How Big Should Your Rain Garden Be?

plant_01Goal: Calculate rain garden depth and area. Research plants.

Objectives: Students will…

  • Use criteria to select an answer from a table
  • Calculate area of oval
  • Determine dimensions of oval or rectangle with certain area
  • Research plant species based on criteria
  • Record research in table

Materials: Rain Garden Handout, calculators, plant catalogs, copies:
2 per group: Rain Garden Species List and pp.19-22 Rain Garden Guidelines for Southwest Ohio
1 per student: pp17-18 Rain Garden Guidelines for Southwest Ohio and Rain Garden Species Selection Worksheet

Advance Preparation: : Collect plant catalogs (perennials, grasses and shrubs) for student research, reserve computer use if necessary

Timing Estimate: 1 class with homework
10 min work through depth, slope and area calculations
10 min sketch garden shapes and sizes that would work on site
5 min garden theme brainstorm, break into groups
25 min small group garden planning and plant research (continue as HW)

The class has collected all the information they need to figure out the depth and area of the rain garden:

  • Area feeding into it
  • Volume of runoff coming off area
  • Slope of rain garden site

In this lesson, your students will use the information they have collected to determine the depth and area of the rain garden. First, they will use the slope to get a depth range. Then, they will use soil type and garden depth to select a soil factor.

Depending on how much space you actually have available for the garden, you may need to rethink the area feeding the garden in order to make the garden smaller, once you have worked through the calculations.

Determine Rain Garden Depth from Slope

If your slope is: Then your garden depth should be:

Use Rain Garden Depth and Soil Type to Find Soil Factor

Use what you have discovered about your soil type and garden depth to select a soil factor, which you will use to calculate the area of the rain garden.

Calculate Rain Garden Area

Multiply total area draining to the rain garden (from previous lesson) by the soil factor from the table above to get the total surface area of the rain garden.

drainage area (ft2) * soil factor = rain garden area (ft2)

Will this size garden fit in the area you had in mind? Have students figure out dimensions of a garden with this area (a simple rectangle will work for this purpose) so they can get an idea of how practical it is. If it is too large, go back to the runoff area calculations and rethink what areas you want to drain to the rain garden.

Ask students to sketch some shapes and dimensions that would have the area they calculated. If you are working on a bioswale, it is going to have a long, narrow shape (probably rectangular). Rain gardens typically have an oval or kidney shape. You may need to remind them that the area of an oval can be calculated using the formula:

If you want to get fancy and calculate the area of a kidney shaped garden, there is a nice animated tutorial available at here

calculate_image3Draw for the class a cross section and contour type drawing of a rain garden that shows the different depths of the garden. Use this to explain that there are three moisture zones in a rain garden: moist at the bottom, average on the sides and dry around the top edges. They will use this information as part of their plant selection process.

Plant Selection

RainGardenThe main purpose of a rain garden is to absorb rain. Like any kind of garden, it can serve other purposes as well, including: beautify the landscape, provide food sources for birds, butterflies, bees and other wildlife, be organized around a theme like scents, textures, colors, herbs, history or other educational purpose. Lead a group brainstorm of additional purposes your rain garden could serve.

Divide students into planning groups of four. The groups can be based on students’ interests in the different theme ideas, or other criteria that works for you. Ask each group to select a palate of plants (mix of shrubs, grasses and perennial flowers) that meets their criteria (soil and sun conditions plus theme of garden they are working on) by researching plants on the Rain Garden Species List and in the OSU Extension Handbook for Rain Gardens. Review with the class how the lists are organized so they know where to find height, bloom time, color, etc. The groups should use their class time to decide which plants they want to research and who will research each plant (minimum of four per student). You may wish to have computers or plant catalogs available to allow students to do some research during class time.

Homework (start in class if time): Read pages 17-18 of Rain Garden Guidelines for Southwest Ohio. Research four plants from the rain garden list and record data on Rain Garden Species Selection Worksheet. Print or assemble electronic images of each plant to bring to class.

Click here to download PDF file.
Rain Garden Species Selection worksheet
Rain Garden Species List