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Monday, April 9, 2018

When to Change the Irrigation Clock and How Often

Q. You mentioned increasing how often you water, not the number of minutes, as the growing season changes. When are these changes made and how often should they be watered?

A. Irrigation schedules can be important but they can also cause problems if some flexibility in this schedule isn’t provided when the weather isn’t cooperating. This cool, wet spring was the perfect example.

Irrigation Should Follow a Seasonal Pattern

            Scheduling irrigations should follow a seasonal pattern that increase the frequency of applied water as the season moves from spring to summer to fall. This is much easier to do than adjusting the number of minutes of applied water.

These are daily averages of water use of plants in the Las Vegas Valley as they vary from month to month. The numbers 1 – 12 correspond to the months of January (1) through December (12).On top of each bar is the daily average of water use for the month in inches of water per day.Notice in June (6) and July (7), the hottest months, water use is 4/10 of an inch per day. While in January (1) and December (12), the coolest months, water use is 1/10 of an inch of water per day.This represents a 400% increase in water use from mid-winter to Midsummer.

Basic fruit tree irrigation schedule for Southern Nevada

  • First week of February (irrigate once per week)
  • Last week of April – first week of May (twice per week)
  • Midsummer, late May – June (three times per week)
  • last week of August – September (twice per week)
  • last week of October – November (once per week)
  • second week of December (winter schedule, leaf drop, 10 – 14 days)

Using the Soil Moisture Probe For Fine-tuning

The concept of an irrigation schedule is sound but some hands-on judgment should be applied when irrigation changes are considered. These judgments help “fine tune” a schedule around unseasonal weather conditions.
             Knowing how much water is remaining in the soil is a critical hands-on judgment. It is impossible to look at the surface of the soil and know how much water is around the roots. A piece of equipment I have found valuable is a heavy-duty, soil moisture sensor. The one I like has a 24 inch stem and can be purchased online from stores like Amazon for about $70. I will put a link to it on my blog.
Soil moisture probe with the moisture sensing device located at the tip of the probe. This is a heavy duty model that can be pushed into most desert soils without breaking. Push the probe into the soil slowly to get moisture measurements near the surface of the soil and deeper as it is pushed deeper into the soil. Moisture sensor is located at the tip of the probe. Measure soil moisture around trees at three to four locations for each tree and midway between drip emitters

The moisture sensor is located at the tip of the probe

  • measure soil moisture at three or four locations at each tree
  • slowly push probe into the soil to root depth (2 to 8 inches) as you watch the meter
  • meter needle should indicate more moisture as the probe is pushed deeper
  • irrigation is needed when the average moisture level has dropped to "6" or below
Moisture 8-10 No irrigation needed
Moisture 7-6  Irrigate soon
Moisture 5-6 Irrigate now
Moisture 1-4 Reserved for cacti; too dry, possible plant damage

Water in the soil is like the gas tank of a car

Think of the soil surrounding fruit tree roots like the gas tank of a car. We fill a gas tank after we drive the car for awhile. When irrigating, use about 50% of this “gas” before filling the “tank” again. 

During the winter months, when fruit trees use very little water, we fill the gas tank infrequently. During the summer months when we are “driving around a lot”, fill the gas tank more often. 

Change the watering frequency, not the number of minutes.


  1. How many gallons is an inch of water?

    1. An acre-foot of water is roughly 325,900 gallons. An acre inch is roughly 1/12 of that which is 27,158 gallons. One inch of water in a one cubic foot container is 7.48/12 = 0.62 gallons. One inch of water applied to pure sand will penetrate to a depth of a little over 20 inches deep. Fine sand 14 inches deep. Fine sandy loam 10 inches. Silt loam 7 inches and clay loam 6 inches. The amount to apply to a plant is determined by the depth of its roots. The shallowest rooted plants are lawns, annual flowers and annual vegetables. We assume the rooting depth of these plants is less than a foot. The next depth are perennial plants that are small with a rooting depth of 12 to 18 inches. And finally trees and large shrubs with an effective rooting depth of about 24 inches. Larger plants are given more water but watered less often because their "gas tank" is much bigger. Plants that are shallow rooted like lawns, annual flowers and vegetables are watered much more often because their "gas tank" is much smaller. This is why it is very important to put these groups of plants (lawns/flowers/vegetables, medium sized plants, trees and large shrubs) on separate irrigation valves so they can be wagtered separately at different times. Fourth and fifth categories could be argued; desert plants and cacti.