Thursday, June 5, 2014

Adventures in Building a Rain Garden

When I bought my house last year, I knew I wanted to incorporate green measures like xeriscaping and rain barrels. Little did I know how that would cascade into multiple projects! The week after I moved in, Austin experienced the historic Halloween Flood of 2013 and according to the closest rain gauge in the Flood Early Warning System, our area received 7-8 inches of rain in just a few hours.

I noticed that there were a couple of inches of water in the yard and really started to worry my new house was about to be flooded, argh! Thankfully that did not happen and it got me thinking more broadly about stormwater management. Not long after, I saw a flier in the library—Earth-wise Guide to Rain Gardens—and soon after read a great book explaining the ins and outs of stormwater management—Rain Gardens: Managing Water Sustainably in the Garden and Designed Landscape.

The roof on a small house like mine can fill four 55 gallon rain barrels with a little as 0.33" of rain. In order to even use the rain barrels, we had to install gutters. With only two rain barrels this meant we'd need to direct the rest of the water somewhere. Off the back porch, we installed another section of gutter to direct the downspout into the rain garden. The rain barrel overflow would be directed into the garden as well.

Project Description

Build a rain garden to capture and direct rainwater, including overflow from a rain barrel.

Time: It tooks us a combined total of about 15 days over several months (January to April)
Cost: $150 for gravel and plants

Completed rain garden in Austin, Texas

What is a rain garden?

According to Grow Green, a rain garden is a shallow, vegetated depression designed to absorb and filter runoff from hard (impervious) surfaces like roofs, sidewalks, and driveways. Rain gardens are usually planted with colorful native plants and grasses. They not only provide an attractive addition to the yard, but also help to conserve water and protect our water quality.

How does a rain garden help?

As areas become increasingly urbanized, native landscapes are replaced with impervious surfaces that prevent rainwater from soaking into the ground. Stormwater quickly runs off these hard surfaces, picking up pollutants from the land and carrying them to our creeks. This rapidly flowing water also increases the chances of flooding and erosion.

The goal of a rain garden is to keep water on the land. Rain gardens, with their shallow depressions, capture stormwater and provide for natural infiltration into the soil. This provides water for the plants and helps maintain a constant flow of water in our streams through groundwater. They also help filter out pollutants including fertilizers, pesticides, oil, heavy metals, and other chemicals that would otherwise reach our creeks through storm drains or drainage ditches. By reducing the quantity of water that runs off your property, rain gardens help lower the risk of flooding and erosion.

Supplies Needed for This Project

The lists below reflect what we used for our rain garden design.


  • Large, flat limestone rocks for the border
  • 5/8" river gravel (2 yards) for mulch
  • Native, drought-tolerant plants


  • Shovel
  • Wheelbarrow
  • Gas-powered tiller

Create a Rain Garden in Six Easy Steps

Well, that's what the rain garden fact sheet claims but depending on the size of the garden and the grade and soil composition of your yard, the time and effort needed will vary greatly. My goal for writing this blog post is to show a walkthrough of what we did following the Grow Green project outline.

Step 1: Find the Right Location

The right location for our garden was cut and dry. We have a relatively flat area behind the house where water tends to collect. It gets full sun during the summer and partial sun in the winter. It's also located such that we could easily direct the gutter on our porch and the rain barrel overflow into it.

An important step when determining your garden location is to call 811 and find out where any underground utilities like gas, water, electric, and cable lines are running on your property so that you don't accidently cut them. Call before you dig! It's a free service.

Electric and cable lines marked with spray paint

Step 2: Test the Soil

Most of the soil in this area is composed of caliche (a kind of clay) and rocks formed from limestone. If your yard is located on a bunch of shallow rock, a rain garden won't work for you. You need about 12" of loose soil for water to absorb effectively.

To test our soil, I dug two 6" wide by 6" deep pilot holes on either side of where I wanted the garden to be. You then fill the holes with water and see how long it takes the water to drain. Lucky for us, it took about two hours so our location was good to go.

Pilot hole to test drainage

Step 3: Calculate the Size and Shape of Your Garden

The fact sheet provides a formula for calculating the size, in square feet, a rain garden needs to be to hold 1" of runoff based on the measurements of the roof area. Our back roof and porch came out to 658 ft. sq. which calculates to a 110 ft. sq. rain garden. We experimented with the shape using spray paint (bad idea) and bricks (good idea) until we came up with a shape we liked that was roughly the right size—19 ft. x 6 ft. more or less.

We also made sure that the rain barrel would have ample space and that we would still be able to walk comfortably between the house and the garden and the porch and the garden (not visible to the right) and be able to get the lawn mower around everything.

Rain garden shape before construction

Step 4: Rain Garden Construction

Ooo boy, time to get dirty. We made this a lot harder than it needed to be because we wanted to transplant all the turf from the rain garden area to low spots around the back porch that had been eroded by rain over the years. This extended the project timeline by a lot. After we stopped digging and started using a tiller, things went very fast.

Digging out the garden takes a long time

Because you have to dig out 6" for the rain garden to be able to hold water, we had a lot of soil piled up and covered with a tarp—about 2.5 cubic yards worth! We put an ad on Craigslist for free fill dirt and have gotten rid of some of it, but keep in mind that you will have to find a place for all the dirt.

Tillers are worth every penny

While the soil was nice and loose from tilling, we had to grade the area. One side of the garden is about 6" higher than the other side because of a slight slope in the yard. The important thing is to get the floor of the garden level to 6" at the lowest point so that the water can spread out evenly. At this point, we also lined up the rain barrel overflow and gutter downspout to drain into the garden.

Step 5: Plant Selection and Installation

I selected native, drought-tolerant plants for the rain garden based on some tried and true favorites of mine: salvia bushes, lantana, red yucca, fountain grass, and feather grass. Some of these stay green year round and all are perennial. I used eight plants spaced out so that when they reach maturity, they have enough room. We threw down some leftover garden soil and crushed glass from our pallet planter project as a base.

Luna checks out the rain garden

Next, we filled the garden up with some 5/8" river gravel to act as a mulch. Originally we were going to use crushed glass, but I did not like the look. We went with the 5/8" instead of pea gravel to discourage the cats from using the garden as a litter box, which they did while it was dirt. After adding the gravel, we positioned the border rocks around the edges and filled the spaces with more gravel. We were lucky and got the border rocks for free from Craigslist, saving us hundreds.

Step 6: Maintenance

There's not much to it so far. We had our first big rain storm of nearly 4" over two days and the garden filled and drained twice. I've had to do a little weeding and edging but it has been nearly maintenance free. Through some questionable math, I determined it has about a 300 gallon capacity.

Rain garden full after 1.5" of rain

That's it! For more information, check out the City of Austin's rain garden site. If you're in Austin, you can also check out installations of small-scale green infrastructure around the city. I'll end this post with a short video of our rain garden at work during a storm.