Rain Garden

A rain garden needs to be located in a low spot where stormwater runoff from the surrounding landscape and/or rooftops can drain to it. For the footprint of the rain garden, look for a relatively flat area, as the basin of the rain garden should be as flat as possible. If the slope is greater than 12%, find a different location or consider other stormwater management techniques.

To avoid the risk of seepage into basements, do not install rain gardens up gradient or adjacent to home/building foundations. Rain gardens must be located at least 10 feet away from a home/building foundation if it is down-gradient from the building. Rain gardens must be set back at least 10 feet from the property line of a down-gradient property and 5 feet from the property line of an up-gradient property. For lots with individual wells, locate the rain garden at least 50 feet away from the well.

Always call Miss Dig at 811 or submit and online request at missdig811.org before digging. Interference with underground utilities should be avoided whenever possible, particularly water, sewer, and gas lines. Excavation should be done very carefully to avoid damaging marked utilities. Also, be aware that Miss Dig may not always mark private cable, propane, electric, and similar lines, so some additional site work may be necessary to locate these.

Ideally, rain gardens should not be placed within the dripline of a tree because digging the rain garden may damage the tree roots.

The steps below should be followed to determine the rain garden size.

Step 1: Determine the slope of your lawn area. For slopes between 0 to 4%, it is easiest to build a 3 to 5 inches deep rain garden. For slopes between 5 to 7%, it is easiest to build a 6 to 7 inches deep rain garden. For slopes between 8 to 12%, it is easiest to build an 8 inches deep rain garden.

Step 2: Measure surfaces. Measure the area of the impervious surfaces that will flow to the rain garden. Typically, this is the area of the roof drainage being collected by a gutter downspout.

Step 3: Infiltration rate. Test the infiltration of the soil by digging a hole that is about 6 inches in diameter and 18 inches deep. Fill with water and wait until it is completely drained. (This should drain within 24 hours or this site may not be suitable for a rain garden without the complete removal of soil and/or a sub-drain installation). Fill the hole again with 12 inches of water and time how long it takes to soak into the soil. This is your infiltration rate. For help calculating the rate of infiltration, use the soil infiltration test worksheet.

Step 4: Using the tables below, take the infiltration rate from the infiltration test to determine the basic soil type and use the slope measurement to determine the sizing multiplier. To calculate the size of the rain garden, take the impervious area previously measured and multiply it by the sizing multiplier. This will be the approximate size of the garden.

If your hole does not drain within 48 hours, email rainsmart@oakgov.com before proceeding.

Where topography allows, it may be possible to incorporate a rain garden by constructing a berm to create the necessary depth, rather than excavating the entire depth. In these cases, it is very important that the berm be constructed properly to retain water. First, all vegetation must be cleared away before the berm is constructed (it cannot be placed on top of existing turf—the decaying turf will create a route for water flow, which will eventually erode the berm). The berm should then be constructed with high-clay-content soil (sandy soil will not hold water back). The berm soil should be compacted to ensure compaction. Plant the berm with native plants that are adapted to drier conditions, preferably a quick-rooting/spreading ground cover (like wild strawberry, for example).

Slopes extending from the existing grade to the surface of the rain garden are recommended to be no steeper than 1 foot of vertical drop for every 3 horizontal feet, referred to as a 3:1 slope.

Plan for a place to dispose of or use excess dirt when the rain garden is excavated. Some of the soil may be used to make a berm on the lower edge of the rain garden, but there will likely still be leftover soil. If some of the excavated soil is used on the lot, make sure to add some topsoil, as that will help grass or other vegetation to establish.

Pre-treatment refers to a device that will remove leaves, grit from the roof, and larger particles of dirt from the runoff before it enters the rain garden, helping to keep the rain garden from clogging. Pre-treatment can consist of gutter leaf screens (for rain gardens that treat roof runoff), a filter strip, which is a strip of grass about 5 feet wide around the rain garden, or a stone and/or a shallow grass channel that leads into the rain garden. Some form of pre-treatment must be included for every rain garden.

Ideally, runoff water should enter the rain garden over land, also known as sheet flow. However, inlets with concentrated flow (e.g., from a downspout or uphill area where water becomes more concentrated), are common. It may be necessary to incorporate a pad of cobble stone to reduce flow velocity (energy) at these inlets to the rain garden to prevent any erosion issues.

When the rain garden fills up during large rain events, it will need to have a place to safely overflow without causing erosion, damage to building foundations, or water problems for a neighboring property. A spillway can be created by adding a notch to the downslope berm for overflow water to go to a safe location. The spillway can be made of compacted soil topped with filter fabric and cobble stone, or similar material.

If an existing garden on the property has perennials that are adapted to both wet and dry conditions, they can be transplanted into the rain garden. If new plants are bought for the rain garden, it is recommended to use plants in pots because they have root systems. Seeds are not recommended for rain gardens because they will often get washed away. As a basic guide to calculate the number of plants needed, take the garden square footage and divide it by 2.25, placed about 18 inches apart.

Plants should be specified for different areas of the rain garden, as the bottom of the rain garden will be wetter than the side slopes or upper edges. Plants in the bottom of a rain garden must tolerate both periodic inundation and very dry conditions. The slopes and especially the upper edges of a rain garden are inundated rarely, so plants that can tolerate drier conditions should be specified for this zone.

The rain garden must consist of at least 75% native species. Lists of recommended native plants for dry and wet conditions are provided below.

Recommended native plants for the sides of the rain garden, where it is the driest:

• Aster novae-angliae—New England Aster
• Anemone canadensis—Canada Anemone
• Geranium maculatum fulgida—Wild Geranium
• Rudbeckia—Goldstrum Black-eyed Susan
• Physocarpus opulifolius—Ninebark
• Cercis anadensis—Redbud
• Fragaria virginiana—Wild Strawberry
• Liatris spicata—Kobold blazing star

Recommended native plants for the bottom of the rain garden, where it is the most wet:

• Echinacea purpurea—Purple Coneflower
• Panicum virgatum—Switch Grass
• Allium cernuum—Nodding Wild Onion
• Metteuccia struthiopteris—Ostrich Fern
• Potentilla fuiticosa—Goldfinger Potentilla
• Carex vulpinoidea—Fox Sedge
• Geranium maculatum—Red-Osier Dogwood
• Hibicus moscheutos—Rose Mallow
• Chelone lyonia—Pink Turtlehead
• Onoclea sensibilis—Sensitive Fern
• Lobelia siphilitica—Blue Lobelia
• Iris virginica—Blue Flag Iris

Invasive species must not be planted in the rain garden. Additional information about invasive species is available online at michigan.gov/invasives. Useful information about invasive plants and landscape alternatives is available at mipn.org.

Call Miss Dig at 811 at least 3 days before any excavation and check for private cable, propane, electric, and other lines. Make sure to have a plan and phone numbers of who to call in case there is any damage to utilities.

Mark the boundaries of the rain garden excavation area. Confirm the flow of water into the rain garden, checking the areas that contribute runoff to the practice. Tip: Garden hoses are a helpful tool to conceptualize the shape and size of your rain garden.

It is best to dig the rain garden when the weather is expected to be dry for several days. Cover stockpiles of soil temporarily with a tarp to prevent the material from washing away.

Elevations are key. Excavate the rain garden area and side slopes. The rain garden surface area should be as flat as possible while side slopes are recommended to be no steeper than 3:1.

Remove the grass in the defined space and 12 inches to 18 inches more than the border to minimize grass growth in your garden. Grass competes with the plants and creates more weeding work. Do this with a sod-cutter or kill the grass by laying down cardboard and mulch for at least two weeks.

Once the grass is gone, dig a shallow depression (based on the size calculations) with a level bottom. Build a berm on the downhill side to hold the water within the garden. Add a notch to the down slope berm for overflow water to go to a safe location.

Dig the rain garden 4 inches deeper than the final intended depth to make room for the compost mixture and mulch layer.

Add 2 inches of compost mixture (this mixture will depend on your soil infiltration test results) to amend the existing soils.

Till the bottom soils to a depth of 6 inches to promote greater infiltration and incorporate the compost amendment. Ensure that the bottom is as level as possible.

Prepare holes for trees and shrubs on the rain garden basin surface, install vegetation, and water accordingly. Plant trees and shrubs after they go dormant in the late fall, if possible. Avoid trees and shrubs on the berm if it is present. Install any temporary irrigation. Place the mulch around the plants, as per the design. Water plants during weeks of no rain for at least the first two to three months after planting.

Install any pre-treatment features associated with the plan, such as a stone pad at the mouth of downspouts.

Seed and straw or place sod on all side slopes, berms, and disturbed areas outside the rain garden area. These areas may also be planted with native plants, although grass often works best on constructed berms.

Inspect the rain garden after several rain events to look for any needed adjustments: ensure that runoff is entering the rain garden, the garden is draining properly, there is no erosion at inlets and outlets, and plants are surviving.