Creating Rain Gardens

As you wind your way through the exhibits of the Milwaukee County Zoo, you’ll come across several bricklike walkways and colorful gardens nearby. These aren’t your average gardens. They’re rain gardens. They collect rainwater, filter it and release it into the ground. And they’re making a big difference with water conservation at the Zoo! Each rain garden is a part of the Zoo’s continuing efforts to create a greener environment. Read all about it in Setting a “Green” Example in the April 2011 issue of Alive. In the meantime, learn more about rain gardens and their benefits to both the Zoo and the planet with the Rain Garden Q&A below.

Meet Philip Hung, the Man Behind the Green

Philip Hung with permeable pavers at the Milwaukee County Zoo

Philip Hung with permeable pavers at the Milwaukee County Zoo

Philip Hung has quite the green thumb. Hung, Milwaukee County’s managing architect, oversaw the design and construction of the Milwaukee County Zoo’s rain gardens. He has also helped plant the seeds for other “green” projects at the Zoo.

“In relation to the rain gardens, I am leading the effort in the design and construction of two roof water-reclamation and recovery systems,” says Hung. One is at the Australian Building, and one is at the Otto Borchert Family Special Exhibits Building. These projects will install storage tanks, pumps and filters. The tanks will receive and store rainwater from the roofs of these buildings. The pumps will transfer the water from the tanks to the filters, where the water will be cleansed and then used to wash roadways. Afterward, it will be collected by the permeable pavements and rain gardens.

“This combination of roof-water reclamation and recovery systems, permeable pavements and rain gardens forms an experiment to create a three-stage total rainwater management system,” Hung says. “Through this experiment, we can study how rainwater from the roofs, parking lots and roadways can be effectively managed in the future to benefit the environment.”

Hung is involved in all construction projects at the Zoo, from green initiatives to animal exhibits to participation in master planning. Below, he explains what a rain garden is, how to create one and its benefits.

Rain Garden Q&A with Philip Hung

Q. What is a rain garden? What is its primary purpose? What are the different elements of a rain garden?

A rain garden is a special garden that collects surface water from rainstorms, filters it and slowly releases it into the earth. It has three main elements:

  1. A depressed area called a basin
  2. A bed of carefully designed sandy soil mix 
  3. Native plants.
Rain garden illustration

A rain garden with native plants is situated a little lower than permeable pavers (right). Illustration provided by Philip Hung

The basin allows water to flow into the rain garden. It must not have a hard surface that won’t let water filter through. The sandy soil is called "engineered soil." It allows water in the basin to settle at the bottom and doesn’t form puddles of standing water at the top. It also acts as a filter to remove unwanted matters from the collected water. The engineered soil bed must fill the entire basin and be thick enough for planting. The native plants’ roots hold the soil together to prevent erosion. They even help the soil drain extra water. Native plants can grow in the engineered soil without artificial fertilization. They add color to the Zoo, too!

Q. How do you create a rain garden?
This rain garden next to permeable pavers borders the asphalt drive of the Zoo entrance gates. Rain water can filter through the garden instead of into the storm drains.

This rain garden next to permeable pavers borders the asphalt drive of the Zoo entrance gates. Rain water can filter through the garden instead of into the storm drains. Photo provided by Philip Hung

  1. Select a rain garden’s location. Water must be able to easily flow into each rain garden location from roadways, yards or roofs.
  2. Determine the rain garden’s surface area and depth. If a rain garden is the only means of emptying rainwater from an area of a certain size at a public facility, it is called a “Subsurface Infiltration Plumbing System.” Its size must meet Department of Natural Resources’ (DNR’s) standards and state plumbing codes.
  3. Design the engineered soil mix. The mix is determined by the thickness of the engineered soil bed, the rate the rainwater filters through the soil and the plants involved. If a “Subsurface Infiltration Plumbing System” is to be constructed, then the soil mix must also meet DNR standards and state plumbing codes.
  4. Select which plants to use.
Q. What kinds of plants typically go in a rain garden?
Plants must be suitable for growing in the engineered soil. They don’t require additional fertilization. For this reason they are called native vegetation or native plants. They may include:
golden Alexanders, oxeye sunflowers, showy goldenrods, little bluestems, prairie dropseeds, selected hedge plants, fox sedges, cordgrass, annual rye ground cover, redtop seed ground cover.

Q. What are the benefits of a rain garden? How do rain gardens contribute to the Zoo’s “green” efforts?

In a standard storm system, rainwater is collected by inlets (entryways) connected to storm pipes. The pipes transport the rainwater and empty it at locations far from the places it fell. This standard storm system damages the environment in three ways:

  1. It requires major construction that uses a lot of resources.
  2. When the water collected is greater than the amount of water the storm pipes can handle, it causes flooding and backups on roadways and in buildings.
  3. Since rainwater is released far away from where it hits the ground, the area ground water is not replenished. The surface water is not distributed evenly. 
    A rain garden or permeable pavement collects rainwater where it falls and then returns it to the earth where it fell (locally). It requires fewer construction efforts, reduces the risk of flooding and helps replenish ground water.
Q. Where are the Zoo’s rain gardens being created? How were these locations selected?

The Zoo’s five rain gardens and permeable pavement areas are at:

This is one of the rain gardens built at the Milwaukee County Zoo in winter 2011.

This is one of the rain gardens built at the Milwaukee County Zoo in winter 2011. Photo provided by Philip Hung

  • The Zoo entrance between the entry gates and the first parking lot;
  • Between the Apes of Africa Pavilion and the Herb and Nada Mahler Family Aviary—look for a stretch of bricklike pavers and then a rain garden (decoratively framed by landscape rocks) next to the aviary pond (see photo of Philip Hung with pavers);
  • The Dohmen Family Foundation Hippo Home, where rain from the roof drains into a garden;
  • East of the MillerCoors Giraffe Experience a section of permeable pavers drain into two rock-rimmed rain gardens;
  • Permeable pavers without rain gardens are found between the Alaskan brown bear exhibit and Australian Outback Picnic Pavilion and also at the Zoo Terrace.

These locations were chosen based on project funding tied to each location. The conditions of the ground in nearby areas also affected the locations. (For example, water must be able to easily flow into each rain garden location from roadways, yards or roofs.) For the purpose of public education, the locations were based on their visibility to visitors, too. 

Q. What is most challenging about creating a rain garden? What has been most challenging about creating the Zoo’s rain gardens?
The most challenging task is selecting the rain garden location and resolving conflicts between underground utilities and the rain garden or permeable pavement. At the Zoo, our greatest challenge was working around the underground utilities. There’s a lot of underground piping at the Zoo, and we had to factor that into both designing and constructing the permeable pavements.

Q. How long does it take to create a rain garden from start to finish?
The time required for designing a rain garden depends on both the knowledge of the designer and the kinds of problems the designer must take into account. The time required for the construction of a rain garden depends on its size, the equipment used and the underground utilities encountered. For the smaller gardens east of the MillerCoors Giraffe Experience, which measure about 20 feet by 30 feet each, excluding the construction of the permeable pavement, the gardens were completed in two days. Planting usually takes one to two days.

Q. How do you maintain a rain garden, particularly from season to season?
A rain garden is supposed to be maintenance-free. Fertilization is not required. There is a potential for weed growth. So weed removal by hand may be required. 

Q. When will people be able to see these rain gardens?
We are planting the gardens in the spring of 2011. Depending on the maturity of the plants, we may be able to see some rather attractive gardens by summer 2011.

Q. Does Milwaukee County have any information for the public on creating home rain gardens?
The best resource for creating home rain gardens is the Milwaukee Metropolitan Sewage District (MMSD). It should be noted that the projects at the Zoo are partially supported by a grant from MMSD.