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How To Choose

By Larry Keesen — March 2004

There are five things every homeowner should know when it comes to watering the lawn. How does the grass use water? What kind of soil type is it and what is its moisture holding capacity? How much is the allowed water depletion from the root zone, and when & how much should you water.

But first some plant watering myths. Deep root watering does not work. The active root zone in turf grass is usually 3 to 4-inches deep at most. True a few roots may go deeper, but roots do not grow through dry areas to search out moisture, and most of the water uptake by roots occurs in the top 2-inches of the turf root zone. The active root zone for trees and shrubs is in the top 12 to 18-inches of the soil, so watering below that level with a root feeder or surface watering can be a waste of water.

Soil water extraction by plants will vary, but a safe assumption for calculating soil water extraction is the 40-30-20-10 rule. This rule states that 40 percent of the water will be withdrawn from the top 25 percent of the root zone, 30 percent will be withdrawn from the second 25 percent of the root zone, 20 percent from the third 25 percent, and 10 percent from the lowest 25 percent of the root zone.

Every third day watering throughout the growing season does not save water. The turf needs to be watered when 50 percent of the moisture is gone from the active root zone. This time span varies depending on the time of year, how hot it is, and other climatic factors.

Transpiration occurs when the plant moves water through the roots and stomates into the air as a result of a difference in tension or atmospheric suction. The plant uses this water to draw up nutrients, maintain structure and to cool the leaf surface. The description of this process and how much water it requires is called evapotranspiration (ET). ET is the combination of transpiration and water that is evaporated from the soil surface. It is usually defined as the quantity of water (in inches) that needs to be replaced in the soil in order to maintain ideal plant growth and appearance. Evaporation from the soil surface usually affects only the top few inches of soil. Daily irrigation of a clay soil with turf grass can result in a higher soil evaporation rate than every other day or every third day watering. This is not much of a problem in sandy soils because the water drains faster. Crop cover and mulches also reduce the amount of soil evaporation.

The climatic factor that affects ET is air temperatures, wind speed, relative humidity, and solar radiation. Solar radiation is one of the primary climatic factors that effect ET. This is the source of energy necessary to convert water to vapor in both plants (transpiration) and soil (evaporation). Solar radiation peaks in late June in the Northern Hemisphere at latitudes above 22 degrees. Temperatures usually peak in June or July, and after these months, the ET rate begins to decline leading to reduced irrigation through the remainder of the season.

Soil is the water storage reservoir or ”fuel tank” for most plant material. Additionally, water movement into the soil is important in order to prevent runoff and conserve water. How plants interact with soil and their available water supply is critical to understanding how water conservation practices can be achieved.

SOIL types and texture are determined by the particle size of minerals that make up the soil. Soil types range from very course sand with a particle size of up to 20 mm to clay soil with particle sizes below 0.002 mm. For our purposes we will group the soil categories into three types:


 Sandy soil  Course  Loose, single-grained; flows
 through fingers and 
 when squeezed.
 Compressed in the hand,
 it may form a ball, but
 will crumble easily.
 Loamy soil  Medium  Slightly moldable, compressed
 its form is fragile when touched.
 Moldable: when compressed
 its form can be handled
 without breaking.
 Clay soil  Fine  Forms hard clods, surface
 appears cracked with loose
 crumbs on surface
 Very modable & generally


Soil pore space is the area between particles that is available to hold water and air. Sandy soil has the least amount of pore space at approximately 35 percent of total volume, while clay soils have up to 60 percent pore space. A high percentage of pore space results in a greater soil water holding capacity. When compacted, clay soils have reduced pore space that can be as low as 25 percent of the total volume. Tilling the soil prior to planting is important to create space for needed air and water.

THE SOIL MOISTURE HOLDING CAPACITY of a soil type is defined as the amount of water that remains in the soil after the gravitational water has drained away and after the rate of downward movement has decreased to a significant extent.

Water holding capacities of soil will vary by soil type, and are generally measured in inches of water per inch of soil thickness as shown in the table below:


Per Inch of Soil
 Sandy soil  0.083
 Loamy soil  0.125
 Clay soil  0.167


If the plant root zone is 4-inches deep in a loamy soil the water holding capacity would be 0.50-inches (4 x 0.125 = 0.50), a clay soil 4-inches deep would have a water holding capacity of 0.668-inches (4 x 0.167 = 0.668).

ALLOWED SOIL WATER DEPLETION. Wilt results when plants can't retrieve enough moisture out of the soil. The difference between a full saturation and the wilting point for the plant is known as available water holding capacity and is the percentage of moisture available to the plant from the soil. This is usually 50 percent for turf grass. This is the amount of water that can be removed from the soil by the plant before irrigation should occur.

HOW MUCH & WHEN TO WATER. The turf cover as well as compaction, slope and thatch will decrease the water infiltration rate. During the first few minutes of irrigation the infiltration rate is very high at 5 to 10 inches per hour and then it rapidly decreases to very slow rates. Small fixed arc pop-up sprinkler heads with a 15 foot radius or less should run no more then 7 minutes at a time, and then the cycle repeated after 30 minutes if additional water is needed. These heads have an average application rate of 1.5 to 2-inches per hour. During the heat of the summer the maximum runtime should be about an hour per week. If you’re watering 3 days per week, apply a maximum 21 minutes per day or three 7-minute cycles. Larger rotor heads with a single stream (20 foot plus radius) will need to water about 3 times as long as sprays, or 20 minutes or more per cycle. Longer water cycles will cause runoff.

Small fixed arc pop-up sprinkler heads with a 15-foot radius or less will apply approximately .20-inches in 7 minutes or .60-inches in 21 minutes. Rotor heads will apply about .20-inches in 20 minutes or .60-inches in one hour. You can put out cans and catch the water for 7 or 20 minutes (pop-ups or rotors) to determine the actual application rate for your system.

The best time to water is between the hours of 10 PM and 6 AM because of less evaporation loss, less wind, and its better for the turf.

1. Potential soil compaction is reduced by the absence of foot traffic on wet grass.
2. The ET rate is much lower at night because it is usually cooler, there is less wind (especially in the early morning hours) and little solar radiation.
This is why at least a ten percent savings in water is achievable by watering at night.
3. Less water is wasted because of wind deflection.
4. Improves public relations by reducing the visibility of water waste.
5. Avoid watering before 10:00 P.M. to avoid fungus problems.

The average historical ET rate in Denver, Colorado for various months is as follows:


 April  .23 inches  1.0 inches
 May  .69 inches  3.0 inches
 June  1.04 inches  4.5 inches
 July  1.50 inches  6.5 inches
 August  1.39 inches  6.0 inches
 September  .92 inches  4.0 inches
 October  .23 inches  1.0 inches





PAY ATTENTION TO THE SEASONS: Plants need little or no irrigation water through the winter and an increasing amount each month till the demand peaks in July, and begins to taper off each month as the season cools. Be sure to adjust your irrigation clock for the differing water requirements each month.

Plant roots need oxygen to function and grow. A soil that is saturated most of the time is not conducive to healthy plant growth because oxygen movement in the soil is limited. In Clay soils this can be a problem and can be avoided by infrequent or ”just in time” irrigation, and in sandy soils this is not much of a problem.

OVERWATERING causes runoff & pollution, water drains below the root zone, leaches nutrients from the soil, and causes disease in the turf.


1. If you walk on the grass and can look back and see your footprints, its time to water.
2. Look at the grass with “blue blocker” sunglasses and if you see a gray tint in some areas it may be time to water.
3. Probe the soil to see if it’s dry or not.
4. Water utilizing short runtimes and 2 or 3 repeat cycles. Repeat cycles reduce runoff and




A number of years ago, a homeowner hired a contractor to install an irrigation system. Sometime after the contractor was done and paid, the homeowner called him because the system was not working. The phone was disconnected, the contractor had disappeared. After calling another contractor to look at the system, the homeowner was shocked to learn that only the controllers and a few heads were installed. The system had no pipe, valves or wire and was not connected to the water supply. This is a true story.

When I was asked to write this article about how homeowners can protect themselves from fly-by-night or unscrupulous irrigation contractors (same as a sprinkler contractor), I recalled an article my father, Lawrence Keesen wrote for the July, 1958 issue of The Green Thumb (a Denver green industry publication). I would like to quote a portion of this article titled:


“Here today - gone tomorrow is an old saying that is particularly applicable to many firms and individuals who have contracted with homeowners to install lawn sprinkler systems one day and folded up and disappeared the next - or a few days or weeks later. As a result, some homeowner’s find they paid part of their money in advanced for installations that were never completed. Others have discovered, Too late that they have paid for inferior workmanship and materials and that entire jobs must be done over. And still others, confronted with sprinkler system failure due to defective parts, find they are unable to replace or repair them with standard items.

All such cases represent losses to homeowners which could have been avoided. Since city codes frequently afford no protection to the homeowner in regard to standards to which he may refer, he is therefore "on his own" insofar as protecting himself is concerned."

After 55 years, homeowners are still facing the same problems when selecting irrigation and landscape contractors. Associations like the Associated Landscape Contractors of Colorado (ALCC) and the international Irrigation Association (IA) have developed irrigation certification programs for the design, and installation of irrigation systems. This assures homeowners that the individual has passed a test indicating a certain level of expertise.


1. How many years in business?
2. What is the education and/or work experience?
3. Is this person certified by a recognized association as an irrigation designer, installer, or technician. (ALCC or IA)
4. Obtain several references and check them out.
5. Obtain copies of certificates of insurance for liability and workman’s compensation insurance.
6. Call the Better Business Bureau.

Much of this information can be obtained from the ALCC at 303-757-5611. ALCC can provide you with referrals, and verify membership, insurance, and other information about contractors. ALCC provides this information as a public service to homeowners and property managers.



By Larry Keesen

Overwatering occurs on most turf sites around the country that we have audited and evaluated. I shudder when I think of the huge waste of this precious resource and the potential environmental damage from irrigation system runoff and damage to structures and pavement.

When contractors install a new irrigation system, the initial irrigation schedule is set for total saturation or the "rice paddy effect." Additionally, many contractors and designers fail to provide the owner with proper scheduling guidelines and techniques or the owner doesn’t take the time to find out about proper scheduling. Hopefully what follows will help you save water and have a better looking lawn.

GETTING STARTED. The first step in scheduling is to determine the application rate of the sprinkler heads for each zone throughout the irrigation system. This can be done as follows:

Place several catchment devices (cans or cups) in the landscape areas of one of the sprinkler system zones to measure the amount of water that the system applies for 15 minutes. Next measure the depth of water in inches in the can and multiply by four. This will be the application rate in inches per hour for each container. If three containers are set in one zone area, and the readings are .5", 1", and 1.5" then add the three together and divide by three and the average application rate is 1" per hour. This rate of application can then be used to calculate accurate irrigation time schedules.

If the sprinkler head application rate is 1.0 inches per hour and 0.50 inches needs to be applied, the runtime for the zone will be 30 minutes (0.50in./1.0-inches = .50 x 60 min. = 30 min.).

TURF WATER USE. Turf water use varies widely through the seasons. One way to predict plant water requirements is to use historical evapotranspiration (ET) data. Historical data is an average of up to 30 years, and is useful for predicting general monthly water requirements. The following are Weekly water requirements in inches per hour for each month during the irrigation season:

Inches per week .25" .80" 1.2" 1.7" 1.6" 1.0" .25"

Small radius pop-up spray heads (10 to 15’ spray radius) often have an application rate in excess of 1.5 inches per hour. To avoid runoff and promote more efficient irrigation, the runtimes for pop-up spray heads should be reduced to five to seven minutes with repeat cycles as needed. Most single stream rotor heads have an application rate ranging from .30 to .80 inches per hour. Avoid watering every day if possible. Once a week is enough in April and October while July may require every other day or every third day watering.

WHEN TO WATER. The best time of day for irrigation is between the hours of 9:00 P.M. and 7:00 A.M. for the following reasons:

1. Potential soil compaction is reduced by the absence of foot traffic on wet grass.
2. The ET rate is much lower at night because it is usually cooler, there is less wind (especially in the early morning hours) and little solar radiation.
3. Less water is wasted because of wind deflection.
4. Avoid watering before 9:00 P.M. to avoid fungus problems.
5. At least a ten percent savings in water is achievable by watering at night.


* If you can see your own footprints in the grass you have just walked on, you need to water!
* Use short run times and several cycle starts per irrigation period and water as infrequently as possible.
* Water between 9 PM and 7 AM to decrease water loss through evaporation.
* Pay attention to the seasons: Plants need little or no irrigation water through the winter and an increasing amount each month till the demand peaks in July, and begins to taper off each month as the season cools.