Rain Garden: a boon to save and utilize rainwater runoff
A rain garden is a garden of native shrubs, perennials, and flowers planted in a small depression, which is generally formed on a natural slope. It is designed to temporarily hold and soak in rain water runoff that flows from roofs, driveways, patios or lawns. Rain gardens are effective in removing up to 90% of nutrients and chemicals and up to 80% of sediments from the rainwater runoff. Compared to a conventional lawn, rain gardens allow for 30% more water to soak into the ground.
A rain garden is not a water garden. Nor is it a pond or a wetland. Conversely, a rain garden is dry most of the time. It typically holds water only during and following a rainfall event. Because rain gardens will drain within 12-48 hours, they prevent the breeding of mosquitoes.
Why is Rainwater runoff a problem?
Every time it rains, water runs off impermeable surfaces, such as roofs or driveways, collecting pollutants such as particles of dirt, fertilizer, chemicals, oil, garbage, and bacteria along the way. The pollutant-laden water enters storm drains untreated and flows directly to nearby streams, ponds and river. It is estimated that pollutants carried by rainwater runoff account for 70% of all water pollution.
Rain gardens collect rainwater runoff, allowing the water to be filtered by vegetation and percolate into the soil recharging groundwater aquifers. These processes filter out pollutants.
What makes A Rain Garden different from a traditional Garden?
In the design of a rain garden, typically six to twelve inches of soil is removed and altered with tillage, compost and sand to increase water infiltration. The type of alteration to the soil depends on the current soil type, so it is a good idea to obtain a soil test.
Rain gardens are generally constructed on the downside of a slope on your property and collect rainwater runoff from the lawn, roof and/or the driveway. Once water collects in the rain garden, infiltration may take up to 48 hours after a major rainfall. Also, rain gardens incorporate native vegetation; therefore, no fertilizer is needed and after the first year, maintenance is usually minimal.
Benefits of Rain Garden
- Improves water quality by filtering out pollutants
- Aesthetically pleasing
- Preserves native vegetation
- Provides localized storm water and flood control
- Attracts beneficial birds, butterflies and insects
- Easy to maintain after establishment
Suitable Plants for Rain Garden
As a rule, native vegetation should be incorporated into a rain garden. Native plants don’t require fertilizer, have good root systems, and are better at utilizing the water and nutrients available in their native soils than non-native species. Perennials, shrubs, wildflowers, or a mixture of all three can be planted. Check with your local nursery for a list of rain garden approved plants for your hardiness zone, or search the rain garden plant database.
As a rule of thumb avoid planting trees in your rain garden, as trees generally absorb more water than surrounding plants. Also, never plant invasive or noxious species in a rain garden.
Size & Placement
A rain garden should have an area about 20% the size of the roof, patio, or pavement area draining into it. A typical rain garden for a residential home or small building is between 100 and 400 square feet. Regardless of the size, big or small, each rain garden can make an impact.
Rain gardens are shaped longer than they are wide and positioned perpendicular to the slope of the land in order to catch the maximum amount of rainfall. Rain gardens should be placed at least 10 feet away from building foundations and should not be located where water ponds for an extended period of time.
Site suitability Check
- The site is fed by one or two downspouts
- Soil tests show the site does not have heavy clay soils (conduct a ribbon soil test)
- Infiltration tests show the site infiltrates water one-half inch per hour or more (conduct an infiltration test)
- The water table is at least 2 feet from the surface at its shallowest
- The slope of the site is not more than 12%
- The site is at least 10 feet from buildings with basements
- The site is not over any utilities (like telephone, electricity or other cable)
- The site is not over or near a septic tank, drain field, or wellhead
- The site does not interfere with any trees. If there are trees in the area, make sure they can handle wet soil conditions for lengthy periods of time.
If the site that you have chosen does not meet all of the above criteria, it does not necessarily mean that a rain garden cannot be established there.
Maintenance of Rain Garden
When native plants are used, your rain garden will not require pesticides or fertilizer. As the rain garden is establishing during the first and second year or during periods of little to no rainfall, occasional watering of the plants may be necessary. Likewise, weeds will need to be removed while dead plants may need to be replaced. Once the plants in the rain garden have become established and grow larger, they will eventually out-compete the weeds.