New England Wild Flower Society's step-by-step guide to building your own rain garden
Horticulture & Botanic Garden Director
What is a rain garden?
Designed to capture water from impervious surfaces such as roofs, driveways, walks, and patios, rain gardens are depressions built into the landscape and planted with native plants. They allow water to pool during and after a storm. The storm water then slowly filters into the ground, undergoing a natural microbial process that results in cleaner water.
Rain garden benefits
Rain gardens are a hands-on, action-oriented solution that enlists the homeowner’s participation in reducing environmental degradation. They can lesson storm water runoff, a leading cause of water pollution. Beautiful, green, and functional rain gardens can help, and have many additional benefits:
- Reduces storm water runoff from impervious surfaces such as roofs, driveways, walks, and patios
- Allows the natural microbial cleaning process to occur before the water reaches open waterways
- Aids in recharging groundwater
- Reduces flooding by preventing excessive water flow to streams and rivers during storms
- Creates habitat for birds, butterflies, and beneficial insects.
- Provides a good family or group project and introduces active green living
- Stimulates creativity and enhances your property with limitless design possibilities
Rain gardens are also becoming a necessity as many state and local municipalities require new construction projects to have a reduced (or zero) run off.
Locating your rain garden
- Choose a spot directly off the rainspout of your house or
- Create a garden further out in the landscape to capture site runoff
- Be sure to place your rain garden at least 30 feet from a foundation to prevent seepage into the basement
- Avoid using wet or poorly drained areas
- Avoid placing rain gardens in deep shade
- Do not place a rain garden over a septic tank or leaching field
- Avoid placing rain gardens in close proximity to a well
- Observe protective zoning regulations, especially if you live on waterfront property
Starting a rain garden
1. Determine the size of your garden
A rain garden should be 30-50 percent of your impervious (non-permeable) drainage area. Calculate the size of your roof, driveway, patio, or other impervious surfaces into square feet. Your rain garden should be half of that size.
Example: If your roof size is 3000 square feet, then your rain garden should be 1,500 square feet. You may go bigger or smaller, but never scale the design to less than 30 percent of your impervious size.
2. Determine how you will channel water
A rain garden located directly off a down spout may require a gutter extension. A rain garden designed to catch driveway run off may utilize drain pipes or a grass swale.
Tip: To prevent washout, be sure to stabilize the area where water enters your rain garden.
3. Plan ahead
Select your garden location and choose the appropriate options for clearing the site.
For existing lawn sites:
- Remove or kill the grass
- Consider digging and transplanting grass to utilize the turf
- If you plan to eliminate the lawn, use smothering as an eco-friendly solution which requires two months lead time
- Avoid using herbicides for lawn removal
For borders and near existing landscape plants:
- Dig and transplant existing material
- Time transplanting for times of lowest environmental stress with cool temperatures, adequate moister and dormant and non-flower periods
4. Choose your native plant palette
Create a native plant list from New England Wild Flower Society, keeping in mind the level of available sunlight for the location. Rain garden plants also need to be able to handle inundations of water and periods of dryness. Place your orders early with the Society’s nurseries to ensure a wide choice of plant species.
Preorders with a minimum of $500 are accepted at email@example.com.
Do a perc test
Dig a hole 18 inches deep. Fill the hole with water and observe how long it takes for the water to seep into the ground. If it drains within 24 hours you are in good shape for a rain garden location. Water that stands longer than 24 hours will require additional drainage plans.
Layout and dig
Using water-based spray paint or a garden hose, delineate the size and shape of your garden. Dig in! Work from the middle outward towards the sides of the site, creating sloped walls. Loosen the soil at least one foot deep to increase drainage. If your rain garden is on a slope you will need to create a berm (or small ridge) to build up the downward-sloping side of the garden to maintain stability.
Tip: All rain gardens should have an emergency overflow pipe. One foot below the top of the rain garden, install a 1-3 inch diameter pipe through the berm, to prevent overflow
Plant your garden
Now the fun begins. Place plants into your rain garden starting in the center and working your way out. Locate the plants that can handle the greatest inundation of water at the lowest point and move up the slope with plants requiring less water. Water immediately after planting and mulch three inches with compost or shredded leaves.
Tip: If you are installing woody plants, be sure to keep the mulch three inches away from the trunks.
A rain garden, like any garden, requires weeding, deadheading, and water during prolonged drought periods. You will also want to keep your emergency overflow drain free and clear of debris. Leave plants standing over winter to encourage wildlife and provide a food source. Cut back in spring and enjoy the next season!
Rain gardens that function properly will drain within 24 to 48 hours. This prevents mosquitoes from using your rain garden as a breeding ground. Mosquitoes will not survive in areas that dry out in fewer than seven days because development from larval stage to adult takes longer than seven days.