How big should your rain garden be?

Rain gardens typically range from 30-90 square meters (100 to 300 square feet). They can be smaller or larger depending on your resources. Even if you reduce the recommended size of your rain garden by 30%, you will still control 90% of the runoff.

If your rain garden site is within 9 meters (30 feet) of a downspout, most of the storm water will come from the roof so you will only need to calculate its area to obtain the capacity. The area of the roof is the same as the area of the first floor. If you do not know it, use a tape measurer to find the length and width of your house. The product of the two is your area. Now take a walk around your house, how many downspouts do you have? Most houses have 4, each taking care of 25% of the roof runoff. By the shape of the roof, estimate the percent of water your downspout is responsible for. Now multiply the percent of water flowing down the downspout by the roof area. That is your roof drainage area.

Example: You have a building that is 5 m long and 5 m wide and has 2 downspouts that share the load equally. The ground floor area is: 5 m x 5 m = 25 m². If your 2 downspouts are equal, one will take care of 50% of the water. Your drainage area will then be: 25 m² x 0.5 =12.5 m²

If the site is over 9 meters away from a downspout you will need to consider drainage from the lawn. From your rain garden site, look up towards the house and identify which parts of the lawn slope towards your garden. You will need to measure the length and width of the uphill lawn and multiply them to get the lawn area. Now you simply need to add that lawn area to the roof drainage area (calculated in the previous step).

Example: Your building is surrounded by a lawn that slopes towards your garden. It measures 2 m long and 3 m wide. 2 m x 3 m =6 m² as your lawn area. Your total drainage area will then be: 12.5 m² + 6 m² = 18.5 m².

Now you need use your slope, drainage area, and suggested depth to get the surface area of your garden. Use the table below to determine your size factor, then multiply it by your calculated drainage area.

Location	Depth	Sandy	Silty	Clay
Within 9.1 m
	8-13 cm	0.19	0.34	0.43
	15-18 cm	0.15	0.25	0.32
	20 cm	0.08	0.16	0.20
Beyond 9.1 m	All Depths	0.03	0.06	0.10

Example: You have a drainage area of 18.5 m²  sandy soil, and a depth of 8-13 cm. The surface area of your rain garden should then be: 18.5 m² x 0.19 = 3.5 m².

Once you calculate the size of your rain garden, you can divide it by the intended width to find its length. You will want the garden to be twice as long as it is wide, with a max width of 15 feet especially for slopes greater than 8%.

Slecting the Best Site for the Rain Garden: Slope and Soil

Before you get started on building your rain garden, you want to know the answer to two basic questions: "What's my slope?" and "What kind of soil do I have?".  This article will help you get to the bottom of these two questions.

What’s My Slope?

 To calculate the slope of the intended rain garden, pound one stake uphill of the site and the other at the downhill end approximately 5 m (15 feet) apart. Tie the string to the stakes and make sure it is horizontal with a carpenter’s levels. Once the string is properly secured and level, measure the length of the string between the two stakes and then the height of the string from the ground at the downhill stake.   The percent slope will be equal to:

Percent Slope = Height x 100

                         Length

Slope <4%, easiest to build 8-13 cm (3-5 inch) deep rain garden.

Slope 5-7%, try to build it 15-18 cm (6-7 inches) deep.

Slope 8-12%, build it 20 cm (8 inches) deep.

If the slope is greater than 12%, it would be easier to install a garden elsewhere.

How Do I Know What Kind of Soil I have?

Before you even think of putting a shovel in the ground, you also need to figure out what type of soil your rain garden site has. Sandy soils have the fastest infiltration; clay the slowest. Sandy soils are described as gritty and coarse, silty as smooth but not sticky, and Clay as sticky and clumpy. Clay garden must be bigger to handle infiltration and should have sandy pits installed in them to help infiltration. Here is an easy test to help you see if your rain garden will work well at your selected spot:

Soil test: Dig a hole 15 cm (6 inches) deep and fill with water and allow it to filter away. If it takes more than 24 hours to drain, the soil is more clay-based and is not good for a rain garden. You can mitigate the lack of drainage by adding sandy soil pockets during installation, but that takes time and money.

Planning the Rain Garden

The first thing you need to know is where the water goes. Where do the downspouts from your gutters point? Do you see small rivulets of water passing through the grass? Do they join the stream from the downspouts? Does the water pool anywhere? Where can you catch the most water?

This downspout allows stormwater to flow away from the property.

Once you have an idea of where the water flows, you can start eliminating areas around your house. Do not install a rain garden where the water naturally pools, that is where filtration is slowest; think about installing it where the rain garden can catch the water before it pools. You also want to avoid large tree roots. If there are branches overhead, there are roots beneath your feet.

Try to put the rain garden close to a downspout but it is important that you keep the rain garden at least 3 meters (or 10  feet) away from the building to protect the foundation of your house. It is also important to keep your rain garden at least 7.6 meters (or 25 feet) away from your septic system.

After that, it is more a matter of practicality. Installing a rain garden in a flatter portion of land will be easier to dig and level. Make it easier for yourself and do not decide to dig a rain garden where the slope is more than 12%, you will need retaining wall or a high berm to keep the water in and it will take a lot of earth to level out the garden.

In terms of design, crescent-, kidney-, or teardrop-shaped gardens work well. You will want the longer side of the garden to face upslope. Water will hit the berm at the back of that long line, and then flow back towards the garden like a receding wave at the beach. A larger surface will catch the most water possible.

Consider the height, bloom times, colours, and textures of the plants you are planning to use. By choosing plants that bloom at different times, your garden will have a longer flowering season. If you want a traditional, more formal look, try clumping individual species in groups of 3-6 plants and repeat the pattern throughout the garden. Try mixing sedges, rushes, and grasses with flowering species to keep the root network strong and keep some plants from overpowering each other. 

What is a Rain Garden and Why build one?

Rain gardens function as a stormwater absorption and filtration site.  They are beneficial to the local environment by improving water quality and absorbing excess runoff. Rain gardens function by directing stormwater into a plant-filled channel or shallow depression filled with a variety of native plants.  A berm or raised edge at the lower end of the rain garden prevents the water from spilling out.  This allows the water to slowly filter into the soil instead of running out into the streets and down storm drains.  Besides all these environmental benefits, they are beautiful and for the most part they water themselves!

What are the benefits of a rain garden?

• Absorbs 30% more water than a patch of grass.
• Helps reduce stormwater runoff.
• Enhances the aesthetic appeal of your property
• Provides habitat for birds and other critters.
• Recharges groundwater aquifers
• Improves the water quality of your community.
• The plants’ root networks will stabilize the soil on your property and reduce erosion.
• Plants provide a buffer to winds, acting as wind-break for your property.

These photos show the stages of construction of a rain garden.  This rain garden was built in September 2012 at the George Dixon Centre in the North End of Halifax.

For an overview of rain garden, please visit our previous article on rain gardens