Planting with a Purpose – Planning a Rain Garden

by Cassandra McCullersAugust 9, 2016

When it comes time to think about and design your landscape, one of the more popular trends these days is to develop and nurture a beautiful rain garden. Rain gardens are more than just filler for an empty yard – they can be a quiet refuge after a busy day at work, they can be a place of exploration and wonder for young children, and they can make a meaningful statement about living an environmentally friendly life.

Rain gardens are shallow depressions in the ground planted with carefully chosen vegetation, that trap and filter storm water. Rain gardens help reduce storm runoff dramatically, and they can reduce the need for irrigation in some areas. Once fully established, rain gardens usually require less maintenance than traditional flower beds. Depending on where you live, they might rarely require watering. Rain gardens with adult plants will need to be weeded only infrequently.
Beautiful cosmos flower with rain
Worried about mosquitoes? A properly designed rain garden won’t serve as a breeding ground – the water drains away too quickly for mosquito eggs. In some areas, rain gardens can actually help reduce mosquito numbers, by improving drainage and reducing the amount of water that can pool in an area. A rain garden can also attract dragonflies, especially if near a breeding ground. Dragonflies absolutely love mosquitoes – love eating them, that is!

Some counties and municipalities have programs in place to encourage installation of rain gardens and can help with the design. Some universities and organizations also have lists of plants that are ideal for rain gardens in their area.

Rain gardens aren’t suitable for everyone. Areas with less than 10 inches/year of precipitation likely won’t be able to maintain one. Soils that are almost entirely clay might not drain well enough. Rural areas that lack large swaths of impermeable surfaces like roads, parking lots, and roofs might have no need. Properties on very steep slopes may likely not be suitable.

Planning Your Rain Garden

Image of soil after a rain.
In most areas, the best time of year to plant your garden is in the Spring, but pay attention to the specific requirements of each plant that you want to use.

Always call 811 before digging. You will need to check the location of any utilities as well as local regulations for excavating. Most states also require you to give advanced warning before any digging project.

Plant at least 10 feet away from the edge of any buildings, since the draining water can damage foundations. If uphill of a septic system, field, or tank, plant at least 50 feet away, and 10 feet away if downhill of a septic system. The rain garden should be set at least 100 feet away from wells for drinking water. Do not dig or plant over utilities.

Think about planting where water will be able to naturally flow into your garden. A rain garden planted uphill of the drainage area won’t do much good, after all. However, avoid putting your garden where water tends to pool during a storm, since that area likely has poor drainage.

Avoid planting near or on steep slopes, cliffs, and bluffs, as water soaking into the ground can increase the risk of a landslide. If your rain garden is on or within 50 feet of a slope with a gradient greater than 10% to 12%, consult with a qualified geo-technical engineer. In wetter areas especially, it’s better to be on the safe side and aim for a shallower slope.

  • To measure the slope of your land, place two stakes fifteen feet apart such that one is directly uphill of the other.
  • Tie a string to the bottom of the uphill stake and run it to the downhill stake, tying it so that the string is level.
  • Measure the length in inches of the string between the uphill and downhill stake.
  • Measure the height in inches between where the string is tied to the downhill stake and the ground.
  • Divide the length by the height, then multiply by 100 to get the percent slope.

Determining Ponding Depth

Decide approximately how deep you want your ponding depth to be. The ponding depth is the vertical distance from the lowest point in the rain garden to the top of the rain garden. Washington State University recommends either a 6-inch or a 12-inch ponding depth, while the University of Wisconsin recommends a 3-inch to 8-inch ponding depth. A shallower rain garden is less effective and will need to cover more area, while a deeper rain garden may be more difficult to build. The ideal ponding depth also depends on the slope of the land, how well the soil drains, and how much rain you receive. If your soil drains poorly, especially in areas prone to heavy rain, consider keeping ponding depth at 6 inches or lower.

The University of Wisconsin recommends specific minimum depths for different slopes.

  • For a slope less than 4%, the minimum depth should be 3-5 inches
  • For a slope between 5% and 7%, the minimum depth should be 6-7 inches
  • For a slope between 8% and 12%, the minimum depth should be 8 inches

Be aware that if your area receives more than 30 inches of rain per year, or if your soil is particularly loose, you should restrict yourself to a slope of 10% or less.

Determining Soil Texture

To make sure that your garden will be a lush, beautiful, and vibrant feature of your home, you will want to test the soil for quality. Most soils come in three basic textures – sandy, silty, and clay soils. Sandy soils drain well, silty soils drain moderately to poorly, and clay soils drain poorly. It’s best to check your drainage rate during the wet season, to better mimic actual conditions. To test the texture and drainage rate of your soil, dig a hole where you intend to put your rain garden. A simple test can be performed with a shallow hole of just 6 inches.

  • As you dig, gather a clump of soil in your hand. If the soil is moist, try to squeeze it into a ball. If the soil falls apart easily and feels gritty, then the soil is likely sandy. If the soil is smooth but not sticky, then the soil is likely silty. If the soil is smooth and sticky and can easily form a ball, then the soil is likely clay-heavy.
  • For a 6-inch hole, fill it with water, preferably during a wet season, but when the ground is not water-logged. If the water doesn’t drain within three days, the site isn’t suitable for a rain garden.

Determining Your Garden’s Drainage Area

Once you know where you want your rain garden to be located, determine the drainage area, or the area that will drain into it. Rain gardens within thirty feet of a drainage spout or spout extension will more easily collect the water from that portion of the roof. Depending on roof design, most drainage spouts collect water from one-fourth to one-half of the roof.

  • Measure the length and width of any portion of your property that will be draining into the rain garden, including your lawn and driveway.
  • Multiple the length by the width to get the area of the ground draining into the garden
  • Find out the area of the part of the roof that will drain into the garden by measuring the length and width of that part of the house, taking into account roof overhangs. The slope of the roof doesn’t matter
  • Multiply the length by the width to get the roof area, and add it to the ground area you found two steps ago

Size Does Matter! – Determining Your Garden’s Size

rain barrel
Exactly how large and deep your rain garden will need to be will depend on the drainage area, the amount of water falling on your property, the soil texture, and the slope of the land. It won’t hurt if your rain garden is slightly too large, though you might need to water more often. Be aware that these are generalized guidelines. Different states have different regulations and recommendations for rain garden size and for how rigorous testing needs to be.

Finally, choose a shape that fits your needs and aesthetic. Kidney bean, crescent, and tear-drop shaped rain gardens are popular choices. Rain gardens generally should be longer than they are wide, with the longer side perpendicular to the slope. Wider gardens are more difficult to keep level, especially in areas with steeper slopes.


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About The Author
Cassandra McCullers
Cassandra is a writer with a background in engineering, enjoying the rural life in the Virginian Appalachians. When not working, she enjoys writing fiction, running a blog, camping, working in the garden, and tending to her flock of chickens! In addition to writing, she has a passion for art and graphic design. Her interests include disaster preparedness, homesteading, landscaping, cooking with natural ingredients, history, and animal husbandry.

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