Correct watering is the single most important thing you can do for your desert landscape plants. But most homeowners water “quick & shallow” which stresses their plants and wastes water. Learn a better way to water for healthy, resilient landscape plants.
Most homeowners and gardeners water their plants too frequently and don’t give them enough water with each watering. If you’re new to the region or new to gardening, it’s understandable why you might get this wrong.
The default irrigation schedule set up by most landscapers or irrigation technicians is every 3 days for 20 minutes. All year long. And that is absolutely the wrong way to water.
How Deep Watering Grows Healthier Plants
While new plants need water more frequently, once plants are established, they should be watered deeply but not frequently. Here’s why…
Increased Resilience to Heat and Drought
Plant roots actively seek out water and grow towards a water source, a pretty amazing feat!
When you give your plants a trickle of water from an emitter, the roots remain close to the soil surface where the ground is hot and moisture quickly evaporates. This leaves your plants more susceptible to stress from heat and drought.
Watering deeply is training your plants’ roots to move downward where the ground is cool and there’s a reserve of moisture. This makes your plants more resilient to heat and drought.
Increased Resilience to Storm Damage
Every monsoon, an alarming number of desert trees are uprooted. This is due in part to improper watering. Many fallen trees had shallow roots and never developed a strong, deep anchoring root system. Watering deeply will encourage your tree to develop anchor roots.
While watering isn’t the only cause of storm damage, it’s a significant factor that you can control.
Watering Landscape Plants: How Often, How Much?
Learning how to correctly water breaks down to understanding two factors.
- First is how often to water.
- Second is how much water to give your plant with each watering.
How Often to Water
How often to water varies with the type of plant and the time of year.
A look at the “Seasonal Frequency” columns in the chart below might come as a bit of a shock if you’re used to watering every three days. To get your plants on this new schedule, it’s important that you gradually reduce the frequency to give your plants time to acclimate.
How Deep? Follow the 1-2-3 Guideline
This chart also tells you how much to water in the far right column labeled “Water This Deeply.”
You might be surprised (or maybe relieved!) to see that so-called “deep” watering isn’t all that deep — usually 1 to 3 feet. But it’s deep compared to the typical default irrigation setting.
Here’s a recap of the suggested watering depths:
- Trees — to 3 feet
- Shrubs — to 2 feet
- Everything else* — to 1 foot
*succulents, vines, ground covers, perennials, annuals
But Don’t Tree Roots Grow Deep?
You might be wondering whether your tree’s roots go much deeper than this. It’s a commonly held belief that a tree’s root system mirrors that of the canopy, but that’s an outdated model that’s now known to not be true.
This illustration is a more accurate representation of how a root system grows.
According to the University of Arizona, a tree’s root system generally extends 1.5 to 4 times beyond the width of the tree canopy.
And it’s estimated that 90% of tree roots are found in the top foot of soil where oxygen, water, and nutrients are readily available and essential soil microbes reside.
Use a Soil Probe (or This Chart)
So, now you need to make sure you’re watering to the desired depth of 1, 2, or 3 feet — but how do you do that?
You can physically check using a soil probe. It doesn’t need to be anything fancy. Pima County Master Gardeners sell handmade probes made from rebar that are pointy on one end and have a handle on the other.
You simply insert the probe in soil after you’ve watered and measure how far down it goes. In theory, the probe will easily slide through wet soil and stop when it reaches dry soil.
You may or may not have luck with this. I’ve tried and no matter how soaked the ground is, I hit too many rocks to get the probe down more than a few inches.
So instead, I use this chart published by Water Use It Wisely. It tells you how many gallons of water are needed to wet the root zone per plant canopy size. I’ve used these numbers for years and they’ve worked well for me.
Water at the Dripline
And finally, be sure to water a large plant at its dripline. Too many irrigation systems are set up with emitters next to the trunk of a tree or shrub, and that’s where they stay.
Emitters need to be moved outward to the plant’s dripline as the plant grows. The number of emitters needs to grow, as well.
Of course, you don’t have to water large plants with emitters, even if you have an irrigation system. Since mature trees need to be watered infrequently, I use a soaker hose placed in concentric circles at the drip line. This wets all the roots at the dripline, not just those near an emitter.
See these articles for more in-depth information:
How to Water Desert Trees: How Often? How Much?
How to Water Desert Shrubs: How Often? How Much?
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Author Bio
Deane Alban is the creator of Southwest Gardener. She is a science writer with a bachelor’s degree in botany from the University of South Florida. Gardening is her lifelong passion. She’s been gardening in Tucson for 15 years.
