Do not till under the dripline of trees and shrubs or you will cause serious damage to their root systems. If plants that cannot tolerate wet roots must be planted on a site with poor soil drainage, a mound known as a berm can be created to elevate the plant's root systems. Although its use may be the only option, making a berm is not an ideal solution because of the various problems it entails. Successful use of this landscaping technique requires intelligent advance planning. Soil that has been loosely spread over a compacted soil creates special challenges.
Roots often grow only in the loose soil and fail to penetrate the compacted subsoil.
The resulting shallow root system can create unstable and potentially hazardous large trees. Consequently, only small and medium-sized trees are recommended for planting where less than 2 feet of loose soil will be spread over compacted subsoil. In landscapes with subsurface compacted layers, the lowest areas are likely to be wet during certain times of the year. Within a day or two after a significant rainfall, evaluate the site and decide if choosing plants tolerant of wet sites will be necessary. If this kind of evaluation is conducted during a drier time of the year, you may mistakenly conclude that your drainage is fine.