When plant materials are to be used in the wall, place clean topsoil between the stones. If desired, small pockets in the wall will permit vegetation to establish a firm root system throughout the wall and provide added erosion control. However, large woody plants are not desirable. Too much soil between rocks. Tamp clean topsoil firmly behind each layer of stone.
Firm soil is important to the strength of the wall.
An alternative method is to tamp and back fill with gravel or stone chips to help drainage behind the wall. Remember, loose soil will settle irregularly and wash out freely, so during construction tamp the soil firmly behind the wall to insure a solid foundation. The secret of designing a solid retaining wall is in laying each stone firmly in place. The quantity of rock used in the wall will generally reflect the amount of soil placed between each stone. Using less rock is false economy.