Poor draining soil, often termed waterlogged soil, is a condition where water is removed so slowly that the ground remains saturated for prolonged periods, sometimes days, following rainfall. This excess moisture displaces the air pockets within the soil structure, which is detrimental to plant life. Without oxygen, plant roots effectively suffocate and become highly susceptible to fungal diseases like root rot. Additionally, the presence of standing water or persistently soggy soil creates an ideal breeding habitat for mosquitos. Addressing this issue is important for plant health and the usability of your yard.
Diagnosing Poor Drainage
The first step in fixing a drainage issue is to accurately determine the severity and cause of the problem, which can be accomplished with a simple percolation test. Dig a hole in the affected area approximately one foot wide and one foot deep. Fill this hole with water and allow it to drain completely to saturate the surrounding soil. The following day, refill the hole with water and measure the drop in the water level after one hour.
A drainage rate of one to three inches per hour is considered acceptable for most landscape plants. If the water level drops less than one inch per hour, the area is poorly drained and requires intervention. Slow drainage is frequently the result of an underlying condition such as a high percentage of fine clay particles, which hold water tightly and impede flow. Other causes include soil compaction from heavy foot traffic or machinery, which crushes the pore spaces, or an impermeable hardpan layer beneath the topsoil.
Improving Soil Structure with Amendments
For localized areas, the most direct solution involves physically altering the soil composition to create more space for water and air. Incorporating large amounts of organic matter, such as compost, shredded leaves, or well-rotted manure, is the most effective method. As organic matter decomposes, it encourages the formation of soil aggregates, which are stable clumps of soil particles. These aggregates create larger, interconnected pore spaces that allow water to filter through more freely and reduce the tendency of clay soil to compact.
For soils with a high clay content, especially those that are alkaline or sodic, a mineral amendment like gypsum (calcium sulfate) can be beneficial. Gypsum works through a chemical process called flocculation, where the calcium ions replace sodium ions on the surface of the clay particles. This exchange causes the minute clay particles to clump together into larger, more porous clusters. When adding organic matter or gypsum, work the materials deep into the soil profile, ideally by deep tilling or using a broadfork, to break up compacted layers and fully integrate the amendments. Incorporating fine sand into clay soil should be avoided because the small sand particles can fill existing pore spaces, creating a dense, nearly impermeable mixture.
Addressing Water Flow Through Structural Changes
Structural modifications are necessary when soil amendments alone are insufficient or when dealing with large volumes of surface runoff.
Raised Beds
Raised garden beds are an effective solution, as they lift the planting area entirely above the existing saturation zone. These structures create a new, well-draining environment filled with imported topsoil and amendments, isolating the plant roots from the underlying poorly draining native soil. For beds that are 12 inches high or more, the soil volume is typically sufficient to prevent water from wicking up from below.
Surface Grading
Managing the flow of water across the landscape with surface grading is another structural approach. Shallow, broad channels called swales can be dug across a slope to intercept runoff, slow its velocity, and encourage gradual infiltration into the soil over a larger area. The excavated soil can then be used to form berms, which are raised mounds that divert water away from vulnerable spots like building foundations or planting beds. Swales are often lined with grass or stone to prevent erosion within the channel itself.
Subsurface Drainage Systems
For severe issues or concentrated low points, subsurface drainage systems actively divert excess water. A French drain is a trench containing a perforated pipe surrounded by coarse gravel and covered with a permeable fabric to prevent clogging. This system collects water below the surface and redirects it, using gravity, to a suitable discharge point away from the problem area.
Alternatively, a dry well is an underground chamber, often a large pit filled with stone or a specialized plastic crate. It collects a significant volume of water and holds it temporarily. The dry well allows the collected water to slowly soak into the subsoil over time, preventing immediate surface pooling.
Gardening Successfully in Wet Areas
When site limitations or cost prevent a complete fix for poor drainage, adapting your gardening strategy is the next best option. Selecting plants that naturally tolerate “wet feet” is a simple and effective adaptation. Examples of water-tolerant species include the vibrant Cardinal Flower, Swamp Milkweed, and shrubs like River Birch or various dogwood cultivars. These plants have evolved to thrive in conditions where their roots are frequently saturated.
The technique of mounding soil is a practical way to raise the root zone for individual plants or small beds without constructing full raised beds. Creating a mound of soil and organic matter approximately four to six inches high provides the plant with a pocket of well-drained medium that elevates the root crown above the saturation level. In areas that remain perpetually moist, be careful with supplemental irrigation. Avoid watering during the rainy season, and only apply water once the top few inches of the mounded soil feel dry to the touch.