By Elinor Bennett Markle
“Rain, rain, go away. Come again some other day.” The problem of getting too little or too much rain is hardly new. The nursery rhyme that begins with those words originated in the days of Queen Elizabeth I, during the late 1500s. If only we could cause rain to fall in amounts and durations that suit our individual purposes, there probably would be no floods, no droughts, no ruined concerts, ball games or picnics. Nearly 500 years later and with all our technological advances, we still cannot control precipitation. What we can do is manage precipitation when it gets to earth.
Previous engineering practices have provided safe streets and dry basements, but with the increased amount of water runoff caused by high-density development, we are learning that overly speedy redistribution of water causes flash flooding of streams and rivers, increased non-point source pollution, and the gradual lowering of ground water tables. When slopes are steep and rain velocities are great, water has little chance to percolate down to recharge the groundwater table. Wet and wild, it shoots from downspouts into gutters and then to storm drains and, in a matter of minutes, may reach a discharge point such as a local stream. Too much water running too fast erodes the very soil it is meant to nourish, ruining its structure and exposing plant roots and storm water collection features such as headwalls, curbs and gutters. If there is no pollutant removal system in place in the storm water system, pollutants picked up by runoff enter the stream system unchecked, further degrading water quality.
We can take a leaf out of nature’s book and create “rain gardens” to slow storm run-off, reduce pollution, and recreate wildlife habitat and beauty. The idea of rain gardens came not from an environmental watchdog group, but from a developer of a new housing subdivision in Prince George’s County, Md. in 1990. He thought bio retention areas could be used effectively for storm water detention rather than the traditional grass-lined storm water detention pond. During the design development phases of his project, he worked with a local governmental agency to implement his idea. At construction completion, each home within his subdivision had a rain garden of about 300 square feet in size, and the cost savings realized in the resulting development was so substantial that other developers began to use the system. Rain gardens are now one of the fastest growing trends in home landscapes, and have great applicability for commercial sites as well.
Key to designing the successful rain garden is reliance on locally native plants that can tolerate wet or waterlogged soils as well as drought. They provide wildlife with food and cover, and the great variety of plant forms and blooms are fun to work with. Sedges and reeds, grasses and perennials are all good choices, and shrubs such as Hibiscus, Hydrangea and several Ilex species are just a few of the interesting plants that love life in the rain garden. One of the reasons native perennials are integral in the rain garden is because a portion of their roots die each winter. The resulting organic matter left in the soil creates spongelike characteristics that increase each year, and a rain garden can absorb 30 percent more run-off than a typical lawn.
Rain gardens mimic naturally occurring situations because water collects and gradually infiltrates into the ground. As run-off passes through the soil and is absorbed by the plants, naturally occurring microbes attached to the plant roots capture and break down various pollutants. Studies by universities and municipalities show that 80 percent of total suspended solids, 50 to 60 percent total phosphorus and nitrogen, and 80 percent of the heavy metals of cadmium, copper, lead, zinc, and lead are intercepted and removed.
Strategic locations for rain gardens are down-slope of the target drainage area in mostly sunny areas away from foundations and trees (whose roots would interfere with and be hurt by digging). Rain gardens are good for treating small drainage areas such as individual homeowner lots, and are easy to retrofit into existing home landscapes. They are also well suited to parking lot islands, but not recommended for areas with steep slopes.
Investigate the site and soils before committing to materials or labor pricing. Dig a hole 8 inches deep and fill with water. If after 24 hours the hole has not completely drained, you may need to amend the soil, depending on the size of the area available for the garden. Sandy soils need a rain garden between 20 and 30 percent the size of the drainage area collected; heavy clay soils require more — up to 60 percent. A rain garden should drain completely within 24 hours after a rain of up to 1.25 inches, which is the amount of 80 percent of current rainfall events. As the soil’s organic characteristics increase, water will stand for less and less time.
You may think “mosquitoes” when you consider rain gardens, but this will not be a problem, as their life cycle takes at least 10 days from laid egg to flying adult. The principle places water stands that long include gutters, birdbaths and pans under plant pots. Maintenance needs of rain gardens, once established, are less than lawns as they eventually need only monthly maintenance of the type any perennial garden needs — shrub pruning and the cleanup of dead leaves and stalks.
Elinor Bennett Markle, RLA, ASLA, is a landscape architect practicing in Kentucky and Tennessee. She can be reached via e-mail at firstname.lastname@example.org