A sunken garden is a depression dug into a yard that uses rainwater for hydration and improves drainage in your lawn. You can build a sunken garden in a weekend's time and with relatively basic tools. Although the process is relatively free form, you must balance the size of your garden with the amount of rainfall you generally receive. It is easy to overwhelm a sunken garden with too much water and it is easy to starve a sunken garden of water if you do not receive much rain.
Mark the outline of a 150- to 250-square-foot area with marking paint. Of you choose, you can make the area round or even kidney shaped. If you receive little rainfall each year, keep towards the smaller size. If you receive a lot of rainfall, stay towards the larger scale. The site should be away from your home and, ideally, in a natural depression, where you notice water pooling after heavy rains.
Dig a hole inside the marking paint approximately 3- to 6-inches deep. Keep the hole shallower if you receive little rainfall and keep the hole deeper if you receive a lot of rainfall. Sunken rain gardens in yards that are primarily clay soil should be dug more on the shallow side as well, according to rain garden expert Sue Ellingson.
Create a 3-inch lip around the perimeter of the area with dirt you removed from the centre. Plant grass seed on the lip. This will prevent erosion and help to add beauty to your sunken garden.
Plant perennial flowers of foliage plants in the manner listed on the packaging for the plant. Ellingson recommends plants native to the area and those that don't require either overly wet or dry conditions, as the amount of water in your sunken garden will vary.
Place a 1- to 2-inch layer of bark mulch around the plants to create a covering over the entire garden.
Slide the downspout extension tube onto the downspout. Use an extension that is long enough to reach the lip of your garden. When it rains, the water from your gutters will flow into the garden to increase hydration.
If the garden is too far away from your house for a downspout extension to adequately redirect runoff water, consider digging swales or laying underground pipe to drain into the sunken garden.