Rain Gardens

Here in Wilmette, many of us have engaged in what you might call a Water War, but instead of a fight caused by water scarcity, it’s a dispute about who’s dumping stormwater where.

It’s easy to see why people get mad. When water flows out of a downspout, sump pump, or drain tile, it can create a slippery mess on sidewalks, kill plants, and grow moss or mushrooms in yards. When it’s cold, dangerous ice slicks can form.

Rain gardens can help prevent these problems.

What is a rain garden?

A rain garden is a landscape feature that’s shaped as a depression in the ground, usually located on a gentle slope, and filled with deep-rooted native plants.

Diagram from University of Illinois, Engineering Your Rain Garden.

At the high side, there’s an inflow of runoff from impervious areas (like roofs, walkways, or parking lots). At the low side, a berm (or embankment) holds the stormwater inside the depression. This setup allows stormwater to infiltrate the soil, so it can be absorbed by the plants. During heavy rainfall, any overflow can safely exit at a low side, or via a drain.

According to the Illinois Extension, a small DIY rain garden can be “low-tech, inexpensive, sustainable, and a beautiful addition to home, municipal, and business landscapes.”

Rain Garden Benefits
Rain gardens can:

  • Restore habitat in intermittently wet areas.
  • Filter and settle sediment.
  • Capture some stormwater and pollutants in plant roots.
  • Slowly return some moisture to the atmosphere via plant transpiration.
  • Reduce landscaping maintenance–if installed and established properly.
  • Develop a beautiful wildlife habitat that attracts butterflies and birds.

Rain Garden Limitations
On the minus side, the capacity of a rain garden is limited:

  • Generally most helpful during mild rainfall (less than 1 inch).
  • Cannot be installed if the water table is high (< 2 feet below ground surface).
  • Does not work well in clay soil, unless soil is amended.
  • May require a connection to a storm sewer to handle overflow.
  • Can supplement, but not replace, adequate drainage.

What’s required for a rain garden?

Rain gardens are generally small and don’t require a big investment of time or money.

Native Plants:

  • The plants should be adapted to the local environment, so they can tolerate wet conditions, as well as drought.
  • The plants should thrive naturally, so no pesticides or herbicides are added to the environment.
  • The root systems should be deep and expansive to handle biofiltration.
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Typical lawn grass (left) does not have the deep, expansive root system characteristic of native plants (right), which function better in rain gardens. (FEMA, Reducing Damage from Localized Flooding, p8-12)

Optimal Rain Garden Location and Size:

  • Gently sloping area.
  • Location 10 feet from all buildings.
  • Small size, typically 100-300 sf.
  • Has full or partial sun.
  • Away from underground utility lines.
  • Must create a well-formed depression to catch and detain runoff.

Source of Stormwater:

  • Downspout
  • Drainage pipe
  • Sump pump outflow

Path for Overflow:

  • Overflow via a low side, safely away from buildings.
  • Or overflow into a underdrain, connected to the storm sewer (if permitted).

Appropriate Soil:

  • Sandy, loamy, or rocky soils are best.
  • Water infiltration should be greater than 0.5 inches per hour (in/hr).
  • If the soil is inadequate, use engineered soil. (The City of Chicago recommends 40 percent sand, 30 percent topsoil, and 30 percent compost.)

FAQs

Is a rain garden a pond?
No. A rain garden intermittently floods during rainstorms, and then slowly drains until dry again. During low rainfall in mid-summer, it can even go through periods of drought.
Will a rain garden breed mosquitos?
No. Water usually drains in less than a day after a storm. Mosquitos need seven to 12 days of standing water for their eggs to hatch, so any eggs won’t survive. In fact, rain gardens may attract a variety of beneficial insects like dragonflies that can reduce the mosquito population.
Are rain gardens high maintenance?
No. Weeding and frequent watering are required at the start, to help the plants get established. After that, maintenance is minimal.
How much stormwater can a rain garden hold?
One estimate is that a properly constructed 300 sq. ft. rain garden could absorb up to 12,000 gallons of stormwater annually.
Could Wilmette solve westside flooding with rain gardens?
No, although rain gardens could make a slight difference, especially for light rainstorms, and they could help keep our waterways cleaner.

According to the January 2015 Separate Storm Sewer Study, flood levels during a 10-year storm currently range from 0.3 feet to 2.2 feet. If every residential lot in the entire westside Wilmette added a two-feet deep, 10 x 20 feet rain garden, then flooding could be reduced by .2 feet.

Rain Garden Resources for Homeowners

I consulted a wide range of experts to find out more about rain gardens, and they delivered an amazing array of resources.

They also had a few tips for getting started with a project:

  • Keep it simple, so it’s not too hard to maintain.
  • Start small, and find “extra hands” to help with installation.
  • Carefully select a few types of plants, appropriate for the soil and sun conditions.
  • Add more plants after you see which types work best.
  • To handle a challenging location, soil conditions, or drainage issue, consult a professional landscaper who has experience installing rain gardens.

Make sure there’s no conflict with underground utilities. Call JULIE before you dig!

Quick How To Video: A great place to start is with a video, Learn how to install a rain garden, which is posted on this informative rain garden page, by MMSD, the Milwaukee sewer district.

Home Flooding Questionnaire: Center for Neighborhood Technology/RainReady provides an online tool to offer some basic, semi-customized floodproofing information for homeowners at My RainReady.

DIY Rain Garden Guide: CNT/Rain Ready created a step-by-step general guide on how to plan, install, and maintain a home rain garden.

Detailed DIY Guide: Here’s a more detailed guide on building your own rain garden, from the Wisconsin Department of Natural Resources (download the guide here). This guide includes a couple tests for soil types, and some sample layouts using native plants.

Guidelines & Plant List: The University of Illinois Extension has a Conservation@Home project that includes resources for rain gardens. You can find detailed technical guidelines for managing soil types and sizing a rain garden, as well as a list of recommended native plants for rain gardens.

Tool to Find Your Soil Type: The USDA Natural Resources Conservation Service (NRCS) provides general soil surveys, so you can look up your home address and check out the soil on your lot: Web Soil Survey (WSS).

Rain Garden Starter Plants: Go Green Wilmette will be offering a Rain Garden Starter Kit at the annual plant sale, Saturday, May 12, from 9:00 am to 2:00 pm at the Lakeview Center in Gillson Park.

Rain Garden Visit and Plant Lists: Visit a rain garden in Libertyville and check out additional resources, including an extensive plant list, prepared by the University of Illinois Extension Master Gardeners, in partnership with the Lake County Stormwater Management Commission.

Mail Order Plants: Prairie Nursery in Wisconsin offers mail order plant kits for creating pre-planned rain gardens.

Chicago Botanic Garden Inspiration: The Chicago Botanic Garden provides a few tips and plant recommendations, as well as a list of resources for rain gardens. At the Botanic Garden, there’s a Rainwater Glen, which might be fun to visit for inspiration, in the spring or summer.

Model for Municipal Support

The Village of Glenview has a Rain Garden Program. Glenview residents can propose a rain garden that provides drainage benefits, using primarily native plants. Qualifying projects are reimbursed by a Village grant for 50 percent of the project costs, or a total of up to $1,000 per project.​​

Integration with Green/Gray Infrastructure

The Delta Institute, a non-profit working to solve Midwest environmental challenges, has created a guide called Green Infrastructure Designs: Scalable Solutions to Local Challenges. The guide shares expertise and inspiration with local governmental entities faced with flooding problems, with a flowchart to select the most effective solution for a particular challenge.

The EPA Municipal Handbook also offers some great ideas for green infrastructure, as well as case studies of successful implementations around the country.

Sources for this Post: Thanks to Elizabeth Hoffman, Manager/Owner of West End Florist and Garden Center; Rebecca Raines, Outreach Associate Planner, RainReady Community, Center for Neighborhood Technology; and many helpful civil engineers.

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