The rule of thumb is that a tree’s roots are as wide as the tree’s canopy (branch spread). But this doesn’t account for all species of trees, or the depth of the roots. When it comes to a septic system, it is best to only have grass planted above and around the system. Shallow rooted plants, small shrubs and flowers usually can be safely planted near and above the system.
Trees really should be avoided. A weeping willow tree, or other species that really have a massive root system should NEVER be planted ANYWHERE near a septic system!