Image is not available
Image is not available

What is a rain garden?

A rain garden is a stormwater feature, usually a shallow depression with plants, that captures stormwater and infiltrates it into the ground.

It's Creek Smart® and an attractive feature of any property!

Rain Gardens

Photo Credit: Tip of the Mitt Watershed Council

Image is not available
Image is not available

There are three basic steps in constructing a rain garden:

Step 1: Find a site for your garden
Step 2: Design your garden
Step 3: Properly install your garden

Let's begin with Step 1.

Rain Gardens

Photo Credit: Mitch Woodward, NC Cooperative Extension

Image is not available
Image is not available

Because a rain garden will be designed to overflow in the event of extreme rainfall, it should always be placed downhill of any existing structures on your property.

There are several additional constraints on where a rain garden should go.

Rain Gardens - Step 1: Finding a Site

Photo Credit: Milwaukee Metropolitan Sewerage District

Image is not available
Image is not available

Rain Garden Constraints:

1. Must be at least 10 ft downhill from any crawl spaces or basements
2. Must be at least 10 ft downhill from any existing wellheads
3. Must be 25 ft downhill from any existing septic systems

Rain Gardens - Step 1: Finding a Site

Photo Credit: EPA

Image is not available
Image is not available

And, as with any garden, it should be located in full or partial sun for happy and healthy plants!

Rain Gardens - Step 1: Finding a Site

Photo Credit: Public Domain Image

Image is not available
Image is not available

Now that we've got the constraints out of the way, we need to find the 'pinch point' - that is, the place on your property where your water naturally flows. To capture as much water as possible, it's best to place the rain garden at a 'pinch point'.

To find your 'pinch point', simply watch your property during the next storm and watch where the water flows. This will give you a good idea of where your rain garden should go!

Rain Gardens - Step 1: Finding a Site

Photo Credit: Mitch Woodward, NC Cooperative Extension

Image is not available
Image is not available

In a storm, the majority of runoff comes from impervious surfaces – such as your driveway, or the road. Any downspouts that deliver water directly to one of these surfaces are considered ‘priority downspouts’, and should be re-routed into your rain garden if possible.

Rain Gardens - Step 1: Finding a Site

Photo Credit: Mitch Woodward, NC Cooperative Extension

Image is not available

Other downspouts can be used as well, but a downspout that already discharges onto a lawn is considered a Disconnected Downspout, and is already a Creek Smart® feature!

Rain Gardens - Step 1: Finding a Site

Image is not available
Image is not available

Now that you’ve identified a pinch point and your priority downspouts, you may have several spots where a rain garden can go.

To choose the best location, we need to do a infiltration test by digging a ‘test hole’.

Rain Gardens - Step 1: Finding a Site

Image is not available
Image is not available
Image is not available

Before you dig the test hole, or perform any rain garden excavation on your property, please call NC 811 – it’s a free service that will mark utilities on your property!

Any spot where gas or electric utilities cross your property is unsuitable for a rain garden.

Rain Gardens - Step 1: Finding a Site

Photo Credit: NC 811

Image is not available
Image is not available

Perform an infiltration test by digging a test hole at least 12 inches deep. Fill it with water, and mark how long the hole takes to fully drain.

It’s best to dig 2-3 test holes near your potential rain garden sites.

Rain Gardens - Step 1: Finding a Site

Photo Credit: ©2012, Five Counties Salmonid Conservation Program and Green Girl Land Development Solutions

Image is not available
Image is not available

Perform an infiltration test by digging a test hole at least 12 inches deep. Fill it with water, and mark how long the hole takes to fully drain.

It’s best to dig 2-3 test holes near your potential rain garden sites.

Rain Gardens - Step 1: Finding a Site

Photo Credit: ©2012, Five Counties Salmonid Conservation Program and Green Girl Land Development Solutions

Image is not available
Image is not available

After your test hole has fully drained, fill it with water 2-3 additional times, and again mark how long each test hole takes to drain.

This will simulate saturated soil, and let you know how long your rain garden will take to drain in a worst case scenario.

Rain Gardens - Step 1: Finding a Site

Photo Credit: Mitch Woodward, NC Cooperative Extension

Image is not available
Image is not available

Once all the test holes are completed, choose the one that infiltrates best. This should be the best spot to put your rain garden.

Note the longest amount of time it took the test hole to drain at that spot.

Rain Gardens - Step 1: Finding a Site

Photo Credit: Durham Go Maps

Image is not available
Image is not available

If your test hole drained within 12 hours, you should design a ‘Quick-draining Rain Garden’.

If it takes between 12-72 hours, you should design a ‘Standard Rain Garden’ .

And if it takes longer than 72 hours, consider a ‘Wetland Garden’.

Rain Gardens - Step 1: Finding a Site

Photo Credit: Durham Go Maps

Image is not available
Image is not available

Now that we have a site for your garden, and we know how long it takes to drain, we can move on to Step 2: Designing your Garden!

Rain Gardens - Step 1: Finding a Site

Photo Credit: Low Impact Development Center, Inc

Image is not available
Image is not available

To design your rain garden, we must first determine what size it should be.

Rain Gardens - Step 2: Designing Your Garden

Photo Credit: Durham Go Maps

Image is not available
Image is not available

The size of your rain garden will be determined by the combination of impervious area (roof, driveway, etc.) and pervious area (lawn) that feed into your garden.

Note the downspouts you chose earlier. Measure the roof area that feeds into these downspouts.

Rain Gardens - Step 2: Designing Your Garden

Photo Credit: Durham Go Maps

Image is not available
Image is not available

To measure the area, consider using a tool like Durham GoMaps.

Simply search for your address and use the Area Measurement tool to determine the square footage of your roof, lawn, and/or driveway that will be feeding into the garden.

Rain Gardens - Step 2: Designing Your Garden

Photo Credit: Durham Go Maps

Image is not available
Image is not available

Now that you have a square footage for your pervious (lawn) and impervious (roof+driveway) surfaces, we are going to size the garden to handle a 1-inch rain storm – this will allow your garden to fully infiltrate 90% of the storms in a given year.

Rain Gardens - Step 2: Designing Your Garden

Photo Credit: Durham Go Maps

Image is not available
Image is not available

To size for a 1-inch storm, we use the 10-by-10 rule.

Take 10% of the impervious area you measured earlier, and add it to 1% of the pervious area.

A rain garden 10-inches deep at this new square footage can now handle a 1-inch rain storm!

Rain Gardens - Step 2: Designing Your Garden

Image is not available
Image is not available

For example, if your rain garden is capturing water from a 10’x30’ roof, a 12’x30’ driveway, and 20’x30’ of lawn you have:

660 sq ft of impervious surface

600 sq ft of pervious surface

Rain Gardens - Step 2: Designing Your Garden

Image is not available

We then take 10% of the pervious surface area (66 sq.ft.) and add that to 1% of the pervious surface area (6 sq.ft.) for a total of 72 sq.ft.

A rain garden with dimensions 7.2’x10’ dug 10 inches deep will now handle a 1-inch rain storm!

Rain Gardens - Step 2: Designing Your Garden

Calculate your rain garden size

Image is not available
Image is not available

Now we know where your rain garden is going and how big it will be.

It’s time to move on to
Step 3: installing your garden!

Rain Gardens - Step 2: Designing Your Garden

Image is not available
Image is not available
Image is not available

When installing a rain garden, the most important thing to keep in mind is where water will be entering the rain garden, and where water will be leaving the rain garden.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: Geauga SWCD

Image is not available
Image is not available

Water will most likely enter your rain garden from the downspouts you identified earlier.

It may require some trenching and corrugated pipe to re-route the downspouts to your garden area.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: Geauga SWCD

Image is not available
Image is not available

When trenching, remember to use a level – to avoid standing water in the pipes, the level of the pipes must always be traveling downhill.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available

As you reach the garden area, try and have the bottom of the pipes come in as high as possible with respect to the surrounding landscape – best case scenario, the bottom of the pipe is level with the top of the garden. This is called an inlet.

Place stones around the inlet to prevent erosion.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available

If you are building a rain garden on a slope, you may be required to build a berm on the back side of the garden. This will allow you to store as much water as possible in your garden.

If you build a berm, be sure to compact it using a tamper to avoid erosion and water loss!

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available
Image is not available

In the event of a major storm, the rain garden WILL overflow, and it’s important that we control where that water is leaving the garden.

This will be your outlet.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: Geauga SWCD

Image is not available

The most important thing to remember is that your outlet must be below the level of your inlet.

In doing so, water will be guaranteed to overflow from the garden where you want it to, and not back up into your pipes.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available

To prevent erosion, you must stabilize your outlet.

For the outlet, place a capstone at least 1ft wide to form a stable weir. On the back side, place small, loose rocks to channel the water.

These can be purchased at your local hardware store.

Rain Gardens - Step 3: Installing Your Garden

Image is not available
Image is not available

Now that your inlet and outlet are set, it’s time to make sure your garden is 10 inches deep across its entire area.

To do this, we will use a tool called a line level. You will also need two stakes and a strong string – all of which can be purchased at your local hardware store.

Rain Gardens - Step 3: Installing Your Garden

Image is not available
Image is not available
Image is not available

Place a stake at one end of the garden, tie the string around it with a strong knot, and run it to a stake at the other end.

Make sure the string is taut, and place the line level.

Adjust the string at either stake until the bubble reads level.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: Organic Transit

Image is not available
Image is not available

Next, take your tape measure and check the garden depth at various points along the garden.

A shorter depth means you need to dig that portion deeper.

A longer depth may require more soil.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: Organic Transit

Image is not available
Image is not available

Once your garden is level, it’s time to plant it!

Place the plants you chose in the design phase in the garden, but don’t plant them too deep. The next step will be adding mulch, and the top of the mulch should be no higher than the crown of the plants.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: Organic Transit

Image is not available
Image is not available

When mulching, it’s recommended that you use triple-shredded hardwood mulch.

This type of mulch tends to settle better and not float away and clog your outlet during major rain storms.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available

With the inlet, outlet, plants, and mulch, you have a working rain garden!

Keep in mind that there will be some additional maintenance required.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available

For example, it’s important to weed your garden every season.

This will keep your plants happy and healthy.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available

You may be required to water your garden, especially during the drier months.

If it hasn’t rained at least an inch in the last week, go ahead and water.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available
Image is not available

Every few years, it’s recommended that you replace the mulch in the garden.

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available

And finally, it’s very important that you NEVER fertilize your garden. Excess nutrients from fertilizer can cause algal blooms downstream!
Learn more about the problems with fertilizing or learn how to install a cistern!

Rain Gardens - Step 3: Installing Your Garden

Photo Credit: ECWA

Image is not available
Image is not available

Questions? Comments?
Share a photo of your project?

Rain Gardens

Photo Credit: Tip of the Mitt Watershed Council


ECWA has created a maintenance guide to help you get the most out of this Creek Smart® addition to your property.

View or download the

ArrowArrow
ArrowArrow
ArrowArrow
Slider