Rain Gardens

Overview: Rain gardens help collect stormwater to reduce runoff, providing numerous environmental benefits. They are excellent tools for teaching students about the water cycle, storm water drainage, water quality and the broad environmental impacts of urbanization. This lesson provides instructions for designing your own rain garden

Grade Level/Range: Grades 9 to 12

Objectives: Students will research the issue of stormwater runoff and the impact of water pollution on the environment. They will design a rain garden to further understand the ecological impact of stormwater and why rain gardens are an important solution.

Time: 1- 2 hours to discuss and design

Materials:

  • graph paper
  • pencil
  • tape measure
  • long handle shovel
  • ruler
  • hose or watering can
  • Water Cycle: The Rain Garden Handout

Background Information:

Rain gardens provide a perfect setting to teach about water quality, habitat creation and the impact of our actions on protecting our natural resources. A rain garden is a garden planted in a depressed area to encourage water collection. This design enables rain gardens to trap stormwater before it becomes runoff and filter it before it’s absorbed into the soil.  The plants in a rain garden have high tolerance for excess moisture and the increased levels of nutrients often found in stormwater. Rain gardens are most useful if situated downhill from impervious surfaces, such as rooftops and roads, and are designed to collect runoff from those surfaces. They slow down the flow of stormwater by collecting it in the sunken garden area and allowing it to absorb into the soil rather than cause erosion and carry pollutants into our waterways.

Why are rain gardens beneficial? To understand the importance of rain gardens, students must first learn how the urban environment impacts the natural water cycle. In nature, rain falls directly onto vegetation and is evenly distributed over the land's surface. The plants slow the velocity of raindrops and also help decrease the volume of water reaching the soil, which helps with absorption (approximately 50% of the rain will be absorbed by the soil) and decreases runoff (approximately 10% of rain will run off to local waterways).

In suburban and urban settings, however, much of the rain that falls hits impervious surfaces such as roofs, parking lots and roads, where it cannot be absorbed. It becomes runoff, moving across the ground to areas where it can be absorbed or into local waterways, either directly or via storm sewers. In urban settings as little as 15% of the water may be absorbed where it falls and up to 55% will runoff. Not only does this result in lower groundwater reserves which endangers drinking water supplies and can ultimately cause cities to sink (subsidence), it also creates a significant amount of water to deal with above ground. To prevent flooding of houses and roads, cities install drainage systems to move storm water to municipal water treatment facilities or to streams, lakes and rivers.

Although rain is an important contributor for recharging local waterways, the problem with runoff from urban environments is what the runoff is carrying. As the water moves across surfaces such as streets, parking lots, and roofs, it picks up all sorts of pollutants, from nutrients like nitrogen and phosphorous that fuel algal blooms to pesticides, herbicides, oil, grease, heavy metals, and harmful bacteria, . These pollutants can kill water life and interfere with the delicate balance of the aquatic ecosystem. Scientist estimate that 70% of the pollution in streams, rivers and lakes is from stormwater runoff.

To decrease the amount of runoff flowing directly into local waterways, some is diverted into treatment facilities to remove the contaminants and then deposited into waterways or used for drinking supplies. However it is not feasible for treatment facilities to catch and process all storm water. Rain gardens are another solution.

Rain gardens help catch storm runoff and then aid in absorption and filtration of the water. Benefits of installing rain gardens include:

  • decreased amounts of polluted stormwater runoff reaching local streams, rivers and lakes
  • increased absorption of water to recharge groundwater supplies
  • filtration of pollutants by soil and plants helps improve water quality in groundwater supplies
  • installation is less expensive than other drainage techniques
  • an attractive addition to your landscape and a low maintenance alternative to lawns
  • creation of a habitat for birds and butterflies.

Laying the Groundwork:

Begin this lesson with an observation activity. If weather cooperates, ask students to observe a rain event from a safe location (such as from a window or covered pathway). If this is not possible, there are numerous videos online that can be viewed. As they watch the rain, ask them to observe how and where the rain falls. Ask, what do you think happens to the water once it hits the ground? How do you know?

Visit your schoolyard right after a rain event and create a map showing areas of flowing and standing water. Return after an hour and document and compare your findings. Repeat after two hours. Ask, how well does our schoolyard manage rainwater? Are there any areas that need help?

Exploration: Planning a Rain Garden

  1. You begin your rain garden by identifying a good location. Use your from the Laying the Groundwork determine the natural patterns of runoff. In order to capture rainwater, the garden must be planted down slope from buildings and other surfaces that increase stormwater run-off, but up slope from municipal storm drains and natural waterways. If you don't find a good spot along the current travels of runoff, you may need to install drains to help move the water to the location of your rain garden.

The garden must be located at least 10 feet away from a building foundation and should not be placed over a septic system. Also, avoid locations under mature trees, because the digging of the garden could cause serious damage to their root systems.

Also, since the ground of a rain garden is intended to become water-logged, it is important to plan for maintenance and student access around the perimeter to avoid soil compaction.

  1. Once you identify a possible site, test the drainage of the soil. It is important for the garden to contain well-draining soil so that the collected water dissipates within two to four days. If water sits for too long, plant roots will suffocate and insect breeding will become a problem. Ideal rain garden soil is comprised of:
  • 20-25% leaf mulch or compost
  • 50% sandy soil
  • 25-30% topsoil

To test the drainage of the soil in each a potential rain garden location:

  • Dig a hole 6 inches wide and 18 inches deep in each location
  • Fill each hole with water and measure depth with a ruler.
  • Check on water depth every hour and record results.
  • If all the water drains within a few hours, the site has excellent drainage. If the water drains within 24 hours, then it is still an acceptable site for a rain garden. If the water has not drained in 48 to 72 hours, then you should choose a different location.
  1. After a location is identified, call your local utilities hotline to have them mark any underground lines on the property. Your soil will need to be cultivated by a tiller or by hand 1 to 2 feet deep to break up any existing soil compaction and you want to make sure to avoid any unnecessary entanglements.
  1. The next step will be to design your rain garden. As mentioned earlier, gardens can vary greatly in size, shape and types of plant material. However as a general rule rain gardens include the following components: (from the Virginia Department of Forestry Rain Garden Guide):
  • Ponding area or depression. To help capture runoff, the garden base should be shaped like a saucer with the middle deeper than the edges. The grading between the middle (generally 6" deep) and edges should be gradual though so that water is spread out throughout the garden. Because of this shape, the edges of the garden will usually be drier than the middle which will need to be considered when selecting plant materials.
  • Well-draining soil. Well-draining soil is important to ensure quick absorption of runoff. During planting and maintenance, it is important to avoid compacting the soil, which will decrease its effectiveness.
  • Tough plants. Plants chosen for the rain garden must be able to tolerate extremes of wet and dry soil. Rain gardens are typically planted with shrubs and perennials. Because the rain garden functions better with deep rooting plants, annuals are not part of the usual design. Although native plants are not mandatory, they are often the best choices, as long as they are adapted to the conditions your site offers because they will be well suited to the environmental conditions of your climate and do not need additional fertilizer.
  • Mulch. Mulch is needed to protect the soil from erosion and insulate the garden from extreme wet and dry conditions. Shredded bark mulch is preferable because it does not wash away as easily as lighter bark chips.
  • A grass buffer strip. A grass buffer strip around the garden is important to slow the speed at which the runoff enters the garden and to decrease soil erosion.
  • A berm. A berm made from at least six inches of soil or rocks helps to keep the runoff in the garden long enough to allow it to be absorbed into the soil. Make sure that if your garden does overflow, the overflow will head to storm drains rather towards structures.

Download the handout Water Cycle: The Rain Garden for further instructions.

Making Connections:

  • Dig in! If resources allow, turn your rain garden plans into a reality. Use your new design to gather support and resources for your new space. Your rain garden will serve as a living laboratory to teach young gardeners about the importance of protecting and conserving water resources.
  • Visit a local water treatment facility to learn more about the sewer and storm drains in your community. If a field trip is not possible, invite a staff member from the facility to the class for a presentation.

Branching Out:

Science - Learn about other ways to protect water resources in your area. Check out the EPA's water resources to get started.

Science - Research good rain garden plants for your area. Native plants adapted to varying amounts of water are best. Create a brochure or presentation to share your findings with parents and community members.

Social Studies - Bring in magazine and newspaper articles focused on water quality or quantity. Ask students to evaluate and debate current issues related to water.

History - Lead students in researching the Clean Water Act. Discuss the role of government in protecting natural resources.