Building a Rain Garden

Michael Judd
Tuesday, 25th February 2014

Michael Judd shares how he created a permaculture rain garden that enabled a home-bound retired couple to once again enjoy their garden.

Sandi Gill-Smith is a retired scientist who lives in the Washington D.C. area with her ailing husband Ivan. As he slowly loses his mobility, the couple continues to become more home-bound. She commissioned Ecologia Design, founded by author Michael Judd, to convert her yard to an edible one that would lure them outside with its beauty and bounty.

The site's first challenge was finding a way to harvest the massive amount of water than runs off a neighbor's yard and funnels along their house and driveway to the road. To combine function and form, Ecologia installed a french drain that collects the water and sends it to a three-tiered rain garden. Michael centered each rain garden bed with edible beach plum trees accustomed to fast draining soils, while Sandi planted up guilds of native companions that absolutely flourish with the harvested water.

The projects grew from there – terraces, swales on contour, a hardy kiwi arbor, herb spiral, and pond all blossomed in her yard. And with each addition, she became increasingly involved right outside her door. Ecologia's latest installation is a wheelchair-accessible flagstone path that winds through the garden. Now Sandi can push Ivan through the garden to enjoy the fruits and flowers. Sandi's garden is bringing an abundance of new found energy and sunshine back to her life. 

Building a rain garden

Rain gardens basically are upside-down baseball mitts in the ground, water-sinking depressions that come in many shapes and forms. They are simple imitations of how a forest floor absorbs and sinks rainwater through porous soil. Since society has chopped down our forests and erected suburbia in its place, it's now our role to sink the rain. Lets explore how it's done with function and style. 

During a good rain, our homescapes are hit with thousands of gallons of water that rush downstream lickity split. Maybe you have a rain barrel or two that catch a few drops, but most of it is going down the gutter across the landscape and into the watershed, picking up all the funky funk on the way. Hopefully, you are catching some of it with swales on contour, but by adding rain gardens to your landscape you increase your site’s multifunctional design while stewarding our watersheds.

Usually rain gardens are sized for the amount of runoff from a site, but you can start small, sink what you can, and build in an overflow. Perhaps you'll decide to create a series of small rain gardens overtime and assure every drop of water is staying on your site. The only rule I will give is to start at least 10 feet from your house foundation, unless you are wanting to grow mushrooms in your basement! 

Lack of water-harvesting design loses valuable water and contributes to unfiltered runoff.

Digging it Out

Smaller rain gardens can be dug by hand, such as the one below, which is approximately 8-by-6-feet-by-24 inches deep. If you go any bigger than that, you’d best be eating your beans or renting a mini-excavator. Yes, you can rent machinery with nothing other than a credit card, and they are fun. The mini-excavator can fit through a 50-inch-wide opening and sneak into tight spaces.They cost around $150 a day, and one day is plenty of time to dig out a small rain garden or two. The three-tiered rain garden and French drain shown at the very bottom, was dug out with a mini excavator in just one day. Regardless of width, deeper is usually better with 2 feet deep an average for home scale rain gardens. Leave 6 to 12 inches from ground level down to the soil mix; this is called the pooling area, and it allows water to accumulate and sink. If you build the rain garden and fill it to the surface with the rain garden soil mix, it will only harvest what falls on it and runs across it.

Note: Remember to call your utility company before digging. Most will come out for free and mark where you have electric, gas, and water lines running on your property.

There are equations for factoring rain water runoff – volume relative to rain garden size – but more often than not, residential rain gardens are factored by the available area, budget, and interest of the owner. This is totally fine. A rain garden does not need to capture the full amount of water running off your property; any water capture is a benefit. Just plan where the overflow will go.

Small teardrop rain garden excavation by hand harvests a considerable amount of runoff.

This small rain garden at the head of an edible woodland garden I designed for Volt restaurant sinks the runoff from the parking lot and overflows into the swales behind.

Soil Mix Recipe

There are two approaches to creating your rain garden soil mix: (1) either mix sand and compost into your existing soil or (2) remove the existing soil altogether and replace it with a pre-made soil, sand, and compost mix from a local landscape supplier. Both are called “engineered” soil, but that’s just fancy jargon. They are simply fast-draining soil mixes that imitate the forest floor. Depending on the site, there are pros and cons to both. I prefer to excavate the soil, using the removed sod and soil on my hugelkultur beds or berming it around the rain garden to add to the rainwater capture. Then I order a soil that I know has been well and evenly mixed.

Whether you mix it yourself or order it to be mixed and delivered, your goal is to have a well-draining soil. There are many engineered mixes out there; most are designed to very quickly drain out huge amounts of water and be planted with super drought-tolerant plants because the mix is mostly sand. This can be played with and tailored to the plants you want to grow. I like my rain gardens to pump fruit and other lush plants while sinking the runoff, so I balance my soil mixes to both drain and hold some moisture. My favorite combo to date is 50% sand, 25% compost, and 25% top soil.

Even if your local landscape material supplier has never heard of a rain garden or soil mix for one, they are usually happy to custom mix whatever you want. Just ask for the above ratio well-mixed by the cubic yard. A smaller rain garden, such as the one shown just above, used approximately 2 yards of mixed soil. Remember to leave the soil surface a good 6 inches below the surrounding ground level and place rock or stones where the water is flowing in and out.

This illustration shows the minimum recommended depths for a rain garden. The 18-inch depth is where the soil mix sits and the top 6-inch drop for water to temporarily pool and sink. Small stones placed at inflow and outflow to avoid erosion. Larger stones can make a nice bed edge and safety from the slight dropoff. Total depth is 24 inches.

The client on the left, ninja bog gardener, Sandi Gill-Smith, had a home built next to her that thoughtfully drained all their water to her side yard. She also had considerable runoff coming from her roof drains and back yard and was dealing with water sitting alongside the house. See how we harvested all the water on below. 

To capture the large volume of runoff, we designed a three-tiered rain garden that perfectly fit the slope, allowing the water to overflow from one garden to the next, assuring not one drop left the site. Note the French drain coming in from the back yard and moving the stagnant water along the house and leading it into the first bay. The second image on the left shows the pre-mixed rain garden mix. The image below is the completed look with pea gravel surrounding for low maintenance and cut wall stone laid around the edges to help mask the bed edges where the soil drops a good 6 inches once settled. The picture at the very top of the page is the same design in only its second year of planting! 




The irony of a rain garden is that it is more often than not very dry, thanks to its fast draining character. Unfortunately most rain gardens are only planted up with a handful of drought-tolerant natives when there are loads of function and harvest to be had from all the water capture and loose soil. Some of my rain garden favorites, such as Beach Plum, Juneberry, Elderberry, Aronia, and low bush Blueberry, thrive in the loose and occasionally wet/dry soil. Other beautiful and functional plants to try are Swamp Milk Weed (more attractive than it sounds), Echinacea, Hibuscus moscheutos (edible petals), Winterberry, Spicebush, and Bee Balm to name just a few. For a low-care rain garden, native clump grasses will fill the niche. To balance the periods between rains in the first year, I mulch well and set up a simple drip irrigation or sprinkler as backup.

Michael Judd's new book Edible Landscaping with a Permaculture Twist, features several of the projects completed at Sandi's home. The above is an excerpt on rain gardens, with Sandi's three-tiered rain garden as a prime example.


Brad Lancaster’s Rainwater Harvesting for Dry Lands and Beyond volumes 1 and 2 are fantastically written and illustrated. Volume 2 is the real nitty gritty on swale and rain garden design and building.

Permaculture: A Designer's Manual by Bill Mollison and David Holmgren is the bible on overall permaculture design but also detailed in earthworks, swales, rain gardens, etc.

Most states have rain garden publications and non-profits dedicated to helping you get started; many even offer financial support toward your project. See what is going on locally, hopefully you’ll be surprised.‘Greening the Desert’ YouTube video by Geoff Lawton is an excellent short video o nthe regenerative benefits of swaling.

Michael Judd has worked with agro-ecological and whole system design throughout the Americas for the last 20 years focusing on applying permaculture and ecological design to increase local food security and community health in both tropical and temperate growing regions. The founder of both Ecologia, LLC, Edible & Ecological Landscape Design and Project Bona Fide, an international non-profit supporting agro-ecology research. 

Further resources

Edible Landscaping: With a Permaculture Twist by Michael Judd is available from our Green Shopping site for just £15.99 (for European customers). US customers can buy it from Chelsea Green for just $24.95

Read more about the experimentations at Project Bona Fide

DIY rainwater fed toilet made easy

English drought in February - does permaculture offer any solutions?

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