How to build a rain garden: pick a low, sunny spot at least 10 feet from the house where a test hole drains within two days. Dig a flat basin 4 to 10 inches deep, sized at 10 to 20% of the roof or paving draining to it. Plant wet-tolerant natives in the center, drier ones on the rim.
This is the Potting Shed, the project blog run by TN Nursery, a third-generation family nursery in Altamont, Tennessee, growing native trees and plants since 1959. Here is how to build a rain garden: a shallow, planted bowl that catches runoff from a roof or driveway and lets it soak in. We cover each step, from site to first-year care, and how to plant the basin with the bare-root, dormant plants and live stakes we ship.
How to build a rain garden: tools and materials
- String and stakes, or a garden hose, to mark the outline
- A line level (a small level that hooks onto string) to check the bottom is flat
- A spade, a flat shovel and a garden rake
- A tape measure and a bucket of water for the drainage test
- Coarse sand and compost, if your soil is heavy clay
- Flat stones or river rock for the inlet, where water flows in
- Shredded hardwood mulch
- Your native plants, sorted by zone before you start
Watch out: Call 811 a few days before you dig so buried utility lines can be marked.
Rain garden steps, from site to soil
1. 1. Pick the spot
Watch where water runs in a hard rain. Choose a low spot downhill from a downspout or driveway, in full or part sun. The University of Minnesota Extension advises at least 10 feet from buildings, so water stays away from the foundation and basement.
Tip: Skip spots where water already stands for days. That is a wetland, not a rain garden site.
2. 2. Test how fast the soil drains
Dig a hole about 10 inches deep and fill it with water. The University of Minnesota Extension says the site works if the water is gone within 48 hours. Penn State Extension is stricter: past 24 hours, it suggests another spot. If your hole is slow, plan on the clay soil mix in step 5.
3. 3. Size the basin
Measure the roof or paved area that drains to the spot. Penn State Extension sizes a rain garden at 10 to 20% of that area. Its method for a 1-inch rain: multiply the area by 0.083, then divide by the depth in feet. For example, a 400-square-foot roof section gives 33.2 cubic feet of water. At 6 inches deep (0.5 feet), that needs a basin of about 66 square feet.
Tip: A long, narrow shape set across the slope catches more water than a round one of the same size.
4. 4. Dig the basin and build the berm
Mark the outline with string or a hose. Strip the grass. Dig the basin 4 to 10 inches deep with a flat, level bottom, so water spreads out instead of pooling at one end. On a slope, pile the dug soil on the downhill side as a low berm, a raised lip that holds the water in. Line the inlet with stones so runoff does not wash out the soil.
5. 5. Mix the soil
Loose, loamy soil can go back as it is. Heavy clay drains slowly, so dig a few inches deeper and refill with a lighter mix. UC Agriculture and Natural Resources gives a mix of 50% sand, 25% soil and 25% compost. Fill to your finished depth, then rake the bottom level again.
Native plants for a rain garden, by zone
A rain garden has three zones. The center holds water longest after a storm. The slopes get wet, then dry out. The rim and berm stay dry most of the time. Put each plant where its roots like the water.
Wet center
- Swamp milkweed: our Milkweed Plant in its standard form is Asclepias incarnata. The Missouri Department of Conservation lists it for rain gardens as a monarch caterpillar host.
- Blue flag iris: the Blue Flag Iris Plant has blue-purple blooms and long, sword-shaped leaves.
- Buttonbush: Buttonbush Live Stakes are dormant cuttings of a wetland shrub that bears round, spiky flower balls. They come in bundles, so set a few in the wettest spots and use the rest along a ditch or stream bank.
Slopes
- Ironweed: the Ironweed Plant carries dense clusters of purple flowers that draw monarch butterflies. Set it on the sides, where water passes through.
Dry rim and berm
- Black-eyed Susan: the Black Eyed Susan Plant has yellow flowers with dark brown centers.
- Purple coneflower: Purple Coneflower has pink-purple petals around spiky orange centers.
Just outside the rim
The Brooklyn Botanic Garden lists pawpaw among its rain garden trees and shrubs. Plant a Pawpaw Tree a few feet beyond the berm, where the soil stays moist but never floods. It is a native fruit tree. It ships bare-root, with a 1-gallon potted option as well.
| Zone | How wet | TN Nursery plants |
|---|---|---|
| Wet center | Holds water longest after rain | Swamp milkweed, blue flag iris, buttonbush |
| Slopes | Wet, then dries | Ironweed |
| Rim and berm | Dry most of the time | Black-eyed Susan, purple coneflower |
| Just outside | Moist, never flooded | Pawpaw |
Planting bare-root plants and live stakes in the basin
Most of our plants ship bare-root and dormant, with no soil around the roots. That suits a rain garden well. Plant them in the dormant season, while the ground is workable and rain is regular.
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Open the box the day it arrives
Check that the roots are damp. If they feel dry, mist them and keep them in the shade.
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Soak the roots
Stand the bare roots in a bucket of water for a few hours before planting. Keep them wet until each one goes in the ground.
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Plant from the center out
Start in the wet center, then work up the slopes to the rim. You stay off the soil you have already planted, and the basin floor stays loose.
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Set each plant at the right depth
Dig a hole wide enough to spread the roots. Set the crown, where roots meet stem, level with the soil. Firm the soil and water it in.
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Push in the live stakes
Push each buttonbush stake into moist soil with its buds pointing up. Bury most of its length so only a short top shows.
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Mulch the slopes and rim
Spread shredded hardwood mulch around the plants, clear of the stems. Keep it thin in the center, where light mulch can float away.
Tip: Plant in groups of three or more of one kind. A drift of one plant fills in faster than a scatter of singles.
First-year care
- Water in dry spells. New roots are short. Water weekly when it has not rained, until the plants are settled.
- Pull weeds early. Weed every couple of weeks in the first season, before the natives fill in.
- Check after big storms. Look for washed-out soil at the inlet, bare spots or plants pushed out. Add stones or replant as needed.
- Watch how long water stands. If water sits far longer than your drainage test, the bottom may be packed. Loosen it before planting again.
- Leave stems standing in winter. Cut old stems back in late winter or early spring, before new growth starts.
Rain garden questions
How deep should a rain garden be?
Most rain gardens are 4 to 10 inches deep, the range the University of Minnesota Extension gives. Penn State Extension uses 6 inches as its standard depth when it sizes a garden. Keep the bottom flat and level so water spreads across the whole basin instead of pooling at one end.
What is the recommended soil mix for a rain garden?
Loose, loamy soil that passed the drainage test can go back in as it is. For heavy clay, UC Agriculture and Natural Resources gives a mix of 50% sand, 25% soil and 25% compost. Dig a little deeper, refill with the mix, and rake it level.
What is the best plant for a rain garden?
There is no single best plant, because each zone holds a different amount of water. Swamp milkweed, blue flag iris and buttonbush suit the wet center. Ironweed suits the slopes. Black-eyed Susan and purple coneflower suit the drier rim and berm.
How do you build a rain garden on a slope?
Dig into the uphill side and pile that soil on the downhill side as a low berm. Keep the basin floor level from side to side so the water spreads out. A long, narrow basin set across the slope works better than a round one.
What are the disadvantages of having a rain garden?
It takes a day or two of digging, plus regular weeding in the first year while the plants fill in. It also fails on soil that drains too slowly, so water stands too long. Do the drainage test before you build to avoid that.
