Designing a Rain Garden 

Most yards, whether urban or rural, are high and dry – and for good reason. No one wants to walk around on a sloppy, muddy lawn each time it rains.

However, such lawns limit the species we can grow to those that don’t require a lot of water, and we have many beautiful native plants that actually appreciate having their “feet wet”, at least occasionally.  At the same time, our rooftops collect gallons of water every time it rains, and often the water is diverted to sewers or ditches, or simply directed onto the lawn. A solution to both these problems is the rain garden. 

What is a Rain Garden 

In its most basic form, a rain garden is a depression in the lawn where water from downspouts is directed each time it rains and in which we plant some water-loving plants. But to be truly effective, the garden needs to be designed to match your soil type and the amount of rain collected from the roof, otherwise you could end up with a mud patch that never dries, or a garden that gets a flush of rain once in a while but soon dries up. And although there are plants that will survive either of these scenarios, a properly designed rain garden will offer a long term, beautiful garden solution. 

This month’s article is on how to design and build such a rain garden. 

The Four Main Considerations – Water, Soil, Calculating the Area, and Plants 

Part 1 – How Much Water Do I Have? 

Calculating how much water you can collect from your roof is actually pretty straightforward (more so when using metric measurements – but I’ll give the formulae for both). A good rain gauge and some long term records are best for accuracy, but you may be able to get enough information from the weather reports. But designing your garden properly requires knowing how much rain you’re apt to get in the growing season, and for this you need historical records.  

In Canada, you can look up historical normal rainfall through Environment Canada’s Website at https://climate.weather.gc.ca/climate_normals/station_select_1981_2010_e.html?searchType=stnProv&lstProvince=ON and simply select your city (or a nearby city, if it doesn’t have data for your town). 

In the USA, try https://www.currentresults.com/Weather/US/average-annual-precipitation-by-city.php  

Once you’ve found what your normal rainfall amounts are (by month is ideal), then you need to know how much of that rainfall your roof is collecting. (If you know of a source of the historical maximum rainfall amounts in various locations across the country, please let me know.) 

Calculating Potential Rain Capture 

Calculating the amount of rain your roof captures is the easy part (sort of). A simple method is to measure the dimensions of your house, then divide it into sections that are captured at each downspout.

For instance, my house is a small wartime bungalow, approximately 9.2 m X 8.6 m or (very roughly) about 80m² of roof area (about 30’ X 28’ = 840 sq ft). There are two downspouts, one on each side, each collecting from 40m² (420 sq ft) of surface area. (We’re not interested in the actual surface area, but instead just the area that intercepts rainfall.) If you don’t know the dimensions of your house, you can use Google maps (satellite view) to measure the area. 

If you’re using metric measurements, it’s simply the area in square meters multiplied by the rainfall amount in mm. This gives you the volume of rainfall in litres.  

If you’re still using imperial measurements, the formula is roof area in sq ft x the rainfall in inches x 0.623m, which provides you with the total (US) gallons of water (to convert to imperial gallons, multiply by 0.83). 

In the example of my house, a 25 mm (1”) rainfall event will collect: 

25 mm X 40 m² = 1000 L of water (or 1” X 420 sq ft X 0.623 = 261.7 US gal) 

(to convert to imperial gallons: 261.7 US gal X 0.83 = 217.2 Imp gal) 

Part 2 – Determine Your Soil Type 

You’ll also need to have an idea of your soil type. Clay soils will need a larger garden than sandy soils because clay does not drain as readily (we’re not building a pond – we actually want the water to soak in and drain away through the soil so that it doesn’t become a mosquito hatchery). There is an excellent article on how to determine your soil type at https://www.rhs.org.uk/soil-composts-mulches/soil-types. Another, slightly more detailed and technical article, can be found at https://www.gardeners.com/how-to/what-type-of-soil-do-you-have/9120.html.  

Once you know your soil type we can put all this information together to calculate the size of your rain garden.  

Building the Right Size Rain Garden in the Right Place 

If your garden is too small for the amount of rain you are apt to get, it will overflow into the lawn. If it’s too big, your plants may not get enough moisture.  

Also, your rain garden should be at least 3 m (10’) from the foundation. If it’s too close to the house, you could end up with a wet basement.  

Large rain gardens will take more time to maintain and money to complete but will be more effective for capturing runoff. However, relatively small rain gardens can still capture stormwater and improve water quality.  

Part 3 – Calculating Area 

Typically, residential rain gardens are between 10 to 30 square metres (100 to 300 square feet) and 10 to 20 centimetres (4 to 8 inches) deep. However, to maximize the efficiency of rainwater use, the surface area of your garden should be about 20% (for sandy soil) up to 45% (for clay soil) of the cumulative drainage area (that area of the roof feeding the downspout going to your rain garden). Note that these percentages are simply a guideline – in fact many sources indicate widely different values – anywhere from 10-20% (sand to clay) all the way to 20-65% (sandy to clayey). These percentages are referred to as the “Soil Factor”. 

Therefore, in my example above, if I am collecting rain from just one downspout and using my suggested Soil Factors, my rain garden should ideally be: 

Sandy soil:  40 m² X 0.20 = 8 m² (420 sq ft X 0.20 = 84 sq ft).  

Loam soil:  40 m² X 0.30 = 12 m² (420 sq ft X 0.30 = 126 sq ft). 

Heavy clay soil: 40 m² X 0.45 = 18 m² (420 sq ft X 0.45 = 189 sq ft). 

And if your rain garden is more than 10 m (30’) from your downspout, you will need to factor in the surface area of lawn and driveway, etc. that are also feeding into the garden.  

What if My Lawn has a Slope? 

It is important to keep the garden level for optimal filtration so if your lawn is sloped, you may need to do some “cut and fill” (removing soil at the high end and spreading it to the lower end – possibly adding a berm to retain water). 

The slope of your lawn should determine the depth of your rain garden and the slope can be determined by following these steps: 

1. Place one stake at the uphill end of the rain garden site and place the other stake at the downhill end. The stakes should be approximately  4.5 metres (15 feet) apart. 

2. Tie a string to the bottom of the uphill stake and run it to the downhill stake. 

3. Using a carpenter’s level, make the string horizontal and tie it to the downhill stake at that height. 

4. Measure the width between the two stakes. 

5. Measure the height on the downhill stake from the ground to the string. 

6. To find the lawn’s percent slope, divide the height by the width and multiply the result by 100. 

How Deep Should the Garden Depression Be? 

• If the slope is less than 4%, build a rain garden that is approximately 7 to 14 centimetres (3-6 inches) deep. 

• If the slope is 5-7%, build a rain garden that is approximately 15 to 18 centimetres (6-7 inches) deep. 

• If the slope is 8-12%, build a rain garden that is approximately 20 centimetres (8 inches) deep. 

Laying Out the Rain Garden 

Finally, you must determine the design of your rain garden. To do this, simply choose a garden width that best suits your property and landscaping. Next, divide the surface area of your garden by the garden width to determine the garden’s length. As for shape, crescent, kidney and teardrop shaped gardens can be quite attractive. 

If your proposed garden isn’t a simple rectangle, and you’re not sure how to determine the area, there’s an excellent tutorial at https://www.hollandbulbfarms.com/garden-area-calculator and a more technical approach at https://www.youtube.com/watch?v=Rogg7stSj6c . 

Getting Water Into and Out of the Garden 

Finally, some consideration about the inlet and outlet to your rain garden. If your downspout empties directly into the garden, you’re set at that end. If you don’t want the downspout running across your lawn, you may decide to make a dry streambed of river stone between the downspout and garden for an added feature, bury some Big-O pipe in the lawn, or simply create a grassed shallow channel. In this last case, it can be helpful to add a few stones at the garden entry area to slow the water flow for days of heavy rain to prevent erosion. 

In periods of torrential rain, the rain garden may overflow. If your lawn is sloped, you’ll want to create a low, stone-reinforced exit so that the berm doesn’t get washed away.  

A Note of Caution on Mulching Your Rain Garden 

Many of us like to use wood chips or other mulch on our flowerbeds. Keep in mind that after a heavy downpour, wood chips and many other mulches will likely float to the top and may overflow onto your lawn. If you need to use a mulch, consider stone or other heavy substance.  

Next Month – Part 4 – Plants for a Rain Garden