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Maintaining Proper Drainage in the Garden

By Nancy Downs, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver County

Ain’t it Swell?

The snowy weather that dogged us throughout February has cleared off, and signs of spring are everywhere: The garden is back, bulbs that weren’t there yesterday are two inches tall today, streets are tacky with sap, male flickers are hammering away on anything metal, bless their hearts (heads?), and tree buds are swelling.

Unfortunately, buds are not the only thing that swell around here. Soils do, too, and cause a lot of damage to houses. Excess water at the foundation contributes to the problem. That’s why when you landscape this spring, you should examine, and fix, if necessary, the drainage around your house.

In the city, we tend to dismiss swelling soils as a suburban problem but, in fact, our houses sit on swelling soils, too. Outdated landscaping practices and soils that have settled often make our drainage situation worse.

Think about it: new houses typically have 5 feet of rock mulch and a 10% slope protecting their foundations. They also have covenants mandating no planting or watering close to the foundation, and builder’s warranties protecting against damage. Around older houses, like those in Park Hill, soil runs right up to the foundation. Typically, it has settled which has created a reverse drainage problem (that’s technical talk for when the water flows into your basement instead of away from your basement). We also are free to plant right up to the foundation, and we do, then we irrigate those plants. No builder’s warranties protect against damage, either; we’re on our own.

So What Can You Do?

You can start by making sure your roof drainage system works properly. That means cleaning the gutters and downspouts if they are clogged. Make sure there are no rusty or leaky spots where water drips over the eaves and lands next to the foundation. Use splashblocks and downspout extensions to carry the water runoff at least 5 feet from the house. If necessary, rig ‘elbows’ and conduits to carry the water over sidewalks and under patios, driveways and sidewalks. Be creative.

Next, figure out the existing slope around your foundation. Then, if you need to, either add dirt (make sure you compact it!) at the top of the slope, or remove dirt at the bottom to achieve the proper drainage.

To figure out your slope, you will need 2 stakes, 12 feet of string, a string level, a pen, a measuring tape and a hammer. A grade school student who needs to do a science project also is handy, but not required.

Pick a likely spot and pound one stake in the ground at your foundation; tie one end of the string to it. Measure 10 feet of string away from that stake and mark that spot with your pen. Tie the other end of the string to the second stake so that there is ten feet of string between the stakes, and pound the second stake into the ground. Attach the string level to the string between the two stakes, and hammer the second stake into the ground until the string level indicates the string is level.

Okay, now you’re ready: with your tape measure, measure the distance, in inches, on the stake near the foundation between the string and the ground. Call that distance ‘x.’ On the other stake, measure ‘x’ distance from string down. Call the distance that is left over ‘y.’

Determine the percentage of your slope by using ‘y’ in this formula: y’ divided by 120 multiplied by 100 = your slope percentage.

Ideally, you should have a drop of at least 2% - 5%. That means 2 to 5 inches of vertical fall for every 10 feet of horizontal distance.

For more information about landscaping on swelling soils, see CSU fact sheet 7.236 Landscaping on Expansive Soils.

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Date last revised: 01/05/2010