How to Design a Rain Garden
- Rain gardens are landscaped depressions planted with adaptive native plants that capture, hold, and filter stormwater, preventing nonpoint source pollutants from entering waterways.
Make your yard more eco-friendly with beautiful native plants that help drainage
The Spruce / Evgeniya Vlasova
Reviewed by Julie Thompson-Adolf
A rain garden is a landscaped, shallow depression planted with adaptive native plants that can capture, temporarily hold, and filter stormwater back into the ground. A good rain garden design prevents stormwater from running off of a property and into storm sewer systems where nonpoint source pollutants (NPS) can enter our local waterways. Rain gardens also provide an effective solution for dealing with occasional flooding, in addition to providing attractive landscape features.
Nonpoint source pollutants (NPS) are pollutants that are picked up and carried to waterways by runoff, such as rainwater or melting snow. Examples of NPS include motor oil, gasoline, fertilizer, chemicals, herbicides, pesticides, and pet waste.
The term "rain garden" is something of a misnomer. A rain garden conjures up visions of perpetually moist landscape features: juicy basins teeming with thirsty plants and perhaps an amphibian or two. In fact, rain gardens experience a wide range of moisture conditions, ranging from boggy to bone-dry. Rain gardens are eco-friendly, but there's so much more to this great garden project that would make you want one in your own yard.
What Is a Rain Garden?
A rain garden is a planting that you install in a low spot in your landscape. This may be a natural depression on your property, or it may be the result of a planned change in grade that you implement in your landscape design. In addition to collecting rainwater, you can also direct downspouts into rain gardens.
A rain garden works somewhat like a retention pond. It collects rainwater from impervious surfaces like roofs, pavements, driveways, patios, parking lots, or waterlogged yards , allowing the water to slowly seep back into the ground, The water slowly seeps back into the ground to be taken up as nutrients by flood-tolerant plants. The ideal plants for rain gardens are not water-hungry plants, but more native plants , which are naturally adapted to a wide range of moisture conditions. Most of a native plant's mass, including its long roots, is located underground, giving it the ability to trap and filter runoff water.
How Does a Rain Garden Work?
The principle of how rain gardens work is simple. You have likely seen water run downhill before and witnessed a puddle on a lawn slowly disappear. A rain garden creates a channel to an artificial depression or serves as an artificial depression, where water will be able to collect and enter the ground slowly. When installing a rain garden, you are not creating a pond; ideally, the garden should drain within 24 hours after a storm.
The rate at which water is absorbed is controlled by estimating the amount of runoff entering the rain garden. The amount of runoff determines the depth of the depression and the size of the rain garden. Absorption will be aided by adding native plants with deep root systems or amending the soil if the type is not ideal.
Perform a simple water percolation test to determine your site's soil drainage rate before you start working.
How to conduct a water absorption test:
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Dig a hole at your rain garden site that is 12 inches deep and 6 inches wide.
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Fill the hole with water, and then let it stand until the hole is empty.
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Refill the hole so that water is one inch from the top. Push a pencil into the side of the hole to show the starting water level. Note the time and use a ruler to measure how deep the water is.
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Measure how deep the water is with a ruler every hour for at least four hours.
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Calculate how many inches of water are drained per hour.
Advantages and Disadvantages of Rain Gardens
Advantages
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Prevents pollution:Rainwater is not pure. It collects pollutants as it flows, especially excess nitrogen and phosphorus from lawn fertilizers as well as oils, salts, and other pollutants dropped onto driveways and walkways. A rain garden keeps these pollutants from harming rivers and ponds with algae blooms and fish kill.
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Nourishes plants:The deep roots of native plants readily absorb those oils, salts, and other nutrients, protecting groundwater quality.
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Reduces flooding problems:Rain gardens also can reduce the effects of flooding . A mature stand of rain garden plants will take up excess water that might normally puddle in your yard, leading to plant or lawn damage, unpleasant odors, and possibly water intrusion issues in your home.
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Reduces mosquitos:When you get rid of standing water with a rain garden, you also reduce mosquito breeding problems around your home.
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Nurtures wildlife:Rain gardens create habitats for wildlife. A lawn's monoculture does little to nurture wildlife, but a mixed planting of native flowers in a rain garden will draw butterflies , bees, other beneficial insects , and birds.
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Conserves water:Native plants thriving in a rain garden typically don't need supplemental irrigation because they're deep-rooted and have evolved to survive periods of drought between rains.
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Prevents erosion:Native plants used in a rain garden can control erosion by stabilizing soils, increasing the recharge rate of water, and filtering any contaminants in the runoff before it enters the groundwater.
Disadvantages
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Poor drainage:If the garden isn't designed or executed properly, a drainage problem could develop. Rainwater runoff will accumulate in the basin, which can lead to backups and flooding
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Erosion:A poorly functioning rain garden can lead to soil erosion if the runoff is not draining properly on the property, but is instead flooding into nearby sewers.
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Slopes: Rain gardens are not suitable for areas with steep slopes where erosion tends to occur.
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High cost:Rain gardens can cost money upfront, including plant material and excavation. Expect to pay between $5 and $40 or more per square foot for a rain garden.
To save on costs, shop at local nurseries specializing in native plants, grow your own from seed, or contact extension services to see if there will be spring plant sales offering native plants.
How to Design a Rain Garden
Determine the Site
The most important consideration when planning a rain garden is site selection. Locating your rain garden in a spot that receives little to no water or too much water defeats its purpose. You want to choose a location between two impervious surfaces, for instance, a roof, a lawn, and a street. Examine where the rainwater pours off the roof via downspouts, watch how it flows to the lawn, and see where the runoff water enters the street.
If you're planning to divert your roof gutters into your rain garden, you won't want a rain garden to be too far away from your home. However, it must be at least 10 feet away from the house to prevent the risk of water intrusion. Underground utilities and tree roots may also influence the placement of the rain garden.
Before beginning any project that requires you to dig, call your utility companies first to identify the utilities in the area. The utility companies will use spray paint or colored flags to alert you where the utility lines are and to avoid digging in those locations.
Rain Garden Sizes
How big should a rain garden be? A small rain garden is better than nothing, but you should plan on a garden of at least 150 square feet to provide enough room to include an interesting variety of plants.
If your yard doesn't have a natural depression, you'll need to create one by digging either by hand or with an excavator. It isn't necessary to create a pit; a gentle basin no more than about 5 inches deep is sufficient. The basin should have a flat bottom and gently sloping sides, and you can use some of the fill dirt to create a berm to hold excess rainfall until the plants can absorb it.
Best Shape for Rain Gardens
Choose a natural, rounded shape for your rain garden, like an oval, teardrop, or kidney shape. If you're starting from scratch with an excavated garden, outline the potential area with a hose or rope first, and view it from all angles as well as from inside your home to envision the finished product.
Choosing Plants for a Rain Garden
Proper rain garden design dictates that all plants must be perennial. This is due to the way a rain garden functions as a water abatement feature, which depends on the deep roots of perennial plants. Here are more considerations when choosing rain garden plants:
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Edges:On the very edge of the garden, the plants you choose must be able to tolerate dry conditions.
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Slopes:Plants installed along the slope should be able to thrive in conditions that are sometimes dry or wet.
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Base:Plants that you install in the center of the garden, known as the base, will experience longer periods of wet soil and must be able to thrive in these moist conditions for an extended time.
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Focal point:Although a rain garden isn't the place to grow a large shade tree, you can add a small ornamental tree or shrub to act as a focal point.
The importance of using native plants can be seen in the following comparison. Compare the watering needs of a native plant like big bluestem grass ( Andropogon gerardii) and a non-native plant like Kentucky bluegrass ( Poa pratensis) . Big bluestem grass roots can grow up to 12 feet deep, whereas Kentucky bluegrass roots will only grow a shallow 1 to 2 feet deep. A deeper taproot means the plant does not need as much watering as one with shallow roots that need an abundance of watering to survive. More watering will create pooling water and runoff because the already-drenched soil cannot absorb or drain any more water.
Consider selecting the following trees, shrubs, and flowers for your rain garden.
Rain Garden Trees
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Pawpaw tree :This tree has a particularly deep tap root and is well-suited to a rain garden.
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Redbud tree :This tree has a pleasing vase shape and will reach a mature height of 15 to 20 feet.
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Hackberry or river birch : These specimens can work in a larger landscape and will greatly increase the water absorption ability of your rain garden.
Rain Garden Shrubs
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Oakleaf hydrangea :This shrub will thrive in a rain garden while providing three seasons of interest.
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Ninebark shrub :This shrub has some interesting cultivars to play with in a rain garden's design, including the chartreuse leaves of 'Nugget' and the moody purple foliage of 'Diablo'. Spicebush, named for its aromatic foliage, will draw swallowtail butterflies, which use it as a host plant.
Rain Garden Flowers
Plants around the edge of the rain garden will rarely have wet feet and won't mind occasional periods of drought. Some attractive flowering choices with big wildlife appeal include:
Use hardy grasses , such as fountain grass or corkscrew rush, in your rain garden as focal points.
Rain Garden Care and Maintenance
Although rain gardens require less maintenance than a typical flowerbed, they do benefit from some of the same care practices. Rain gardens may require more care in their first year as they establish their root systems. Note these rain garden care tips:
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Watering:Water deeply but infrequently during times of drought to encourage healthy roots.
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Weeding:Rain gardens need to be kept free of weeds , both for plant health and aesthetics. The best time to weed is after a rain when the soft soil allows weed roots to release their purchase easily.
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Mulching:A 3-inch layer of mulch will further suppress weeds. Use fine hardwood mulch, as it's less likely to float away after heavy rain.
Frequently Asked Questions
Are rain gardens worth planting?
If you are concerned about water conservation, then a rain garden could be worthwhile. A rain garden allows for more water to soak into the soil, preventing chemicals from seeping into waterways.
Where is the best place to plant a rain garden?
Locate a rain garden at least 10 feet away from your home to avoid damaging the foundation. But it should go in an area where you can easily capture water from a roof, lawn, or paved area.
Will a rain garden attract mosquitos?
Since a rain garden drains in 12 to 48 hours, it does not attract mosquitos or pose a breeding ground for insects.
