About the SFMGA

Saturday, April 26, 2008

Building a Desert Rainwater Garden

©Robert Dailey, Editor, http://desertgardens.suite101.com/

Even with a few inches of rainfall a year, desert gardeners can build a rain garden. Rain gardens are simple, economical ways for desert gardeners to collect rainwater and place it exactly where they want it to go.

When rainwater (or melted snow) runs off a roof, it generally goes down a gutter, onto a splash plate or a French drain and then is shunted away from the house.

For gardeners (and anyone interested in water conservation) there are simple methods to use that water immediately on plants or collect it for later use. The gardener also gets instant gratification the first time it rains.

Building a rain garden

Do you know where your rainwater is?

In general, runoff from a flat roof is 600 gallons of water per inch of rain per thousand square feet of catchment area. Here is the basic formula for calculating the potential amount that can be collected: Multiply the catchment area of building by inches of rain by 600 gallons. Then divide the answer by 1000 to give you the approximate gallons of water coming off your roof.

Example: a 1000 square-foot roof times 2 inches of rain times 600 gallons = 1200,000 divided by 1000 = 1200 gallons of water

Any collection system needs a way to catch runoff. By far the simplest way is to construct a holding area in the landscape: build a rain garden. That's simply a concave area in your yard, garden, hillside, or any other area that will collect any runoff directed to it.

Instead of building a complex drainage system, simply place the collection area where gutter-fed water from the roof will drain into it. Gravity will do the rest.

Next, build a small bermed edge around the concave area. This will help retain more water long enough for plants to absorb the water. Remember that the concave area doesn't have to be circular. It can be elongated (like a dry streambed), irregularly shaped (kidney-shaped for example) or it can meander through other plantings.

Plant the concave area with native vegetation. If you'd like, use true desert plants that tend to bloom when watered by irregular rainfall.

How big do you make your rain garden?

The size of your rain garden depends on the size of the roof or surface area from which the water is drained and the amount of precipitation you receive.

And a rain garden doesn't have to be gargantuan. It can be a few feet or larger in diameter or length.

Rain gardens can also be developed on hillsides. Channels, streambeds, ditches and swales can be built to move and retain water.

A small hillside garden a few feet in diameter can help stabilize a slope. Eventually the vegetation planted there will colonize other areas around it.

Other rain garden catchment areas

Another example of a catchment is the area directly below the drip line from an eave. Make it slightly concave so rainwater will tend to collect along it. Then plant it with vegetation.

Even a slightly sloping sidewalk can be used to direct water into a small holding area where it can nourish plants.

If you've got a paved driveway, this is another opportunity to create a small rain garden at the base of it.