How to Build a Rain Garden

A rain garden can be a beautiful and functional addition to your landscaping. Here’s how to build one…

* The information below is an excerpt from our guide “Rainwater Harvesting in SLO County”. You can get much more information about rain gardens and rainwater harvesting by downloading the guide for free HERE.

A rainwater harvesting system that diverts rainwater into an area where it can permeate into the ground is called a Passive Catchment System. The idea behind a passive system is to slow the rainwater, spread it out, and allow it to sink into the soil.


There are many ways to build a passive catchment system, but for the purposes of this guide, we are going to focus on rain gardens. A rain garden uses rainwater that is diverted from a roof or another impermeable surface to provide the majority of its irrigation needs.

A rain garden is usually contoured using swales (basins) and berms (mounds) in order to maximize the amount of rainwater that can be absorbed by the land.

This system decreases the amount of supplemental irrigation that is required for the plants, helps recharge groundwater reservoirs, and reduces the amount of pollution in our waterways and the ocean. This is especially important on the Central Coast, where we must use water strategically and where water that goes into our storm drains can flow directly to the ocean.

Site Selection

Before you build your rain garden, there are a few considerations to plan out.

First, sketch out and measure the area where the rain garden will be located. Find an area that is near your downspout or catchment surface so that the rain can easily be diverted into your catchment basin.

Take note of the slope of the rain garden site. Preferably it will be located in an area that slopes gently away from the catchment surface or downspout. This will allow gravity to bring the water downhill into the catchment basin. Be careful of slopes that are too steep, however. If this is the case, special care will need to be taken when designing the catchment basin to ensure the water can be slowed as it runs down the slope.


Make sure your catchment basin is set back far enough from sidewalks, retaining walls, and foundations. These setbacks are often mandated by municipalities and can vary from one location to the next. Generally, they are 5-10 feet from foundations and retaining walls, and 3 feet from sidewalks.

Another important aspect of site selection is determining where the rainwater will go if the swale overflows. During heavy storms, it is possible for the swale in your rain garden to become full of water. In most cases, with healthy soil the rainwater will absorb into the ground very quickly and the swale won’t overflow. However, there will be times when it rains a large amount in a short period of time and the soil will not be able to soak up the water quickly enough.

When selecting a location for your rain garden, consider where the water will go if it overflows. You will need to shape the swale in such a way so that when it overflows, the excess water will go somewhere that won’t cause damage. In most cases, this will be towards a driveway or storm drain.

Site selection is very important to consider before beginning your rain garden. By doing your due diligence before starting, you will avoid costly mistakes and achieve better results.


Now that you have your site picked out and know the space you have available for your rain garden, you will need to determine the depth of the catchment basin or swale.

For rain gardens, a good rule of thumb is to size it to catch the first 1 inch of rainfall whenever it rains. Sizing it to catch more than that is sometimes difficult for standard residential areas.

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To determine how large to make your rain garden:

1. Calculate the cubic feet you will need for your catchment basin by dividing the number of gallons you can catch off your roof from 1 inch of rainfall by the coefficient 7.48.

Example: 1,000 Sq. Ft. x 1 inch x .62 (inches to gallons coefficient) = 620 gallons

620 gallons / 7.48 = 83 Cu. Ft.

2.Now you must figure out how deep you will need to make the basin. For this, divide the number of cubic feet from step 1 by the square footage of the area you have available for your rain garden. Then convert that number to inches by multiplying by 12.

Example: 83 Cu. Ft. / 200 Sq. Ft. = 0.41 feet

0.41 feet x 12”/ft = 4.9 inches

3.Using the calculations above, you have determined the depth that you will need to make your catchment basin.

Example: To catch 83 Cu. Ft. of rain in a 200 Sq. Ft. area, you will need to make your basin about 5 inches deep.



With all the pre-planning complete, it’s finally time to get your hands dirty! This can vary in difficulty quite a bit depending on site conditions such as soil type, obstructions, and weather. Also, please be aware of your physical limitations. Digging and planting can be strenuous. Please consult a professional if you need a hand.

Diverting the Downspout

downspout 2In most cases, your rain garden will be fed from the water that comes off your roof. In order to make this happen, you’ll need to divert your downspout into the garden.

Depending on the location of the rain garden in relation to the downspout, this can be as easy as using a downspout extension that is directed towards your swale.

If there is a greater distance between the downspout and your rain garden, or if there is a sidewalk or other obstruction in the way, you’ll need to use an underground drainage pipe.

If this is the case, dig a trench from the downspout to the swale, making sure that there is at least a ¼ inch per foot slope to allow the water to flow. Next attach the drainage pipe to your downspout and lay it in the trench all the way to where the swale begins. Finally, fill in the trench and bury the pipe.

Now when it rains, the water will flow through the drainage pipe and into your rain garden!

Building the Swale

A swale, or catchment basin will be needed to slow down and retain the rainwater. A swale can be a dry creek bed, a wide basin, a depression that is contoured into the earth, or any other earth shaping that is designed to slow, spread, and sink rainwater.

Before you begin digging, it is helpful to know what you want your swale to look like. Sketch out your ideas on paper until you decide on a type of swale that you like. Next, use a spray can of marking paint to mark out the shape of the swale in your yard. This will give you a guide to follow as you are digging.

A rain garden swale dug out in San Luis Obispo, CANow it’s time to get the shovel and start digging. You can use the soil you remove to create berms (mounds) around the swale to add to the contouring of the space. This creates a natural look and adds dimension to the landscape. As you dig out the swale, use your tape measure to determine when you are at the depth you determined in the sizing section above.

Before you start digging, don’t forget to call 811 to get your utility lines marked. Also, watch for underground irrigation lines as you dig, they have a tendency to be where you least expect them.


In the site selection section above, you learned about the importance of shaping and locating your swale to direct overflow appropriately. When digging your swale, don’t forget to incorporate your overflow plan. Shape the swale so that if it overflows with water, the excess will run off towards a storm drain or an area that will not cause damage.

Build a Soil Sponge

A rain garden is not just about the swale, it’s about permeability. The whole idea is to allow rainwater to permeate into the soil and provide water for your plants. In order to make this happen, you need to have sponge-like soil.

Soil in handsSponge-like soil soaks up water quickly, and remains moist and fluffy for long periods of time. This allows for fast drainage when it rains, and it keeps the moisture there for the plants to use long after it’s stopped raining.

Building a soil sponge is simple. It involves adding organic matter in the form of compost and mulch, reducing compaction, and avoiding fertilizers and herbicides.

To do this, place a 2”-3” layer of compost over your entire rain garden before planting. After you are done planting, add a 4” layer of bark or wood chip type mulch. The compost and mulch break down into the soil to loosen it and increase its water holding capacity.

Reducing compaction is done by avoiding heavy traffic or machinery on the rain garden. This helps keep space between soil particles which increases its absorption capabilities.

Finally, by not using fertilizers and herbicides, you keep the microorganisms in the soil healthy. These microorganisms feed the plants, help to aerate the soil, and break down pollutants.


Although it’s not required, many people enjoy the look of having rocks in their swale. These can be boulders, river rock, or smaller gravel (or all of the above!). This can give the rain garden a natural feel that mimics a creek or rocky basin.Building a swale for a rain garden in San Luis Obispo, CA

The rocks also help to stabilize the soil which reduces erosion as water flows through the swale.

When adding smaller rock, try to make a layer that is a few inches thick. When placing boulders, dig them into the soil slightly to make sure they stay in place. Think creatively about your rock use. This is an excellent opportunity to create an artistic and eye catching focal point for your new rain garden.


It’s finally time to plant, but before you start digging holes, you will want to pull out your rain garden plan and make a planting design. Mark down which plants you will be placing where. Ensure proper spacing between plants and that they will all look good together once mature.

Placing plants for a rainwater catchment landscape in San Luis Obispo, CAFor a rain garden to be most effective, it is recommended that you use plants that are well adapted to our climate. These plants are accustomed to our seasonal weather patterns, which means they will require less supplemental water and care. In the natural world, they get the majority of their water during the winter months. That water is what carries them through the dry season. In your rain garden, they will get extra water during the rainy season and this will lower or even eliminate their irrigation requirements in the summer.

After you have decided on what plants you will use and where you will place them, you are ready to put them in the ground. Dig your holes to be 1.5 times the diameter of the pot. After you place the plant in the hole, mix in some of the compost with the native soil to bury the root ball of the plant. With most plants, it is preferable to leave the very top of the root ball just above the soil level. Transplant them all carefully and water them in well to avoid transplant shock.


With a rain garden, the idea is to maximize the rainfall in order to give the plants the majority of their water needs for the year during the rainy season. However, you may still need or want to have supplemental irrigation.

At first, you will need to use irrigation to get your plants established. It can take a couple years before plants have truly established a root system that can enable them to sustain themselves on the least amount of water. In the meantime, they will need a little bit of help getting them through the summer months.

Even after your plants are established, you may want to give some of them just a bit of extra irrigation in the summer months. It doesn’t have to be much, depending on the plants it could even be a watering every few weeks. This will help them stay green and bloom longer.drip emitter

For watering, it is recommended that you use drip irrigation and an automatic irrigation controller. Drip irrigation uses emitters that deliver a specific quantity of water directly to the base of the plant. This minimizes evaporation, excess watering, and run off.

After you install the drip irrigation, put the valve that controls the watering in that space on an automatic controller. These are typically mounted in the garage or on the side of the house. With an automatic controller, you can set an exact watering schedule which will enable you to effortlessly give the plants the precise amount of water they need.


The last step is to apply a thick layer of bark or wood chip type mulch over the planting areas. There are many styles and colors of mulch availableRainwater hervesting and drought-tolerant landscaping in San Luis Obispo, CA, but it is recommended that you use either Petite Walk-On bark or fresh wood chips.

By using a 3”- 4” layer of mulch, you will help to retain soil moisture, reduce the number of weeds that can grow, and provide organic matter that will break down into the soil.

Mulch can be ordered from a tree service company or from a local landscape materials supplier. Spread it out evenly around your plants, being careful not to smother the plants or cover up woody stems and trunks.


That’s about it! Now you can enjoy the beauty of your new garden while knowing that you are doing your part to conserve water and help the environment. 3D Ebook Cover

If you found this article helpful, you will love our free guide “Rainwater Harvesting in SLO County”. It’s full of information you will need in order to have a successful rainwater harvesting project.

Get it in your inbox instantly by heading to the download page HERE.

As always, if you have any follow up questions or need help installing your rain garden, please feel free to contact us. We’d love to hear from you!