This guide is for residential use only. For regulated stormwater BMPs in St. Louis County, please refer to the MSD BMP Toolbox. In other areas, check your municipal regulations.
Now let’s start planning! To make it as simple as possible, our Rain Garden Guide will walk you through the basics from location to soil type to long-term maintenance.
STEP 1: PLAN
A rain garden that doesn’t intercept runoff is really just a pretty hole in the ground, so when thinking about location, also consider the water source and how it will be directed to your garden.
- Observe how water flows on your property. Look for natural depressions or areas that tend to collect water when it rains, or where runoff from a driveway or downspout can be diverted.
- Rock channels, perforated drainpipe buried in the ground, and grass swales are typical methods to ensure runoff from a drainage area or a downspout makes it to the rain garden.
- Other ideal locations include areas susceptible to erosion or at the end of a gently sloping lawn. Stay out from under trees and at least 10 feet away from building foundations.
- Have a utility company locate underground lines before digging (in Missouri, call 800-DIG-RITE).
- Avoid septic system drain fields, areas that pond but do not drain within 24-48 hours, and slopes greater than 12 percent.
The type of soil you have determines how readily water infiltrates and if soil amendments should be made. Soil types are identified by their ratios of sand, silt, and clay.
- Sandy soil drains quickly and holds little water or nutrients.
- Loamy soil is an even mixture of sand, silt and clay. Ideal for gardening, loamy soil retains moisture and nutrients well while allowing water to flow freely.
- Clay soil is rich in mineral content but it is also compact and acts as a barrier to water drainage.
To make sure the runoff your garden catches will drain quickly enough, dig an 8-inch hole, fill with water and let drain completely. Then fill again and make sure it drains within 24 hours.
Here are some links to easy-to-do tests you can do at home to determine soil type and infiltration rates.
You can get a professional soil analysis from your local Extension office.
Size and Shape
A typical home rain garden is 6-8” deep and ranges from 100 to 300 square feet, or generally 25% - 30% the size of total drainage area, and often capturing runoff from a single downspout.
Rain gardens can be any shape, but a general rule of thumb is a ratio of 2:1 where the length of the garden runs perpendicular to the slope of the yard. Whether a free-form kidney bean shape or a more formal design, it should reflect the conditions of your yard and your personal style.
Factors that will influence the size and depth of your garden include:
- Soil type
- Land slope
- Drainage area
- Percentage of runoff you intend to capture
- Available space.
Use the rain garden calculator or follow the step-by-step instructions in the Show Me Rain Garden Guide to get a more accurate estimate.