Wet feet

Poor drainage in soils is one of the most common causes for failure to grow vigorous, attractive lawns, trees and shrubs--plants can suffer from a list of diseases or problems, known as “wet feet”. This can occur if your landscape consists of clay soils which tend to hold moisture well, often causing the roots of a plant to rot.
In heavy clay soils, it is preferable to plant trees and shrubs a couple of inches above the grade of the surrounding soil.