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After the storm- Rain garden revisited
Arkansas Recent rains across Arkansas help provide much needed moisture for plants, but rain can also wash a considerable amount of pollutants into storm drains. You can help improve water quality by constructing a rain garden. A rain garden is a man-made depression, about 6 inches deep in the ground and is used as a landscaping tool to improve water quality. It should be placed strategically to intercept stormwater runoff with a grass buffer around it. Rain gardens form "bioretention areas" by collecting and storing stormwater. While stormwater is stored in rain gardens, it's filtered and slowly absorbed by the soil. The bioretention concept is based on the water-absorbing function of forest habitats. In forested areas, decaying leaves, twigs and other discarded plant materials from trees and other plants produce a spongy layer of litter, or humus. The humus soaks up water and slowly releases it allowing more water to penetrate into the soil profile over an extended period of time. In rain gardens, using a top layer of organic material provides an important role in the removal of metals from stormwater. Shredded hardwood mulch is the preferred choice of organic materials, since it provides maximum surface area for binding metals and resists flotation and washout. Nutrient removal, or filtering, takes place as the water comes in contact with the soil and the roots of trees, shrubs and other vegetation. This accounts for the improved water quality in forested areas. In rain gardens, this second layer of the soil profile should be organic matter in the form of leaf mulch (20 percent) blended into a sandy soil (50 percent) and about 30 percent top soil. This mixture provides a source of soil moisture and nutrients to sustain plant growth. Clay particles in this mixture retain moisture and adsorb heavy metals, hydrocarbons and other pollutants. When rain occurs, the first water reaching the rain garden contains the highest concentration of materials washed off impervious surfaces such as roofs, roads and parking lots. Ponding and storage of stormwater allows for evaporation and settling of material not filtered by the grass buffer. Ponding of stormwater should not exceed four days to avoid breeding of mosquitoes and other troublesome insects. A planting plan for the garden should include species that can tolerate moisture extremes. There will be periods of water abundance and times when the soil will be very dry. Most riparian plant species will do well in rain gardens. The choice of species should include plants that mimic forest habitat and have an aesthetic landscape value such as flowers and berries. Plants with interesting leaves or bark, groundcovers, perennials shrubs and trees can also be incorporated into the planting design. Rain gardens are suitable for any land use situation, residential, commercial or industrial. They should be placed where impervious surfaces will drain into the depression area. Each site is unique. Microclimates (light, temperature and wind), and the size of the drainage area will influence the size of the rain garden and plant selection process. The size of the bioretention area should be 5 percent to 7 percent of the drainage areas. For example, a .3-acre drainage area would use a rain garden of about 600 square feet, or 15 x 40 feet. C 3 7/30/07 Date: 7/24/07
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