Materials: Shovel, Ruler (12 inches or longer), Pencil, Compost, Native moisture-loving plants (see Nat’l Fish & Wildlife Native Plants on Web), Fine shredded mulch, Decorative rock (optional).
Step 1: Contact your Miss Utilities (free) (electricity, gas, phone) to have them mark the location of underground wires or cables.
Step 2: Pick a location. A rain garden should be several feet from foundations, septic systems, utility lines and fence posts. You may wish to extend a downspout to reach the rain garden.
Step 3: Measure drainage rate. Dig a hole about the size of a large coffee can. Insert a ruler or stick into the hole. Fill the hole with water from hose and mark the water level on ruler. Wait four hours, then measure and mark the water level again. To determine the daily percolation, take the amount that has drained in four hours and multiply that by six. (Follow this formula: __ inches every 4 hours x 6 = __ inches every 24 hours)
Your rain garden should empty within 24 hours, so if you can drain 6 inches in that much time, dig 6 inches down. If the water in your test hole doesn’t drain well, consider different placement, or add gravel, sand and then compost (see Step 7).
Step 4: Determine the garden’s depth. It should be no more than 6 to 8 inches deeper than the surrounding soil, but you can place it in the bottom of a larger landscape depression or slope.
Step 5: Outline the garden location. Use string and wooden stakes or a garden hose to mark the general placement. Think about the land’s slope and where heavy rain may come in and flow out; don’t orient the garden so that overflow runs into your foundation or septic system. Dig. Slope the edges and make the garden a natural contour, semi-circle not square or rectilinear.
Step 6: Check the drainage rate again. Fill the depression with water, then measure the rate. If the drainage is poor, remove 3 to 4 more inches of soil and mix in some sand, gravel, and compost to a depth of 1 foot, then check drainage again.
Step 7: Add plants that can tolerate “wet feet” in the lowest places. Add compost, leave it at least 6" below the surrounding surface so it can 'pond' with runoff which will infiltrate.
Step 8: Mulch 2-4" thick (leaving space around plants stems) to keep the weeds out. Water, especially if rain is scarce 1 inch at least once a week. If there’s regular overflow from the depression, you may build a series of rain gardens with connecting drainage notches.