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Remember to Call 811 before Digging

Once the size and location of the garden has been determined, it's time to start building the rain garden. The following steps outline what tools are needed, rain garden shapes, routing water, how to remove the existing lawn turf, digging and leaving the rain garden, mending soil, setting the slope and constructing the berm.

Tools

These tools may be useful when constructing the rain garden. Power tools can make the work easier, but are not essential.

Shape

As mentioned earlier, rain gardens can take a variety of shapes like the ones shown below. Crescents, ovals, teardrops and kidney shapes are popular, but the shape of your garden will be determined by the space you have available, the location and your preferences. Once you have determined the appropriate size for your garden, you should choose a shape that best fits your yard and the existing landscape. To help you shape the garden, mark the perimeter by placing stakes, flags or even a garden hose along the edge of where you want the rain garden to be. Doing this will provide a defined area that you will dig and it will also allow you to better visualize the final size and shape of the rain garden. This is the time to make changes, before you start digging.Images of the shapes can be found here.

Routing Water

If your rain garden will be located more than 30 feet from the home, you may need to plan and construct an arrangement to route water from a downspout to the garden. Although it sounds elementary, remember that water flows downhill, so plan your garden downhill from the water source. Keeping this principle in mind, there are several options for routing runoff from its source to your rain garden:

Removing Lawn Turf

Many rain gardens are constructed in existing lawns. The time and effort it takes to dig out the garden can be reduced by removing the sod first. Sod removal machines are available for rent at some nurseries and tool rental facilities, but a shovel and some hard work can be just as effective. If removed carefully, the turf grass could be reused for patching bare spots around the lawn. As an alternative, you can cover the lawn where the rain garden will be located with black plastic, several layers of newspaper or any disposable material that will block sunlight. Over a period of about a week, the grass will die and it can then be tilled and mulched into the rain garden soil. This can even be done in the fall so that the area is ready for garden preparation in the spring. Using this method, it is not necessary to remove the lawn turf.

Leveling the Garden

Begin by digging into the ground and removing the existing soil from the area where the garden will be located. The garden area should be uniformly deep and have a flat, level bottom. You can check to see if the bottom is level by laying a board across the garden floor with a carpenter’s level on top of the board. Move the board around the rain garden floor to find high and low spots. You can add back soil to fill in the low spots and remove additional soil to level out the high spots. Next, you can use the excess soil to make a berm around the garden area to help contain rain water runoff. Place the excess soil along the downhill edge and the sides of the rain garden as shown in Figure 5.

Figure 5. Excavating a Rain Garden on a Slope

excavating on a slope

Once you have dug out the entire garden area, you may use a tiller, shovel or hoe to loosen the soil that may have become compacted. This will help to promote deep root growth and infiltration of water.

Amending the Soil

Now that the garden is dug, you should decide if you need to amend the soil. As indicated earlier, the type of soil you have affects how fast water will soak into your rain garden. You may contact the Fort Wayne City Utilities for more information about soil amendments.

Developing Rain Garden Slopes

You have just dug out the footprint of your rain garden. Within this footprint, you will dig a slightly deeper depression with gradually sloping sides. This added depth will allow the rain garden to capture water but remain relatively dry between rain storms. The sides of the garden should gently slope downward toward the interior of the garden at about a 3:1 ratio. If the ultimate desired depth for the garden is six inches, the side slopes should be at least three times the depth or about 18 inches long (Figure 6). Because different plants may be more or less tolerant of very wet conditions, you can place plants that like drier soil higher up on the slopes and plants that like more water in the deeper part of the garden.

Building the Berm

The soil that is removed from the rain garden location can be used as a berm around the garden. Mound the soil that was removed from the center of the garden around the outside edge of the garden area on the downhill side. This will create a shallow mound or berm on the downhill side of the garden if you are creating a garden on a slope. Extend the berm out and allow it to gradually taper around the sides of the garden. In a flatter yard, you may need to create a berm around the entire garden to help contain water inside. The berm must be compacted so that it will support the weight of water in the garden. This will also help prevent the berm from eroding if water in the garden overflows. Use a hand tamp to compact the soil for the berm. Grass seed should be planted, or leftover turf grass from the original excavation should be planted on the berm as soon as possible to limit the amount of erosion from the slopes. The berm can also be covered with mulch to help hold it in place.

Click here to see the Rain Garden How-To Manual for City of Fort Wayne Homeowners