I have an issue to deal with regarding trying to hide my septic tanks with vegetation. They are smack in front of the sight lines from the house. They will be covered by about 12 " of soil. Can I grow perennials on top of the septic tanks with that kind of soil depth or is it better to mulch on top of the tanks? How close to the tanks can I plant shrubs? The drainage is excellent. I imagine it is about the same as planting next to a house foundation.