Adequate soil drainage is critical for good plant growth. Some plants such as hydrophytes, or water-loving plants, are physiologically adapted to thrive in saturated soils. Hydrophytes have extensive air channels that allow gases to move freely through the plant. Most garden and landscape plants do not fit into this category.
Poor soil drainage will result in water-logged, saturated soils, which greatly affects plant growth. Saturated soils reduce oxygen-availability to roots, and decrease the plant’s ability to take up water through its roots. Plant resistance to pathogens is also inhibited in water-logged soils.
Soils and Soil Drainage
Soils are composed combinations of sands, silts, and clays. Sand is the coarsest of the soil textures, clay is the finest of the soil textures, and silt lies in between sand and clay. Soils that contain similar portions of sands, silts, and clays are termed loams, and plants grow well in loamy soils.
Very sandy soils may be excessively drained which is problematic in that the soil does not hold enough plant-available water. However, clayey soils hold water so tightly that is difficult for plants to extract the water; clayey soils become saturated very easily as they do not drain well. Soils composed of fine silts or silts and clays may also be poorly drained.
Compaction layers, such as those caused by heavy equipment, will cause drainage problems, as will high water tables and shallow bedrock.
Steps to Improve Drainage
Some landscape and garden plants such as willows, arborvitae, and honey locust that tolerate poorly drained soils better than others. Soil conditioners, such as organic matter can be added to soil to improve drainage (note that organic matter will also improve the water holding capacity of coarse-textured soils).
Organic matter is the single-most important soil amendment and should be incorporated as deep as possible. Besides addition of organic matter, there are also other ways to address soil drainage problems:
- Install subsurface tile drains. Tile drains are sections of perforated pipe buried 12 to 18 inches below the soil surface. Excess soil water is captured in these pipes and is routed away from plant roots. Tile drain supplies can be purchased at home-supply stores and on the internet.
- Install vertical drains in tree and shrub planting holes. Vertical drains are 4 to 6 inch diameter holes dug 3 to 5 feet deep adjacent to the roots. The drains, which can be dug with a post-hole digger or soil auger, are filled with gravel which allows water to vertically drain below the root zone while holding up the soil walls.
- Plant in raised soil beds. Soil can be mounded up 8 to 12 inches, keeping a portion of the plant roots above the surrounding poorly drained soil. Water will also drain away from the raised soil bed. (Sunken beds can be used to address excessively drained soils.)
- Mix layers of soil. If a new soil layer is added, mix it in well with the soil below it. Counter-intuitively, a soil layer may become saturated if the soil below it is of a coarser texture.