How to Make a Sunken Garden: Grade Plus Drainage
Learning how to make a sunken garden means planning for water first. Any bed set below the surrounding grade collects runoff. Mississippi State University Extension notes that water naturally follows the lowest plus easiest course.
The short answer
Decide where water will flow before you dig. Set a shallow bed below the surrounding grade. Edge it with low walls or terraces plus add steps where people enter. UConn notes that rain gardens are shallow depressions planted plus covered with mulch, which is a useful model for a sunken bed.
Why plan drainage first?
A sunken bed is a low spot, so water from roofs plus paving may run into it. UConn notes that home rain gardens can be built to accept runoff from a roof or other hard surface. WSU Extension describes how rain gardens filter stormwater through a special soil mix.
If you do not want the bed to hold water, plan a drainage channel. Mississippi State University Extension advises retaining or creating permanent drainage channels on slopes.
How do walls plus steps shape the space?
NC State Extension describes how moving up or down within the ground plane changes how a space feels. A simple wood terrace with soil piled against it slows rainwater so it can soak in. Used bricks or broken concrete can form retaining walls.
| Feature | Purpose | Watch for |
|---|---|---|
| Shallow depression | Holds plants below grade | Runoff from roofs plus paving |
| Drainage channel | Carries water away | Gullies on steep ground |
| Terrace or low wall | Slows water so it soaks in | Material that suits the site |
Source: UConn plus Mississippi State University Extension.
Which plants suit the bed?
Mississippi State University Extension notes that shrubs, groundcovers plus perennials slow runoff better than lawn. On shady sides, ferns plus low vines carpet the soil. Mulch bare soil between plants, as UConn describes for rain gardens.
How to: step by step
- Note where water flows during rain.
- Add a drainage channel if needed.
- Excavate a shallow bed below grade.
- Set low walls or terraces at the edges.
- Place steps where people enter.
- Plant the bed plus add mulch.
Troubleshooting
Bed floods after rain: runoff flows in. Add a drainage channel or redirect water.
Soil washes from the edges: bare slopes. Plant shrubs or groundcovers to slow water.
Gullies form: rushing water. Mississippi State University Extension suggests stones or check dams in gullies.
For related reading see soil drainage in beds, garden microclimates plus frost damage. Kind Seed Co also stocks cannabis seeds.
Sources
- Rain gardens, Washington State University Extension
- Rain gardens, UConn CT NEMO Program
- Extension Gardener Handbook chapter 19, NC State Extension
- Gardening on steep slopes, Mississippi State University Extension
FAQ
Is a sunken garden the same as a rain garden?
Not always. UConn describes rain gardens as shallow depressions with plants plus a mulch layer that accept runoff from roofs or other hard surfaces. A sunken garden also sits below grade but may be built mainly for design. Both need a plan for where water goes after heavy rain.
How large is a typical rain garden?
UConn notes that a typical home rain garden covers 50 to 100 square feet. The size depends on the area that drains to it. A sunken garden built for design can follow the same idea by matching its size to the amount of water that will flow into it.
What can be used for retaining walls?
NC State Extension notes that used bricks or broken concrete can be used for retaining walls. It also suggests choosing materials that match the site, such as stone for walls in an area where that stone is found naturally. Matching materials ties the sunken space to the rest of the yard.
Why do sunken spaces feel different?
NC State Extension explains that steps or paths that move up or down change the sensory experience of a space. Moving from open to enclosed areas creates a gradual change. Stepping down into a lower bed creates this shift, which gives a sunken garden its sense of enclosure.
Which plants hold soil on the edges?
Mississippi State University Extension notes that trees plus shrubs form a network of roots plus stems that bind soil plus slow rushing water. Lawn binds soil but slows runoff poorly. On shady edges, low vines, ferns plus perennials carpet the soil plus bind the upper layers.
