The roots of building a rain garden
BY Christine Bowen
ON July 2, 2018
Want to help manage run-off? You can! It’s as easy as making a garden. A Rain Garden to be precise. A rain garden is a depression in the ground designed to catch run-off and filled with native growing plants that don’t mind occasional dry and flooding spells. Rain gardens have many benefits, including: providing habitat for wildlife (like caterpillars), slowing run-off to prevent sedimentation and erosion, removing pollutants (natural filtration due to bacteria in the soil), and most obviously, they are beautiful.

We have had a lot of experience building rain gardens! Thanks to an Ontario Trillium Grant and our partnership with Green Venture, since 2016 we have been working to construct 20 rain gardens at local elementary schools by the end of the 2018! More than halfway through, we know how to build a garden and wanted to share our knowledge.
Determining site and size
To start building your rain garden, you need to select a site. First, determine where the run-off flows. You want to pick a spot that water is going to flow into. If building near a retaining wall, check its stability as water saturation in the soil can cause slippage. Lastly, don’t build on top of utility lines or septic tank fields. Also avoid building a garden under an existing tree as you may damage its roots. But feel free to plant a tree, like an oak or willow, in your rain garden!
Once you have a site, you will need to perform a percolation test. To do so, start by digging a hole, 8 inches in diameter and 8 inches deep, in the proposed area. Clean out loose soil from the hole and fill to top with water. Allow water in the hole to soak into soil for 1-2 hours so that it is saturated. To perform the test, refill the hole so that water is 1 inch from the top. Mark that water level (sticks work well) and record the time. Measure how far the water drops after a period of time. Water will drop at different rates depending on your soil type, so depending on your soil type you will want to check the water level at different time intervals. If you soil is sandier, you will want to check every 15 minutes, where if it is more clay based, check every hour. Based on the infiltration rate, calculate how much would infiltrate in 24 hours; that will be your garden depth (to a maximum of 12 inches)! For example, if 1 inch percolated in 4 hours, I would expect 6 inches of water to be absorbed in one day. I would make my garden 6 inches deep.
Once you know your depth, you need to determine how big you want to make your garden. The size of your garden is dependent on how much water you want to collect. Remember that any size does help, for example, say you want to collect the first inch of water that falls on your shed. Your shed measures 18 ft x 20 ft for an area of 360 sq feet. Assuming 6 inches can be absorbed per day (from before), your garden needs to be 1/6 the size of roof = 60 sq feet = 6 x 10 ft garden. If I wanted to collect the first two inches instead, my garden would be 2/6 the size of the shed roof, remembering that the 6 is my percolation rate determined before.
Building and planting the rain garden
Start by removing sod and digging out the dirt. You will want to dig 4-5 inches deeper then garden depth you determine. Continuing our example, for a garden that is 6 inches deep I would dig 10 inches down. Mix some of the soil you removed with top soil or manure. Put that loosened, nutrient-rich mixture back in the hole until you reach your desired depth. You will have to create a berm, as you want the top of the rain garden to be level.

For planting, try to use plugs. The tend to fare better as they are less likely to have transplant shock. If you can, soak the plants before planting them. When picking your plants, there are many considerations be made:
  • Moisture tolerance: Plants that like average to dry soil go on the edges of the rain garden, where plants that like wet soil go in the middle. If your garden is less than 8 inches deep, plants that have an average moisture tolerance can also go in the middle.
  • Sun preference: Is your garden going to be in the sun or shade?
  • Salt tolerance: If your garden is off a driveway, you will want salt tolerant species like honey locust or birch trees!
  • Seasonal interest: You may want to create winter seasonal interest, on top of the typical summer interest. Try planting junipers or red twigged dogwoods. 
Some of the common plants we use include: Big blue stem (heat and salt tolerant), Russian sage (easy maintenance), Wild geranium (recognized by many and thus signifies that your rain garden was done on purpose), Coneflower (great for butterflies), Switch grass (great height and deep roots), and Prairie smoke (provides visual interest). Ultimately though, it is up to you. What do you prefer?
Finishing and maintenance
Once you have finished with planting, you may want to mulch your garden. When filling with mulch, it should be filled to a maximum of 1 inch from the top. We recommend mulching with hardwood as it’s less likely to float off, blow away and lasts longer. Avoid mulching right next to plant as when it moves it can scratch the stem. You can actually use the trays your plants came in, placed upside down over plants while mulching, to protect them.
For the first year, your garden will need weekly watering. It should be getting about 1 inch of water a week. If it gets too much water, you may have to cut an outlet in your berm to avoid drowning plants.
After the first year, you get to enjoy a beautiful garden with minimal work! Just water a little bit in time of severe drought, weed, and split the plants as they become too large. Sit back and relax, watching the visiting butterflies and knowing you made a difference in how our city manages storm water.
Author Bio - Christine Bowen
Program Coordinator for Bay Area Restoration Council
Program Coordinator for Bay Area Restoration Council

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