2 In Do/ Grow

Designing and Building a Rain Garden

Since the removal of the epic cactus earlier this year, which we had to dig down quite a bit to get all the roots out, there has been a hole in the front yard. I had many people walk by and ask if we were building a pond. Nope! No pond, but that depression has stayed because I wanted to use it as the start of a rain garden! This month, now that the weather has cooled down, I’ve been working on the front landscape and building that rain garden.

What is a Rain Garden?

A rain garden is a special planting area that collects the rainwater off the roof (or driveway, patio or some other impermeable surface.) The downspout from the house gutter is directed into this planting bed. Unlike a built rain catchment system, in the rain garden, the water is absorbed into the soil.

Probably to the relief of my neighbors, a rain garden is NOT a pond. However, the rain garden IS slightly lower than the rest of the landscape. This depression allows the garden to hold the water for a short time, allowing the soil to absorb the water. This allows the water coming off my house roof to stay on my property, where it will recharge the groundwater.

The basin is planted with plants that can deal with fluctuating water levels. Some are planted at the bottom of the basin that can deal with standing water. Some are planted along the sides, that can deal with soggy soil, and some around the edge that can deal with more fluctuations. And if you’re in California like me, you’ll have periods of dry seasons, so they can also deal with seasonal drought.

I was going to draw a picture but found these excellent examples online.

source: https://www.thisoldhouse.com/ideas/using-rain-gardens-to-keep-waterways-pollution-free

source unknown

Why is a Rain Garden Important?

Normally, water rushing off a roof simply goes into the street gutter, which then drains to a river or creek. Along the way, it picks up fertilizers and pesticides some people use in their garden, pet waste, litter, and oil and gas residues off the roads. By that time it reaches a natural body of water, it’s a fast-moving deluge of dirty, likely-toxic water, which isn’t healthy for small creatures like birds, fish or any living creature.

Instead, a rain garden gives the water a place to go instead of just sending downstream. It acts as a man-made feature that would be present in a natural, healthy watershed. Most of the water running off the roof will infiltrate slowly, and cleanly, back into the groundwater.

How to Design and Build Your Own Rain Garden

Choosing a Location for your Rain Garden:

The first step is to determine where to put your rain garden. For me, it was an easy answer- where my cactus WAS, because I already had a depression dug out. That also happens to be right next to a downspout. For you, it just needs to be in an area where you have water draining. Perhaps you can dislocate the gutter from the pipe that goes to the street drain and direct to your garden, or perhaps it’s built right next to your driveway to receive the runoff you get when it rains.

You should avoid low-lying areas, which signals poor drainage, which you don’t want. You want the garden to drain well. You should not have standing water for more than 1-3 days. If you’re like me, and never lived somewhere will decent soil, you can amend the basin so it drains better (keep reading for more on that).

Rain Garden Shape:

The shape of your garden depends on what your site needs. If you’re putting it on the side of a driveway or a patio, a long, linear shape would make more sense than a circle. I chose the shape of a wonky- misshapen kidney bean. Thats the fancy design term for this shape, obviously! 🙂

It’s important to note that a rain garden basin doesn’t have straight sides, but sloping or tiered edges, giving you different spots that will be covered in different depths of water. This gives you different planting levels. You can also have “islands” in the middle to give it more shape and dimension.

There are whole equations to help you figure out how large your rain garden basin needs to be to capture the rain from an ‘average’ storm, which is 1″. Basically, if you have loamy soils, your basin should be 30% of the drainage area. For clay, your basin should be 60%. So if you’re draining a 1000 square foot roof and have non-shitty draining soil, you should have around a 300 square foot rain garden.

Because my yard is tiny, I don’t have the space to build a basin the ‘correct’ size. Instead, I just dug until I was tired, and I put an overflow pipe in so the ‘extra’ water can go towards the street.

Rain Garden Depth and How to Amend Soil:

A rain garden isn’t deep, just a 6″ or so depression in the landscape. You’re going to be digging the depression to create three distinct planting areas: the basin, the mid-slope, and the berm.

If you have good-draining soil, simply dig down a depression with sloping sides so the deepest spot is 6″. If you have shitty soil, like I do, you’ll need to dig deeper and amend. For this, you’ll want to dig down about a foot and a half. Then, you add new soil that is a mix of sand and loamy soil to fill about a foot. I used the sand that was from the cactus mound, a bit of the native soil, and compost.

Hard to see because of the shadows, but the amended soil is shaped into three “levels”.


In theory (cough, cough), your garden was built to absorb the water of a ‘typical’ storm. But in California, we often get much more than that 1″ at one time. You need to have a place for that water to go. For me, my downspout was previously just directed to land on the sidewalk, where it then flowed onto the street, and eventually made it’s way to a drain. So my overflow is simply a small depression from my rain garden basin back to the sidewalk. You could use an actual pipe to direct overflow or you could dig a swale to drain it back to the storm drain (or, an additional basin if you’ve got the space).

Next up- planting! Check out the post next week for what kind of plants I put in!

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  • Reply
    October 26, 2017 at 2:03 pm

    Very cool! I can’t wait to see what it looks like planted!

  • Reply
    October 27, 2017 at 8:31 pm

    Looks like it’s going to be really nice! I too look forward to seeing what it looks like all planted.

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