Hi,  good question!  According to Clemson University, " Shrubs with less aggressive root systems should never be planted any closer than 10 feet and small less aggressive trees no closer than 20 feet from the drain field. Planting these less-aggressive species, such as boxwoods, hollies or arborvitae towards the drier ends of the drain lines may also reduce the risk for problems down the road."  To read about why this is  important please refer to: https://hgic.clemson.edu/factsheet/landscaping-over-septic-drain-fields/