How To Build A Rain Garden In 10 Steps - PRINCETON HYDRO

If you’re looking for a way to solve drainage issues in your yard and put rainwater to good use, look no further than a rain garden. Rain gardens are a cost effective, attractive, and sustainable way to minimize stormwater runoff and filter out pollutants. This aesthetic, low-maintenance addition to any outdoor landscape creates a functioning habitat that attracts pollinators, beneficial insects, and birds. And, in a small way, it helps reduce erosion, promote groundwater recharge, and minimize flooding. 

 

 

WHAT IS A RAIN GARDEN? 

A rain garden is an excavated depression in the landscape that is planted with wetland or wet-tolerant plants. It captures a portion of stormwater runoff from a targeted area (i.e., rooftop, walkway, or driveway).  Even if it intercepts the first one inch of rain and the rest overflows, it will still capture about 90% of the total annual rainfall. 

The concept of rain gardens are becoming mainstream and there are many, many online resources to obtain design guidelines. In fact, even “This Old House”, a PBS stalwart, has an instructional page to help guide the design process!

“After repeated stormwater drainage issues near my home’s foundation due to flat topography, I opted to go the environmentally-friendly route of installing a rain garden in my front yard.”

Princeton Hydro President Geoff Goll, P.E. recently installed a rain garden at his home and documented the process and put together step-by-step instructions on how to build your own:

 

STEP 1: CHOOSE THE BEST LOCATION

Where does your stormwater flow? A rain garden is designed to infiltrate water, so it is best to build your rain garden in an area where rainwater will feed into it from downspouts, driveways or low points in your yard. 

 

• At least 15 feet away from your home and downhill from any foundation.
• Should not be placed over a septic tank or underground utility lines. 
• Ensure there is a way to direct the rainwater runoff from the area to be treated (i.e., run a drainage pipe from a roof leader or grade the landscape so the water flows into the rain garden).
• Keep it outside the drip line of any trees you want to keep. 
• Do not select an area that is already wet due to high groundwater or an area that already receives water from other locations, so you do not overwhelm your rain garden, and divert other water away (it will defeat the benefit of the rain garden).
• Consider the soil types. It is best to have soils that are sandy or silty sands. With compost added and blended in, a siltier type of soil can be improved too. However, clay soils are not conducive to a rain garden. A good source to check is the USDA National Database Mapping of Soils and/or dig down 2 feet to view the soils. Click here for a clear explanation for determining your soil type.

 

STEP 2: TEST THE SOIL AT YOUR SELECTED LOCATION

In order to determine if the soil at your selected location will readily infiltrate runoff, dig a few test holes. An optimal location will achieve one inch per hour of infiltration (i.e. the hole empties in 8 hours) and a good location will achieve one-half inch per hour (empties in 16 hours). If it is slower than 24 hours, you should consider a new location.

• Dig a 12 inch diameter by 12 inch deep hole in the selected location.
• Place a 12 inch ruler in the hole and fill it with 8 inches of water.
• Record the total time it takes for the water to completely escape.

 

STEP 3: DESIGN THE SIZE & CAPACITY

Determine the size and shape of your garden. Geoff chose a circular shape for his garden. 

 

• Choose your shape.  A circular shape or kidney shape are the most common. 
• Measure the total area to be treated (i.e. size of water coming rooftop or driveway into the rain garden). This can be done on foot, or even more quickly by finding your home on Google Earth and using the measuring tool. 
• Choose how large the rain garden will be. A rule-of-thumb is to make the bottom surface of the rain garden 1/10th to 1/6th the size of the area to be treated. So, if you have an 800 square foot roof to be treated, the bottom area of the rain garden will be from 80 square feet to 133 square feet. (Note: Even if the size is 1/15th the area due to budget or location constraints, you are still doing good for the environment. Build on!)
• Choose the appropriate depth of your garden. Generally, 6 to 8 inches of ponded depth is used. Using the runoff area calculations above and these depths, you can capture 85% to 90% of all runoff that occurs during the year, or about one inch of rainfall (in the Northeastern US). By capturing this much rain, the rain garden mimics the infiltration rate of a forest or meadow. What a great feeling, right?

 

STEP 4:  DIG YOUR HOLE

IMPORTANT! Before you dig, you MUST call your state’s dig-safe number (811) at least 72 hours before putting a shovel in the ground. It’s the law! They will mark-out your property for free so that you don’t unintentionally dig into any underground utility lines.

Depending on whether you will hand-dig your rain garden (as Geoff did) or rent a small excavator (you can rent one from Home Depot or local equipment rental centers), will dictate how much time the effort will take.  Prepare your garden by removing all of the grass.  If you can, separate the topsoil for later use or for other areas of your property.

Start digging out the area until you reach the proposed bottom depth. In some cases, you may have to dig a little further down to reach more adequate soils for infiltration. The sides of the hole should form a gentle slope toward the middle and create a berm around the perimeter that ensures the garden will hold water in when it rains.

 

Tip: For excavated soil that’s not reused in your rain garden, find it a home by filling in low areas in your lawn or stockpile it for later use.

 

STEP 5: CREATE THE OVERFLOW PATHWAY 

Carve out a spillway by creating a small indentation in the berm of the rain garden, lining it with permeable weed control fabric, and filling it with decorative stone.The spillway will allow excess rain water to overtop and flow out of the garden should it overfill during a storm event. 

 

STEP 6:  ADD SOIL, SAND, &  ROCKS

Use some of the excavated soil to create a berm around the rain garden. Once depth for adequate infiltration is reached, begin filling the hole with sand, and continue to backfill it to the proposed final grade, making sure the bottom of the rain garden is 6 to 8 inches below the surrounding lawn. (Note: Geoff ordered 3 tons of sand for his project, however, this quantity will be custom to your site, so use your best judgement to determine how much you’ll need). Add leaf compost (about 4 lbs per square foot) to the top of the sand and hand-till the compost with a shovel to a depth of 12 inches.

 

STEP 7: SELECT & PLANT THE PLANTS 

Having native wetland or wet tolerant plants that can withstand periods of drought is the key to success. Use hardy perennial species with well-established root systems. For guidance, look to your local garden center and a niche plant nursery. Make the plant palette your own. (Note: Be on the lookout for deer resistant plantings if this is an issue in your area). 

When it’s time for planting, place each plant 12 inches apart and dig each hole twice as wide as the plant plug. The crown of the plant should be level with the ground.  Apply wood mulch over the bed. Water frequently until the plants are established. Later on, no watering will be necessary. The vegetation will fill in nicely over the next few seasons.

 

STEP 8: CONNECT YOUR RAIN GUTTER (OPTIONAL)

If you’re planning to connect your gutter to the rain garden, it’s a good idea to install the drainage pipe last so the project is not inundated with water after a rainstorm while it is still under construction. 

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• Dig a trench for a pipe that will carry water from one or more gutter downspouts to the rain garden.  (Tip: First peel up the turf over the trench, roll it up, and put it to the side. Once you backfill the area, you can simply roll the turf back. It is not a perfect method by any means, but the grass will grow back quicker than re-seeding.)
• Line the trench with stones to prevent erosion. 
• Install the PVC piping. Make sure the pipe has a positive slope for its entire length and that the spillway of your rain garden is at least 12 inches in elevation below the ground surface at your foundation so that water does not back up to the house during rain events. Use a level to make sure there is at least a ¼” per foot of pipe run. The slope keeps the pipe from collecting soil and debris and clogging. This will help prevent the water from shooting out of the connection between the downspout leader and your drainage pipe! 
• Optional: Install a pop up emitter at least 10 feet from the house to allow any backed up water to safely drain away from the house. The emitter acts as a relief valve to allow water to discharge without getting to your foundation or basement.
• Extend the piping into the rain garden basin by approximately one foot. 
• When the piping is in place, fill in the trench with soil.

 

STEP 9: LET IT RAIN! (IS IT WORKING?)

 

It is very important to keep an eye on your rain garden, especially after rain, to ensure that it is emptying in a timely manner. The last thing you want is to have the neighbors complaining about the “mosquito pond” next door. Generally, all the water should be infiltrated into the soil within 24 hours. 

“A week after I completed the installation of the rain garden, we received over 4 inches of rain within a 5 day period, and then, we were hit with Tropical Storm Ida with 4.29 inches of rain (from my home rain gage) in a period of less than 24 hours. The rain garden worked very well, and even after Ida, the rain garden infiltrated all the water retained in the bottom within an 8 hour period (about an inch per hour).”

 

STEP 10: MAINTAIN YOUR GARDEN

Each season, pull weeds, remove debris, and replenish the mulch as needed in order to help retain moisture and block weeds. 

“During the construction, I’m sure my neighbors were judging the ugly hole in my front lawn. And, my wife even marked herself as “Safe from the Hole” on Facebook! However, when it was finished, she was pleasantly surprised at how pretty the rain garden looked and amazed at how it managed stormwater,” said Geoff. “If everyone in my neighborhood installed a rain garden, imagine the positive reduction in stormwater and flooding that would occur downstream.  Plus, we’d have some pretty awesome looking native flowers and grasses around for the pollinators and wildlife.”  

 

• Green Infrastructure
• Natural Resource Management
• Stormwater Management