Plants with shallow root systems will not affect sewer pipes. But plants with longer root systems such as trees will, so never plant them over underground pipes. Before you plant anything, you should contact your local water utility to pinpoint the exact location of the sewer pipes and plant your garden choices according to the pipe location.
Tree roots grow deep into the ground and will grow through the smallest crack in pipe in search of water and nutrients. If you want to plant trees, you must know how far the trees' root systems will spread and then plant the trees far enough away from the pipe so the roots don't come close to entering the underground pipe. Weeping willows, maple trees and ash trees are some just some of the trees with large root systems.
Some woody shrubs have long root systems and will grow deep enough to affect sewer pipes. Some shrubs to avoid planting over sewer pipes include choke cherry, lilac honeysuckle and cottonwood. The burning bush would do very well over sewer pipes because it has a shallow root system.
Flowering plants with shallow root systems are ideal for planting over sewer lines. Climbers and groundcover plants have shallow roots systems. These plants will not affect the sewer pipe in any way. Petunias, morning glories, irises, tulips and junipers are ideal for planting over or near the underground pipes.
If the roots of a tree were to grow down into the sewer pipes, the roots will grow along the length of the pipes and eventually find a way to cause a backup in the pipe. As the roots grow in the pipe, the pipe will become narrower and can eventually become fully blocked. When the tree roots grow into the pipes, a plumber will need to clean the roots out of the pipe with a special auger.