Reduce Stormwater Runoff With a Rain Garden
Booming urban growth, and the concrete that comes with it, has pushed storm drainage to its limits. Most cities channel rain overflow to holding ponds via drains and culverts, which then empty into fresh-water supplies. These manmade systems are designed to move water quickly, but several factors make all this drainage a problem. First, our urban areas produce a lot of runoff: An impervious surface such as a parking lot or rooftop generates nine times more runoff than a wooded area of the same size, according to the Environmental Protection Agency. Second, pollutants such as motor oil, fertilizers, pesticides and debris also are washed down storm drains, accumulating and becoming more concentrated as they enter streams and ponds. This poisonous runoff pollutes drinking water supplies, harms fish and wildlife, kills native vegetation, and makes recreational areas unsafe.
Stop the cycle
In an effort to restore natural drainage patterns to cities across the country, many people are planting rain gardens. Planted in depressions in yards and along roadsides, rain gardens (also called “bioretention” areas) are designed to catch and divert runoff into the ground before it reaches storm drains. Rain gardens include plants, usually natives, that help percolate rain back through the soil. By doing so, they also filter many contaminants.
Municipalities and watertreatment districts nationwide are promoting or subsidizing rain gardens. In Kansas City, where violent storms and flash floods are the norm, city organizers recently launched the 10,000 Rain Gardens project to address storm-drainage issues. This volunteer initiative includes an educational website and how-to classes, links with landscape professionals and hundreds of official participants.
Scott Cahail, environmental manager of Kansas City’s Water Services, believes the initiative does more than prevent flooding. “We should value water as a resource, not see it as a nuisance,” he says. When people add rain gardens to their landscapes, they see the cycle of water conservation. Plus, native plants or flowers attract bees, birds and butterflies. “And on the practical side, it eliminates a patch of grass that needs mowing!” Cahail says. He estimates the cost of building a rain garden at $10 per square foot or even less–the environmental benefit is priceless.
Build a Rain Garden in 10 Steps
Ruler, stick or scrap wood (12 inches or longer)
Pencil or marker
Peat or compost
Moisture-loving native plants (see”Selecting Native Plants”)
Step 1: Call before you dig. Contact your local utilities providers (electricity, gas, phone) to have them mark the location of underground wires or cables.
Step 2: Pick a location. A rain garden should be at least 10 feet from foundations, septic systems, utility lines and fence posts. You may wish to extend the length
of a downspout to reach the rain garden.
Step 3: Measure drainage rate. Dig a hole about the size of a large coffee can. Insert a ruler or stick into the hole. Fill the hole with water from a hose and mark the water level on the ruler. Wait four hours, then measure and mark the water level again. To determine the daily percolation, take the amount that has drained in four hours and multiply that by six. (Follow this formula: __ inches every 4 hours x 6 = __ inches every 24 hours). Your rain garden should empty within 24 hours, so if you can drain 6 inches in that much time, dig 6 inches down. If the water in your test hole doesn’t drain well, consider different placement, or add gravel, compost, sand or peat (see Step 7).
Step 4: Determine the garden’s depth. It should be no more than 6 to 8 inches deeper than the surrounding soil, but you can place it in the bottom of a larger landscape depression or slope.
Step 5: Outline the garden location. Use string and wooden stakes or a garden hose to mark the general placement. Think about the land’s slope and where heavy rain may come in and flow out; don’t orient the garden so that overflow runs into your foundation or septic system.
Step 6: Dig in. The depression should be within your marked outline and to the depth you determined in the previous steps.
Step 7: Check the drainage rate again. Fill the depression with water, then measure the rate as in Step 3. If the drainage is poor, remove 3 to 4 more inches of soil and till in some sand, gravel, peat or compost to a depth of 1 foot, then check drainage again.
Step 8: Add vegetation. Put plants that can tolerate “wet feet” in the lowest places. Lightly cover with additional soil if necessary, but don’t fill the depression completely.
Step 9: Mulch to keep the weeds out.
Step 10: Water. Until the plants are established–especially if rain is scarce–it is beneficial to water to 1 inch at least once a week.If there’s regular overflow from the depression, you may wish to enlarge it or build a series of rain gardens with connecting drainage notches.
Selecting Native Plants
The plants best adapted to your region will thrive in a rain garden. Look for flowers, shrubs and grasses that are not invasive or spreading, that thrive in damp conditions, and that are adapted to wet and dry cycles. For the deepest points in your rain garden, choose plants that can tolerate “wet feet”–that is, their roots enjoy boggy conditions. For suggestions, consult your local Cooperative Extension office, botanical garden or a nursery that sells native plants.
Cooperative Extension office locator
Rain Gardens: A How-To Manual for Homeowners
from the University of Wisconsin Cooperative Extension
Rain Garden Design for Home Owners
from the Alabama Cooperative Extension
Harvesting Rainwater for Landscape Use
by Patricia H. Waterfall
(University of Arizona Cooperative)
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