I recently completed a 16 yr career with Florida Department of Health in the On-site Septic Program, permitting septic systems. Planting a tree on the drain field is not allowed in Florida. Based upon my experience ( not just opinion) if you put a tree on your drain field, the tree will be happy because as previously observed, they love the high nutrient treated wastewater. Your local septic contractor will also be happy because they will be replacing your drain field a lot sooner. I’ve seen roots find their way into tanks, header pipes and distribution lines to such an extent that systems are totally compromised.