Poorly drained soil that remains wet for most of the year is a challenge for landscapers, since many commonly used landscaping plants are prone to root rot and other ailments in wet conditions. When landscaping wet areas, you have a choice between trying to change the soil conditions or using plants suited to existing conditions.
Where poor drainage is caused by a hardpan -- a layer of compacted clay beneath the surface that prevents water from draining down through the soil layers -- digging a dry well at the bottom of a planting hole improves drainage in the immediate area around that plant. A dry well is a narrow hole about 15 inches deep, filled with gravel, that extends all the way through the hardpan layer and allows water to flow down and away from the plant's roots.
If you can't lower the water level around a landscaping site, you can raise the planting level by adding a foot or more of well-drained topsoil and organic matter to keep the plants' roots high and dry. In some areas, it will be necessary to contain the added soil with some kind of retaining wall to prevent erosion. In other areas with no significant runoff during rain events, sloping the added soil down toward the original topsoil level around the edges is sufficient.
Ditches and Drains
Soils that are wet because they receive a large amount of runoff from surrounding land features can be improved by digging ditches and building berms that redirect water around the area to be landscaped. If all else fails, tile drains are an effective, if expensive, way to improve the drainage of a site. Ditches and drains are particularly useful when combined with raised beds, which also serve to physically redirect runoff.
The simplest and least expensive method of landscaping wet soils is to use plant species that naturally thrive in those conditions. A wide variety of trees, shrubs and smaller plants have evolved to grow in marshy areas, including such attractive specimens as weeping willow, red maple, azalea and dogwood. Other plants of interest, such as pawpaws, highbush blueberries and persimmons, are productive as well as water-tolerant, producing desirable edible fruit.