runoff diagramShape and Layout

There are a variety of shapes you can choose from to best fit your yard and existing landscape; click here to see some examples. To help shape the garden, mark the perimeter so you have a defined area to dig and visualize. 

Routing Water

Plan your garden downhill from the water source. If the garden will be more than 30 feet away from your home, consider routing water from a downspout to the garden. Keeping this principle in mind, there are several options for routing runoff from its source to your rain garden:

  1. Grassy swale (or a flat grassy channel) will slow down the water and spread it out as it travels to the garden.
  2. A creek bed feature or small waterfall using a rock-lined channel can slow down the water going into the garden.
  3. A plastic downspout extender can lay on the lawn or be buried to direct rainwater to the garden.

Regardless of how the water is routed, some kind of diffuser should be used at the point where the water enters the garden so plants in the immediate area will not be washed out and to prevent erosion. 

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, use the excess soil to make a berm around the garden area and contain rain water runoff. Place the excess soil along the downhill edge and the sides of the rain garden as shown below.


Excavating a Rain Garden on a Slope

excavating on a slope

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

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. 

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. 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. 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 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. 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 erosion from the slopes. The berm can also be covered with mulch to help hold it in place.