Step 2: Determine the Size and Shape

Determining the size and shape of the garden area
Photo: William Wright
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Your local extension office may have information to help you size a rain garden to suit rainfall patterns typical in your area. The ideal size might be smaller than you expect. On well-draining soil in rainy western Washington, where this project took place, a rain garden just one-tenth the size of a roof handles 99 percent of its gutter water. But if you're short on space or puzzled about how to calculate the size, you can always put in a small rain garden and figure that the good it does will at least be better than what's happening now. If you want an impressive-looking garden, make it at least 150 square feet. Ovals, kidneys, and teardrops often look best, but rain gardens can also be long and skinny. Use a garden hose to test possible shapes.

Once you settle on a design, decide where the water will flow in and where any overflow will exit. Mark the shape with spray paint. On your lawn, mark 18 inches farther out for sod removal, since grass has a way of creeping into planting beds. Also mark any other areas you want to excavate. The outline of this rain garden juts out at the bottom to show the perimeter of a dry well, an optional underground storage basin for excess water when the rain garden overflows.
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    Tools List

    • bucket
      Bucket
    • garden hose
      Hose
    • Gas powered sod cutter
      Sod cutter or spade
    • to remove grass layer. Sod cutter can be rented for $80 a day
    • pointed shovel
      Shovel
    • or rent a backhoe for $230 a day
    • metal garden rake
      Rake
    • wheelbarrow
      Wheelbarrow
    • hacksaw
      Hacksaw
    • hand trowel
      Trowel

    Shopping List

    1. Spray paint

    2. Compost

    3. Stones

    4. Plants

    5. Smooth or corrugated piping to connect to a gutter downspout