The most important decisions in constructing a rain garden are where to build, what size, what composite of soil to use, and which plants to select.
The rain garden should be built at least 10 feet away from any structure, preferably in lower lying areas. It is best to establish the garden in a location where most of your runoff travels to during heavy rainfall.
Ideally the garden should collect all the runoff from your property. The size of the garden should be approximately 25% of the runoff surface area from which the water comes. For instance, if you are collecting water from your roof, the rain garden should be approximately 1/4 the size of your roof surface area. Define the boundaries of the garden with sod, stone, or brick to prevent erosion and help retain water.
The best composite for a rain garden should contain 50-60% sand, 20-30% topsoil, and 20-30% compost. Heavily compacted soils need to be loosened up to a 2-foot depth. In poorly draining soils, shallower basins and a gravel underbed, tiles, and/or an underdrain are suggested. Keep the bottom of the basin flat, along with the top, in order to spread out the water and speed up absorption. Placing mulch on the surface of a rain garden will help to absorb and retain water as well.
As mentioned above, the plants chosen should be native to the area. Deep rooted plants are better for decomposing storm water pollutants. Having diverse plantings is better for survival, and encourages biodiversity.
Birds, bees, butterflies, and insects are dependent on different species of plants for food and reproductive use, thus having a greater diversity of plants yields a greater selection for these opportunities. Plants should also be selected base upon the availability of sunlight. Species require differing amounts of shade for survival.