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Ask a Master Gardener: How Do You Make a Rain Garden?

By: Bob Howard, Mobile County Master Gardener


Time, Talent, and Treasure

 • Time – More than a morning: It will probably take a couple of days.

 • Talent – Beginner level: Basically, just digging and planting.  You should expect to do a little sweating and maybe suffer a blister or two.

 • Treasure – Varies: The size of the Rain Garden determines a lot, but complexity and artistry of design adds expense.  The number and types of plants you want to put in the Garden can add up quickly.

Remember the old maxims: “Less is more.” and “It’s easier to add than to subtract.”

Location: You will need a plan for where you need to locate the garden, and more importantly, how the rainwater finds your garden.  Do you need a short swale, 5 to 10 feet long, to channel the water into the garden?  You can use 1½” or larger river rocks to set the course.  Will you need to bury 4” PVC pipe for a longer run of 20 feet or more, or to go under a sidewalk or other obstruction?

Size: Once you choose the location, you will need to do a little math to determine how big the Garden needs to be.  How much water does your Rain Garden need to capture?  If your source of runoff is the roof of your 2,400 square foot home, but only half of that roof drains toward your Garden, then a 1” rainfall would produce 1,200 sq ft of water at a depth of 1 inch. Divide that 1,200 by 12 and you get 100 CUBIC feet of water for your Garden! If your soil will then drain 12 inches over 24 hours, you will need 100 SQUARE feet of garden area to handle the runoff. If your soil drains at only 8 inches over 24 hours, you will need 150 sq ft of Rain Garden.  Remember to use Yard Math: close is okay.  It is also okay to vary the size.  A smaller garden can still yield big benefits. Rain gardens that are 30 percent smaller than ideal can still handle nearly 75 percent of the storm watershed from a house. Of course, you can also make it larger. Most importantly, make sure the size of the garden fits your landscape.  The shape is very much determined by the yard: square, oval, rectangular, round, bean-shape, or whatever fits. Whether you use regular shapes or irregular forms, defined borders or natural lines, the Rain Garden expresses your choices and brings endless possibilities of beauty into your world.

What to Plant: Now, what are you going to plant? Once you know the size and shape of your garden, draw a diagram of WHAT you want to plant and WHERE in the Garden it is most pleasing to you.  Go Native! Use plants that are already acclimated for our Gulf Coast environment.  You can go sun or shade, so chose you plants accordingly.  Choose plants with “average to moist” water requirements for the deepest parts of your rain garden. On the higher edges of the bed, position plants that thrive in “average to dry” water conditions. While it may seem intuitive to purchase moisture-loving plants for your rain garden, don’t do it. Since your garden is designed to drain in 24 hours, the moisture-loving plants will soon be left high and dry.  Leave space for some rocks or woody outcroppings, and always a space for drinking water for our animal friends.

To a Gardener, everything is a Flowerpot.  Find a spot, and make it bloom.

Editor’s Note: The Mobile Botanical Garden Plantasia plant sale is a great opportunity to select plant material for your rain garden.  You can find more detailed information at Search for ANR 2768 and ANR 2893.

From ANR 2768, Section 6 – a more formal rain garden plan for a LARGE bed

From ANR 2768, Section 6 – a more natural rain garden plan for a LARGE bed. You can scale it to fit your own space.

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