Don't let the septic system deter you from installing a lush landscape. With the exception of the tank access area, you can plant perennials and shrubs anywhere around and over the septic tank and drain field. Of course, when you are digging planting holes, be careful not to damage any of the underground apparatus. Drainage tile fingering out into the drain field is likely not tile at all, but black plastic tubing perforated with holes. Keep any tree plantings off to the sides of the septic field. My 1930 home has trees within 25 feet of the septic tank and lateral lines. Our distribution box collapsed and had to be replaced last year, so we got a chance to see a big stretch of the system exposed and inspected. There weren't any tree roots invading the system.