The Watering Mantra for Desert Landscapes 
You may have heard Arizona landscape experts saying, “water deeply but infrequently,” but what does that mean? If you are originally from a part of the country that gets significant rain throughout the year, watering means running the sprinklers or hose occasionally during a dry spell. Arizona is one of the few places where your landscape will need a consistent irrigation schedule because Mother Nature will not provide enough on its own… well, unless you have all native hardy plants which we whole-heartedly support.
The reason experts say “water deeply but infrequently”, is because it’s very important in the desert to have deeply rooted plants. The heat and high evaporation rates that occur during our hot summer months do not as quickly affect deeper soil levels like it does the first few inches, which can dry out in a matter of hours. Healthy trees and plants sink their roots to a level that is cooler with more consistent soil moisture. This allows the plants to be more resilient during the summer. We help our plants get deep roots by watering deeply so that the roots chase the water downward.
Water to different depths into the soil depending on plant type.
Watering Depth for Different Types of Plants
How deep the water needs to go is dependent on the plant type. The roots of your grass lawn will go to a depth of about 6-10 inches. Annuals, flowers, groundcovers, and cacti roots can get to a depth of about 1 foot, shrub roots can get to a depth of 2 feet, and trees a depth of 3 feet. If you’re wondering what depth of moisture your current watering amounts are providing, get a soil probe and probe the ground about 30 minutes after the irrigation cycle has stopped. If the probe hits dry ground in our soils, the probe will go no further; in wet soil the probe will sink in with consistent pressure. If your probe only goes about 4 inches after watering your grass for 10 minutes, then you would need to double your run time (cycle and soak if needed) to get closer to the goal of 8-10 inches. The opposite is true as well. If your grass water depth is 2 feet, and there are no shrubs or trees on that same valve (and they shouldn’t be), then cut your run time down.
Watering Frequency for Different Types of Plants
Use a soil probe to test watering depth.
The beauty of watering deeply is that the water will hold in the ground and be available to the roots for much longer. This provides a less stressed plant that is much more resilient and healthy. You can use that same soil probe to figure out the frequency, or when you need to water next. Once you are watering at the right depth, come out every few days and check the soil with the probe again. When the probe can only go down an inch or so, and the soil looks pretty dry on the surface, it’s time to water again. The soil should dry out between watering to prevent rot and disease. If you see moss, algae, or have mushrooms growing, it’s a sign the ground never dries enough between watering.
Drip Means Small Quantities Trickle Slowly
For those new to drip irrigation systems, know that it is common to need a run time of several hours for one watering cycle. Each one of those drip emitters is putting out around 1-4 gallons of water per hour, depending on the size emitter you have and how many per plant. Having a drip system run for only minutes is equivalent to spitting at the ground, which does not get water to the plants’ roots. The best way to know you are watering deep enough is to go out there with your soil probe, waive to the neighbors, and start stabbing the ground around your plants – your landscape will thank you!
Remember that a drip emitter typically applies only 1-4 gallons each hour!
Epilogue
We can take a lot of the guesswork out for you in our interactive watering guides of our booklet. Find our site for lawn watering and one for plant watering. We provide estimates of the gallons of water needed to get to the depths mentioned above. If you plug in your irrigation watering rates (we’ll tell you how), we can also provide you with estimated run times. We even have a flipbook version that you can find here Landscape Watering by the Numbers: A Guide for the Arizona Desert.
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Victoria Castor is a Sustainability and Water Conservation Coordinator with the City of Peoria, AZ, one of 18 Water– Use It Wisely partners to offer water-saving advice and programs.
