Meet your Chicagoland Stormwater Storage Requirements with a rain garden!

Determining your Rain Garden Dimensions and Plant Requirements

Design Cheat SheetThere is no one template for a rain garden design.

Any garden that is designed to detain rain water and then soak it up with thriving plants and native soils within 48-72 hours is an excellent rain garden design.

There can be large variations in the dimensions and plant selections of a successful rain garden, even in the exact same location.

All of the different variations can work as long as they understand, and design to, your water sources and site conditions.

To help with this design process, I have included a Design Cheat Sheet below that can be used to formulate your rain garden dimensions and plant choices.

You can download this Design Cheat Sheet here.

This Design Cheat Sheet should be used as a guide; it can be difficult to quantify all of the variables that are found at a site, and experience of the user should be factored into its results.

Below I have included some suggestions on how you might interpret and transfer the Water Source Occurrence, Soil Drainage, and Sun Light from the Site Evaluation to the Design Cheat Sheet, as well as an example situation where I go through using the form to find the basin and plant information.

Water Source Occurrence Examples

Rare –> Downspouts in the Desert, Potentially a Sump Pump in all locations
Intermittent –> Downspouts in the Midwest and Northeast, Potentially a Sump Pump in all locations
Consistent –> Downspouts in the Pacific Northwest, Potentially a Sump Pump in all locations

Soil Drainage

Fast –> Sandy Soils, Loamy Soils on a hill
Medium –> Sandy Soils in a low spot, Loamy Soils, Clayey Soils that are on a hill
Slow –> Loamy Soils in a low spot, Clayey Soils

Sun Light

Full –> 6+ Hours of Sun
Partial –> 3-6 Hours of Sun
Shade –> 0-3 Hours of Sun

How to use Step 3

On step 3 you will first want to transfer over the Design Volume and Available Surface Area from your site evaluation.

With this information, you can use the equation to test different surface areas, ponding depths, and plant water requirements.

Keep in mind you can also do multiple different ponding depth zones in your rain garden.

To do this, insert only the portion of the design volume you want for that ponding depth zone into the formula.

Just make sure that all the portions add up to your total Design Volume.

Example Scenario with Design Cheat Sheet

For our example we will be designing a rain garden in a Chicago suburb with clayey-loam soils. Our rain water will come from the runoff of the roof. Our rain garden is in a relatively flat area, not on the top or bottom of a hill, and gets sun from about 10am-5pm of sun each day in the summer. The Design Volume is 50 Cubic Feet, and our Available Surface Area is 300 Square Feet

For Step 1 we are looking at our Water Source Occurrence, and with our downspouts we are looking at an intermittent source which would mean we would start at MED on our “Plant Water Requirement List.”

Moving on to Step 2, with our clayey-loam soils and no real elevation changes around, we would most likely select “Slow” for our Soil Drainage, which would move us down one notch to MED-WET in the “Plant Water Requirement List.”

Continuing down to Sunlight, our location gets sun for 7 hours, so this would put us in the Full Sunlight category, which would move us back up a notch to MED on the “Plant Water Requirement List.”

At this point, let’s leave a tick next to where we are at on our “Plant Water Requirement List;” we will come back to this sometime during Step 3.

Next we move down to Step 3.

This step encompasses all of the variables we as designers have control over; Design Volume, Surface Area, Ponding Depth, and Plant Requirements.

The choices we make for one of these variables directly effects the others, and step 3 allows us to experiment with these interactions.

We can try multiple combinations to come up with different shapes, sizes, and looks to our rain garden, all of which will still function relatively the same.

But for our first time through Step 3, let’s start by transferring the “Available Surface Area” and “Design Volume” from your Site Evaluation Form; for us this is 300 Square Feet and 50 Cubic Feet.

Next you will want to start with your desired Surface Area for your rain garden. We have 300 Square Feet Available, let’s try that first

50/300 = .16

The Ponding Depth will be in a decimal, which can be matched up on the chart to the right of the formula.

.16 = 2”

This would mean we would have a rain garden that is 2” deep.

This may be good for some locations that drain very slow with constant water sources, but for our situation, we can make the surface area of the garden a little smaller by making the ponding depth a little deeper.

So now let’s try 100 square feet as our Surface Area for our rain garden.

50/100 = .5

.5 = 6”

For an intermittent water source, 6” would be a more appropriate depth.

So now we have our Basin Dimensions of 100 square feet by 6” deep.

We can also use the 6” Ponding Depth to make our final move up or down on the “Plant Water Requirement List.”

Because this 6” Ponding Depth falls in between 4” – 8”, we actually will not be moving up or down on the List, and will use our current location on the “Plant Water Requirement List” of to MED-WET.

So with this information, we will want to put in a rain garden that is roughly 100 square feet by 6” deep, with the plants in the basin requiring Full Sun, Med-Wet moisture, and clay soils.

Right Plant, Right Place

A common mistake made by inexperienced, and even experienced landscape designers, is trying to put a plant in a location that doesn’t suit the plants light, water, or soil requirements; I fell victim to this multiple times early in my career and, to be honest, still do every now and then.

This error is usually made in an attempt to create a certain look or effect with a specific plant.

I have begun to realize that almost any plant is beautiful that enjoys its location.

Conversely, a beautiful plant can become straggly, weak, non-blooming, and just plain lousy when put in a location with light, water, or soil conditions it does not like.

So as you complete your designs be sure to follow the old adage “Right Plant, Right Place.”

Desert Plants

Depending on your geographic location, you probably got a chuckle out of the “desert” option on the plant requirement list.

Well, check this shot out.

Arizona Rain Garden

This is a photo from the Musical Instrument Museum I took recently on a trip out to Scottsdale, Arizona.

This newly constructed site includes multiple rain gardens and bioswales that capture runoff from large expanses of parking lot and roof.

Interestingly, it was actually difficult to find someone who knew where I could find a rain garden out in Arizona.

After calling around for a couple of hours, I finally got a hold of someone who informed me that I was asking for the wrong thing.

I should be asking for a xeriscape garden.

For them rain is rare, about 9” on an average year, and it can sometimes come down many inches at once.

As you can see, their rain gardens, or xeriscape gardens, look very different than ours.

They can be very deep to capture as much rain as possible during a rain event, and they incorporate truly drought tolerant plants like cacti and palo verdes.

I like this photo because it helps to illustrate how important it is to understand the site conditions and design to them; as opposed to assuming that because it is called a rain garden you need to use water-loving plants all the time.

By designing to your site conditions, you can create a beautiful and sustainable rain garden that require minimal inputs to achieve.

So now that we have our rain garden basin figured out, let’s look at how we are going to get the rain water into this basin, also known as conveyance.

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