A rain garden should be located at a low point on your property, the place where water flows or puddles after a storm. A gentle slope (about 1-10%) is ideal to navigate the water into the rain garden. The rain garden should be constructed down slope and away from your house, at least 10 feet away from the foundation.
Using rocks or rope, lay out the shape of the rain garden in the selected location; adjust the markers as needed.
Place stakes around the shape of the garden. Next, use a rope to tie one end to the base of the top stake, then attach the other end of the rope to the bottom stake. Try to keep the rope level. The distance from the bottom of the lower stake, to the top of the rope will show the sloped area that will capture the rain water.
Excavate the soil from the staked area, and bring it to the bottom stake- mound the soil as a berm. This berm will help hold the storm water in the rain garden, allowing it to seep underground.
It is critical to install an outlet for water runoff that exceeds the capacity of the garden. You can direct the water from this outlet into a storm drain or even into another rain garden.
Amending The Soil
Once the rain garden has been excavated, you may need to amend its soil. A rain garden should impound water for 12-24 hours, so the soil needs to be porous enough for water to infiltrate quickly. Test your soil's infiltration rate by digging a hole 8 inches wide and 8 inches deep. Fill with water and see how long it takes to soak into the ground. The water needs to go down one inch per hour- if it takes longer, replace 2” of soil with compost
Selecting the plants
It’s important to find plants that can handle both wet and dry conditions. When choosing plants, you may want to consider the height, bloom time and color of each species. See the UMass Extension’s website for a list of plants suited for rain gardens in Massachusetts
In the first year water the plants in the rain garden regularly to encourage healthy growth. After a few seasons, you may also want to prune the plants to maintain the garden’s appearance. Any sediment build-up in the inlet and outlet of the rain garden should also be removed.
Mulching the planting bed will save both time and energy in the rain garden. The mulch will help block weed growth and retain moisture for the plants. About 2-3” of mulch should be added the first year and supplemented annually.