Rain gardens are an attractive green solution to reduce storm water pollution and improve overall water quality. By directing storm water runoff to a rain garden filled with native plants, pollutants can be absorbed by the deep plant roots instead of contaminating our rivers, lakes and streams.
Step One: Siting & Sizing the Garden
Home rain gardens can be located ten feet or more away from the house to catch roof runoff, or further out in the lawn to collect water from the lawn, roof and driveway. When considering placement of your rain garden, think about how the garden can be integrated into existing landscaping. Also, pay attention to views from inside the house as well as those throughout the landscape. Determine how far away or how close you want your rain garden to be to outdoor gathering areas. For example, why not locate it near a porch or patio where you can enjoy the colors and fragrances?
Garden Size & Shape
The size of your rain garden will depend on the following factors:
- What type of soil do you have?
- How much roof and/or lawn will drain to the garden?
- How deep will it be?
It is very important to identify your soil type: sandy, silty or clay. Sandy soils provide the fastest infiltration; clay soils have the slowest. Since clay soils take longer to absorb water, rain gardens in clay soils must be larger than rain gardens in sandy or silty soils. If the soil feels gritty and coarse, you probably have sandy soil. If your soil is smooth, but not sticky, you have silty soil. If it is very sticky and clumpy, you probably have clay soil.
If you have questions about your soil, call the Leon County Agricultural Extension Office at (850) 606-5202.
A typical rain garden is between four and eight inches deep. A rain garden deeper than eight inches may pond with water too long, resembling a hole in the ground, and possibly creating a safety hazard for anyone who accidentally steps into it. Additionally, a garden less than four inches deep will require a large surface area to contain water runoff generated from heavier rainstorms.
No matter the depth of the garden, the goal is to level the ground surface of the garden. Digging a very shallow rain garden on a steep lawn will require bringing in extra topsoil to raise the downhill border of the garden to the same height as the uphill part of the garden. As your slope gets steeper, it is easier to dig the garden a little deeper to level the ground surface.
Slope of your Yard
• 2 wooden stakes
• String level or carpenter’s level
• Possibly a calculator
- The slope of the lawn should determine the depth of the garden.
- Find the slope of your lawn by following these steps:
- Pound one stake into the ground at the uphill end of your rain garden site, and pound a second stake in at the downhill end, about 15 feet away.
- Tie a string to the bottom of the uphill stake and run the string horizontally across the garden site to the downhill stake.
- Using a string level or a carpenter’s level, make the string level and tie the string to the downhill stake at that height.
- Measure along the string between the two stakes for the horizontal width.
- Now measure the vertical height on the downhill stake between the ground and the string.
- Divide the vertical height by the horizontal width and multiply the result by 100 to find the lawn’s percent slope. If the slope is more that 12 %, it’s best to find a more gently sloped site, or talk to a professional landscaper.
The surface area or square footage of the rain garden can be almost any size, but time and cost will always be important considerations in sizing decisions. Any reasonably sized garden will capture some stormwater runoff, and every bit you capture helps.
A typical residential rain garden ranges from 100 to 300 square feet. Gardens smaller than 100 square feet will limit the number of plants you can plant. The larger the rain garden, the more opportunity you have to plant a variety of plants. A large garden of more than 300 square feet will take longer to dig and will be more difficult to make level.
Step Two: Building a Rain Garden
Digging the Rain Garden
- While digging the garden to the correct depth, heap the soil around the edge to form a berm – a low dirt “wall” around three sides of the garden that keeps the water in during a storm. On a steeper lawn the lower part of the garden can be filled in with some soil from the uphill half and extra soil might need to be brought in for the berm.
- Create the shape of your rain garden by laying a garden hose around the area you wish to use. Remember that the berm will go outside the hose. Next, put stakes along the uphill and downhill sides, lining them up so that each uphill stake has a stake directly downhill. Place a stake every five feet along the length of the garden.
- Begin at one end of the garden and tie a string to the uphill stake at ground level. Tie it to the stake directly downhill so that the string is level. Work in five-foot-wide sections, with only one string at a time. Otherwise the strings will become obstacles.
- Start digging at the uphill side of the string. Measure down from the string and dig until you reach the depth you want the garden to be. If the garden will be four inches deep, then dig four inches down from the string. Refer to Figure 5 for guidance.
- If the lawn is almost flat, you will be digging at the same depth throughout the garden and using the soil for the berm. If the lawn is steeper, the high end of the garden will need to be dug out noticeably more than the low end and some of the soil from the upper end can be used to fill in the lower end to make the garden level. Continue digging and filling one section at a time across the length of your garden until it is as level as possible.
- In any garden, compost will help the plants become established, and now is the time to mix in compost. A roto-tiller can make mixing much easier, but it is not necessary. If you do add compost, dig the garden one or two inches deeper than planned. Then add one to two inches of compost.
Leveling the Rain Garden
• Tape measure
• Shovels, rakes, trowels
• Carpenter’s level
• Wood stakes at least 2 feet long
• Garden hose
• A six-foot 2 x 4
- One way to check the level of the garden is to just “eyeball” it. For more accuracy, follow these steps:
- When the whole area has been excavated to about the right depth, lay the 2×4 board in the garden with the carpenter’s level sitting on top. Find the spots that aren’t flat. Fill in the low places and dig out the high places.
- Move the board to different places and different directions, filling and digging as necessary to make the surface level.
- Once the garden is as level as you can make it, rake the soil smooth.
Making the Berm
- Water flowing into the garden will naturally try to escape over the downhill edge. A berm is critical to help hold the water inside the garden (look back at Figure 5). The berm is a “wall” across the lower border and along the sides of the garden. The berm will need to be the highest at the downhill edge, and should be as high as or slightly higher than the uphill edge. Moving along the sides up toward the front edge of the garden, the berm will gradually become lower and finally taper off by the time it reaches the top of the garden.
- On a more gradual slope there should be plenty of soil from excavating the garden to use for a berm. On a steeper slope, most of the soil from the uphill part of the garden will be used to fill the downhill half and additional soil may have to be brought in for the berm. After shaping the berm into a smooth ridge about a foot across, tamp it down to compact the soil. It is important to have a strong, well-compacted berm, so tamp hard. The berm should have very gently sloping sides; this helps to smoothly integrate the garden with the surrounding lawn and also makes the berm much less susceptible to erosion.
- To prevent erosion cover the berm with mulch, lay sod, or plant a ground cover. You can also use straw or erosion control matting to protect the berm while the grass becomes established. If you don’t want to lay sod or mulch over the berm, you can also plant drought tolerant vegetation or winterize the berm with rye grass.
Step Three: Planning and Maintenance
Planting the Garden
- Select one or more types of plants that have a well established root system. Nursery-propagated plants are best and three or four types of plants should be enough.
- Try to have at least a rough plan for where each plant will be placed. Lay out the plants as planned, keeping appropriate distances between plants.
- Dig each hole twice as wide as the plant and deep enough to keep the crown of the plant level with the existing grade (just as it was in the cell pack or container). Make sure the crown is level and then fill the hole and firmly tap around the roots to eliminate air pockets.
- Apply mulch evenly over the bed, about two inches thick.
- Place plant labels next to each individual grouping. This will help identify your young plants from non-desirable species (weeds) when you weed the garden.
- As a general rule, plants need one inch of water per week. Use a simple rain gauge to measure the amount of water the plants are receiving. Be sure to water the garden immediately after planting and continue to water several times each week (unless it rains) until the plants become established. You should not have to water your garden once the plants begin to thrive on their own.
Rain Garden Grant Application
Click here to view the rain garden grant application!