Build A Rain Garden: Channel Water Runoff & Add Color
by David Salman
Water is the most valuable resource on our planet. And yet, people tend to waste it and not treat it with care. I like to think that gardening gives us a greater appreciation of how plants respond to water and how to more efficiently use our precious water resources on our landscapes and gardens.
The use of rain gardens is an ecologically intentioned way to deal with water run-off from roofs and other hard surfaces. A rain garden is basically a depression in the ground where the run-off water is collected. It's planted with a colorful mix of durable perennial flowers, ornamental grasses and shrubs whose roots filter and clean the water that percolates down into the soil and recharges our aquifers.
Benefits Of Planting A Rain Garden
Rain gardens are a great low-tech way to take water that flows from our roofs and other hard surfaces and allows it to be slowly absorbed by the soil. This helps reduce the destructive flow of dirty storm water run-off into our streams and rivers, and creates beneficial habitat, especially when using native plants.
Basic Rain Garden Design Considerations
Water gardening is a very simple concept, but we do need to take into account a few factors that influence water absorption into the soil. Without turning the construction of a rain garden into a complicated engineering project, attention to some basic design concepts will ensure that your rain garden is successful.
Listed below are some considerations that are helpful to begin thinking about installation of a rain garden. (See "Rain Gardening References" at the end of this blog for more details and construction diagrams.)
Soil Type For Rain Gardens - Identify your soil type.
Plants need to be matched to the soil in the rain garden. In heavy clay soils, only clay-loving species will thrive. Likewise, in sandy soils, choose sand-loving plants.
Heavy clay soils drain slowly while sandy soils quickly absorb water. Thus rain gardens planted in clay will need to be larger in area (square footage) than those installed in faster draining loam and sandy soils. Otherwise, standing water can become a problem.
It's recommended that the soil where a rain garden is being considered be tested for drainage.
To do so, dig 12” wide by 12" deep hole and fill with water to saturate soil; refill in a few hours after water drains.
After filling the hole a second time, make note of how long the water takes to drain away. The water should completely drain within 12 hours. If not, this indicates poor drainage that will require moving the garden to another site to avoid long-term standing water.
Matching Surface Area To Your Rain Garden
Size the rain garden's area (sq. footage) to match the sq. footage of the roof area (or other hard surfaces) that will drain into it. Be sure the depression will hold more than 1" of rainfall so it will hold enough to hold several days of rain. Here are some useful metrics to help size the rain garden depression.
A 100 sq. ft. by 1-foot deep rain garden will hold approximately 750 gallons of water. With one inch of rain, a 1,000 sq. ft. roof will yield 622 gallons of water. So, this 10 ft. x 10 ft. x 1 ft deep depression will be large enough to accommodate 1 inch of rain runoff from a 1,000 sq. ft. of roof.
To calculate the amount of water that runs off your roof (or other surfaces) after an inch of rain, use the following formula: divide the square footage by 12, multiply by rain depth in inches then multiply by 7.48 (# of gallons in a sq. ft.)
A 1,000 sq. ft area that receives an inch of rain yields 623 gallons of water: 1000 /12) x 1 x 7.48
To calculate how much water runs off with more or less than 1 inch, use this modification. divide sq. ft. by 12. Multiply by the rain depth in inches. Then multiply by 7.48.
Start small. A 100 sq. ft. rain garden is a good size to start. As you gain experience, you'll be ready to construct additional ones to accommodate additional water run-off.
Sun Exposure For A Rain Garden
It's a good idea to locate your rain garden in full sun, whenever possible, as this greatly increases the number of suitable plants that can be used.
Rain Garden Soil Preparation
Here are a few basics to help you understand the process. (See " Rain Gardening References" at the end of the blog for much more detailed information.)
Excavate the rain garden to a depth of approximately 8 to 12 inches in depth. Be sure not to leave a vertical edge, but rather a gentle slope down to the bottom.
Take the soil from the depression and use it to mound up the edge and create a gentle slope that slants away from the edge.
Rototill the bottom of the depression to a depth of 8 to 10 inches to mix in a combination of compost (1/2 to 1 cu. yd. per 100 sq. ft.) and Yum Yum Mix (4 lbs. per 100 sq. ft.) before planting.
The Water Cleansing Pre-Planned Rain Garden
We have done the plant selection for you, by choosing good looking plants that do well in alternating wet and dry soil conditions. These gardens have been designed using North American native plants to maximize their resilience and value for habitat creation. They’re suitable for planting across much of the US (zones 4-7) and will color your landscape with shades of pink, yellow and white. There are two garden sizes (9 or 18 plants), each of which comes with premium plants, planting diagram, and plant care guide.
Many state agricultural extension offices will have information on rain gardens and rain garden construction. But if that's not available or you need additional information, listed below are some excellent references to help gardeners gain a more in-depth understanding of rain garden design and installation.