the annual and perennials are fine.
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