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  • Jacky Surber

What’s the difference between a Rain Garden and a Swale?


Rain garden, bio swale, dry creek bed. These aren't the most familiar terms to the average gardener. In the coming years these types of earthworks are crucial in increasing local water supply, water quality, controlling floods, and cleaning up our oceans and beaches. Why? Because contouring the soil in your garden slows water down, spreads it across the landscape and allows it to soak into the soil. Healthy living soil is the best option we have for filtering water to clean it up, better than any manmade technology. The moisture in the soil boosts plant growth and reduces the amount of municipal water you have to use for your garden.

Bridge over a dry creek bed style swale in a front yard in Santa Barbara, designed by Joan Bolton.

A rain garden is created by digging a basin, planting it with plants that are happy with wet or dry soil, and adding a layer of mulch or rocks. A swale is really just a fancy ditch, dug on contour to hold water. Some swales don't have any plants inside of them, just boulders and rocks, with plants on the outside, whose roots can grow to pull moisture from the ground below.

Recirculating water feature in a dry creek bed style swale in Santa Barbara, designed by Joan Bolton.

Large boulders are featured in many swales, because they help to hold the shape, which can shift over time with heavy rains. The boulders are fun for children to climb on, and make a nice seating option for adults too.

A swale with a relatively flat bottom, lined with pebbles, can also double as a garden pathway. Santa Fe, NM.

I hear a lot of people saying “but it doesn’t rain much in LA”. Isn’t it a shame then that most landscapes don’t hold onto any water when it does rain? This is because most hardscape's are not permeable, meaning surfaces like your roof and driveway do not absorb water, so when it rains the water runs off of these areas and on most properties, it is directed to the street as quickly as possible. Instead of solid surfaces, try to incorporate ones that are permeable to water like this pebble lined garden path.

A swale at the edge of the flagstone patio, flows throughout this backyard in Santa Fe, New Mexico.

Take a look at nature, and you will see that most areas have natural contours that are covered in vegetation or rocks which allow water to quickly soak into the ground. Any water that does run off, flows into dry creek beds that fill up with water when it rains. These creeks then slowly meander until they end at a lake, river or ocean, while along the way the rain water is being cleaned and filtered by healthy living soil and plants.

Rain gardens fill up with water during storms but drain within 24 hours, so there is no risk of mosquitos spawning.

Why should rain gardens be required in gardens? Getting rain water to soak into the landscape, before it hits the storm drain, means cleaner oceans and beaches. Sinking rain into the soil will also help to refill the underground aquifers that are all over Southern California, insuring that LA can have a local water source that doesn’t have to be moved from thousands of miles away. A huge amount of electricity is used in CA pumping water long distances, so the less water we import the more we lower our carbon foot print.

Rain water is the cleanest, purest source of water for your garden, so capturing it from the sky is a win win, its better for the environment, saves money on your water bill, and your garden will truly grow better with this fresh source of H20.

California native plants are particularly well adapted to wet and dry conditions. Theodore Payne Garden Tour 2016.

Now that you know how important rain gardens are, you’ll want to know how to incorporate a swale in your yard. It’s best to calculate the amount of water that you are hoping to capture in the soil and then build a rain garden or swale that is large enough to hold this amount of water.

For every 1,000 square feet of roof or driveway, you can capture 620 gallons of water for every 1” of rain that falls. This translates to about 83 cubic feet of water, or a swale that is 3’ x 13’ and 6” deep.

The formula for figuring out these calculations are shown here in this free guide book put out by LA County, on page 33. More information on rain gardens and swales can be found on page 9.

Swales don't have to be a large focal point, once the plants grow larger, you will hardly notice the swale in this garden.

The placement of the swale is important, it should be 10’ or more from the edge of your house, and 3’ from hardscapes, such as driveways, patios and sidewalks. Be mindful of mature trees, when locating swales, keeping at least 5’ from the trunk of mature trees, or consult with an arborist. Swales are ideal for capturing water from your downspouts or gutters. If you’re not sure where to locate the swale, consider having Urbafloria come out to your property for a consultation.

Rain chains are great alternatives to downspouts, not only they are the beautiful, they slow the fall of water.

When you dig out soil to create a swale, place the dirt you dig out on the down slope side of the basin to create a mound, or berm. If you don’t have enough space for a large enough swale to hold the calculated runoff, you can create an infiltration pit, by placing eco rain crates under neath the swale. These are large, plastic, milk crate looking cubes, that get wrapped in special fabric, and are buried underground. This creates a void space for water to flow to. Think of it as a dry well. You can get them at building supply stores such as Bourget Brothers in Santa Monica.

Builders check to see if rain crates will fit into the infiltration pit. They are wrapped in geo textile fabric before being buried (see fabric in left hand corner)

These are some California native plants that don’t mind being submerged in water for a day or so after it rains, and can survive the dry summers too.

Achillea millefolium

Anemopsis californica

Aristida purpurea

Carex tumulicola

Iris douglasiana

Iva hayseiana

Juncus patens

Sisyrinchium bellum

Swales can be a sculptural and creative element in the landscape. The Water Conservation Garden, San Diego, CA.

I hope you have a clear understanding of why swales are important to have in the landscape and that seeing photos of them has motivated you to create some in your garden. Keep your eyes open and you will start to notice them throughout the city, in front yards, parking lots, and out on hikes. Together we can improve the environment, and swales might just be the answer we've been waiting for.

#swales #raingarden #drycreekbed #howto

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