Lay It Out and Dig In
Kill Existing Vegetation
If weeds or invasive species currently occupy the area where you plan to install your rain garden, it is best to kill or remove them. Be sensitive to any potential for erosion and take measures to stabilize the soil. Mow or weed-eat the area to a short height first. Lay a thick layer of newspaper, mulch, old carpet or plastic over moved vegetation. Vegetation should be dead within one month. Leave the dead vegetation to prevent soil erosion until you are ready to plant. If you use plastic or carpet, remove it before planting. When you are ready to plant, do not till or disturb the soil, just dig the planting hole, amend the soil if necessary and plant.
Use stakes, flags or a garden hose to establish the outline of the rain garden and/or bioswale. Include any area where a bioswale and/or piping is required to direct the water to the rain garden. Mark a protective perimeter around trees to prevent damage to the root systems. Mark the ground following the “drip line” of the outer tree branches and do not plan to dig within that circle. Do a final assessment of the layout prior to digging.
Special care should be taken not to compact the existing soil while digging. Use of machinery can inadvertently compact soil and cause your rain garden to be ineffective. Hand digging is recommended. Remove the topsoil and save it aside to put back in place once digging is completed. Dig a basin 4–8″ deep for your rain garden.
Applying well-aged compost will help decompact the soil and improve the effectiveness of your rain garden. (See Conquer Compacted Soils for more detailed recommendations.)
Determine if and where the overflow spillway should be located. Since it has the potential for erosion, consider installing permanent erosion control netting/mat topped with 2–3″ gravel/stone to prevent erosion at this high water flow area.
Determine if any other areas are prone to erosion, such as the input from the downspout, etc. When working on a slope, exposed soil is always prone to erosion while plants are getting established. It is recommended that biodegradable erosion mat be laid down and pinned prior to planting. When planting, cut an ×-shaped slit through the erosion netting/mat large enough to adequately set the plant into the soil beneath.
The sides of a rain garden and bioswale should be sloped and the basin smoothed out so that it is level. You can use a carpenter’s level or line level to establish the grade of the finished basin and the sides of the rain garden. The rain garden should be level from side to side and from front to back. Use a level to determine the slope for bioswales so that water moves properly in its intended direction.
When working on a slope, use excavated soil to build a berm on the downhill side to create a level basin and firm the soil as you construct the berm.
Continue on to Plant the Rain Garden