How to Build a Rain Garden

The function of gardens is usually growing food and flowers, but your patch of green can also serve an environmental function—helping reduce flooding and curb water runoff. 

Growing spaces that perform this function are commonly known as rain gardens. These gardens are shallow, sunken plots of land that act as a reservoir for storm water that is not absorbed by surfaces such as sidewalks, driveways or decks. 

Natural land spaces such as forests and wetlands can capture and filter out toxins or other unwanted material before they reach a stream or river. But these natural landscapes have been increasingly less common over the years due to urbanization. Storm water has been identified in recent years as a leading source of pollution in 13 percent of rivers, 18 percent of lakes and 32 percent of estuaries nationwide in the US. Rain gardens, however, have been touted as a potential solution and can absorb 30 to 40 percent more water than a standard lawn.

There are several components to ensuring your rain garden is able to soak up as much runoff as possible. Here’s a detailed guide to building your own. 

Location, location, location 

You should build your rain garden in a strategic location that has full or partial sun, near a runoff source such as a downspout or driveway. It should also be at least 10 feet away from any house or building foundation and at least 25 feet from a septic system or wellhead. 

If possible, choose a low spot in your yard with a slight, moderate slope. If you don’t have a downspout or a plot of land that is in a location where water can flow naturally, dig a trench about three to four inches deep from your runoff source to where your garden will be.  

You’re also going to want to do a soil test to make sure the area you’ve chosen has good drainage. To do this, dig a hole six to eight inches deep and fill it with water. If the water has disappeared within 24 hours, this means the drainage conditions are good for building a rain garden. 

It’s important that you have located and marked any nearby underground utilities prior to settling on your garden’s location and digging. Utility lines are sometimes located in shallow underground places and you can be held liable if you hit them. Make sure to contact your local utility service to map out any lines. 

Size and depth

Your rain garden should be able to hold runoff from your roof, driveway or other hard surface based on how much water you would have from a one-inch rainstorm

The University of Connecticut Cooperative Extension and the Rain Garden Alliance have both designed comprehensive rain garden calculators to tell you what the dimensions of your garden should be based on your location and environmental conditions if a one-inch rainstorm were to occur.  

Plants and soil

For soil, we suggest using a mixture of 50-percent coarse sand, 25-percent compost and 25-percent topsoil to ensure optimal drainage. You can add this after digging out your garden space. 

When choosing your plants, select ones that are native to your region. They will be better adapted to local climates and growing conditions. They will also require little fertilizer and typically have far-reaching root systems that help send water deep into the soil. 

You’re also going to want to choose a variety of plants that can handle short-term flooding and periods of drought. When you design your garden, you should place plants that tend to be more resilient to saturated soil in the center and grow more drought-hardy varieties closer to the perimeter. 

Staff at your local nursery should be able to point you in the direction of native vegetation. Alternatively, here’s a list of plants commonly chosen for rain gardens that thrive in full sun and partial shade in North America: 

  • Common Camas, Camassia quamash: Grows best in USDA Zones 3-7, dry to moist soil
  • Showy Fleabane, Erigeron speciosus: Grows best in USDA Zones 4-7, dry to moist soil
  • Snowberry, Symphoricarpos albus: Grows best in USDA Zones 2-7, dry to moist soil 
  • Red Flowering Currant, Ribes sanguineum: Grows best in USDA Zones 6-9, moist soil 
  • Salmonberry, Rubus spectabilis: Grows best in USDA Zones 4-9, moist to wet soil
  • Panicled Bulrush, Scirpus microcarpus: Grows best in USDA Zones 2-8, moist to wet soil 
  • Baby Sun Rose, Mesembryanthemum cordifolium: Grows best in USDA Zones 10-11, dry to moist soil
  • Butterfly Weed, Asclepias tuberosa: Grows best in USDA Zones 3-9, dry soil 
  • Wild Bergamot, Monarda fistulosa: Grows best in USDA Zones 3-8, moist to wet soil
  • Cardinal Flower, Lobelia cardinalis: Grows best in USDA Zones 3-9, moist to wet soil
  • New England Aster, Aster novae-angliae: Grows best in USDA Zones 3-8, moist soil
  • Black-Eyed Susan, Rudbeckia fulgida: Grows best in USDA Zones 4-8, dry to moist soil
    Blue Lobelia, Lobelia siphilitica: Grows best in USDA Zones 4-9, moist to wet soil
    Red-osier Dogwood, Cornus sericea: Grows best in USDA Zones 2-7, moist to wet soil
    Sword Fern, Polystichum munitum: Grows best in USDA Zones 4-8, dry to moist soil
    Winterberry, Ilex verticillata: Grows best in USDA Zones 3-9, dry to wet soil
    Dagger leaf Rush, Juncus ensifolius: Grows best in USDA Zones 3-10, moist to wet soil
    Rocky Mountain Iris, Iris missouriensis: Grows best in USDA Zones 5-10, moist soil
    Stream Violet, Viola glabella: Grows best in USDA Zones 4-8, moist soil
    Western Columbine, Aquilegia formosa: Grows best in USDA Zones 3-7, moist soil
    Monkey Flower, Mimulus guttatus: Grows best in USDA Zones 5-9, moist to wet soil
    Switchgrass, Panicum virgatum: Grows best in USDA Zones 5-10, dry to wet soil
    Culver Root, Veronicastrum virginicum: Grows best in USDA Zones 3-8, moist to wet soil
    Common Boneset, Eupatorium perfoliatum: Grows best in USDA Zones 3-8, moist to wet soil
    Purple Coneflower, Echinacea purpurea: Grows best in USDA Zones 3-9, dry to moist soil 

 

Digging and designing 

Rain gardens are typically oval or kidney bean shaped and twice as long as they are wide but still wide enough so that the water can disperse evenly. The government of Snohomish County in Washington State has put together this helpful graphic if you’re having trouble visualizing the shape and details of your design. 

Once you’ve determined the size, depth and shape, you can map it out on your lawn using marking paint before you excavate. You may need to remove grass if you’re building on a lawn. This can be done using a sharp spade or a sod cutter. After grass is removed, you can start digging to your desired depth using a shovel. Make sure that when you’re digging out your garden, the bottom is levelled out. This will encourage optimal drainage. You can check to see if it’s level by laying a wood board across the bottom with a carpenter’s level on it. 

Use the excess soil that you have extracted from your site to build a slight rise along the bottom and downhill sides of your garden. This is otherwise known as a berm. Your berm will help trap runoff and allow the water to have more time to infiltrate into your rain garden. It should be about a foot across and six inches at its highest point. Gradual slopes of the berm will also allow it to aesthetically blend into the surrounding lawn. 

If you need to dig a trench because your runoff source is a considerable distance from your garden, consider lining your trench and the area between the trench and the garden with rocks or stones. This will ensure that water doesn’t eat away at the entry point into the garden. 

When a big storm hits and water overflows, you’re also going to want to have an area that can catch the excess amount. This should be a depression or additional trench attached to the berm on the downhill side of the garden. Ideally, it will attach to an open patch of grass. You can also line this area with rocks to prevent excess water from eroding it. 

Once you’ve dug out your garden, fill your excavated area with your soil mixture, but ensure you’ve left about six to 12 inches of room for your plants, mulch and water. 

Planting 

To gently transport your plant out of its container, tip it upside down and give it a shake so that it falls out root side up. Gently untangle the roots if they are in a ball and spread them out. 

Your plants should be about one foot apart from each other. How deep they are planted in the soil should be based on how deep they were planted in their original containers. 

Once you’ve completed your planting, cover your garden with a layer of mulch that is about two to three inches thick. This will not only help keep out weeds but also help with the absorption and filtration of runoff. 

Maintenance and care 

Most plants will need a healthy watering to establish their root systems for the first year. After that, they will only need to be watered if you’re dealing with longer periods of drought. Give them about one inch of water each week or enough so that the top six to 10 inches of soil is moist. To retain as much of the water as possible, do your watering in the morning or the evening when the sun isn’t at its strongest and the temperatures aren’t their warmest. 

You can add mulch any time you notice exposed soil. We also recommend that you occasionally check for any debris that has built up in your garden as well as the entry and overflow passages. Keeping everything intact will allow as much water to be absorbed as possible. 

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