Time to flush out the drip systems and plan irrigation timing. As the soil begins to dry in the arid summer-rainless gardens, it's time to turn on the system to keep the soil as moist as it was in Spring. Like good comedy, timing is everything.
The photo shows lines of in-line drip tubing (Shown here before planting.) designed for even irrigation for the entire roots system, as described in the April 19th, 2008 blog entry.
I like to think of frequent irrigation as "topping off the tank." Starting with an ideal soil moisture, (not too anaerobically wet and not too parched) the goal is to replace, as often as each day, exactly the amount of moisture lost due to evaporation from the soil and transpiration from the plant's leaves (this amount is called the ET rate, for EvapoTranspiration), plus an amount that represents enough extra water for higher yields or more gorgeous ornamental foliage. I quote from the preeminent book on trees,
Arboriculture, by Richard W. Harris: "In contrast to other systems, drip irrigation must be frequent; waterings should occur daily or every two days during the main growing season...the amount of water applied should equal water lost through evapotranspiration."
Three studies summed up by D. A. Bucks and F. S. Nakayama and A. W. Warrick in “
Principles, Practices, and Potentialities of Trickle (Drip) Irrigation,” “Imply that the best policy is to apply water frequently as possible... assuming that no problems of soil aeration, plant disease or restricted plant rooting will occur.”
This is not to say that experience with other crops or plants wouldn’t produce different results. In a study of cucumbers and melons (“
Effects of Different Irrigation Scheduling and Systems on Yield Response on Melon and Cucumber,” P. Mannini, Boglona, Italy,
Acta Horticulturae #228 [1988], pg. 155-162) it was found that three-day versus six-day intervals of irrigation “had no effect on yield or observed parameters.” Obviously, the details need to be adjusted to the specific plant and any observed response. In another study, as presented on pg. 282 of “
Principles, Practices, and Potentialities of Trickle (Drip) Irrigation,” “Less frequent [12-, seven-, six- or three-day intervals versus daily] irrigation tended to increase yields on cabbage, cantaloupe and grapes. Shallow-rooted dry onions [Editor's note: onions' roots can grow down to three feet] had higher yields with daily than with weekly trickle irrigations.” Nature is diverse, and so should be our response.
An ET-based irrigation schedule means applying the exact same amount of water as used on, for example, a monthly basis, but in a very different pattern of application. If the daily ET requirement is .125 gallons per square foot - as described in my book with the ET chart on page 52 -
Drip Irrigation, For Every Landscape & All Climates - then a weekly watering schedule would apply seven times this amount (.875 gallons) and a monthly watering would apply 30 times this amount (3.75 gallons)—but all equal to the same daily unit of water.
Frequent Irrigation of Xeric PlantsThere are plenty of xeric California native plants which can be killed by improper irrigation, but the range of watering options for xeric plants is greater than many assume. Consider the new cultivar garden installed in 1992 at the highly respected Rancho Santa Ana Botanic Garden in Pasadena, CA, which contains an enormous number of California native plants. To maintain these specimens, the Garden purchased thousands of dollars worth of the same in-line emitter tubing I recommend in my drip irrigation book. This tubing has special pressure-compensating emitters, with a complex, or "tortuous" path which prevents clogging, preinstalled inside the 1/2-inch drip hose. In the Santa Ana garden, the tubing was laid out on a clayey soil in a conservative pattern which allocates only about two to three emitters per plant.
While they didn't irrigate daily, the system, during the first year, dispensed water twice weekly for two minutes. This is, of course, far more frequent than the common drought-resistant gardening concept of no summer watering or only occasional deep watering. According to staff horticulturist Bart O'Brien, "The growth has been tremendous." This success is due to the proper mix and placement of emitters, appropriate irrigation intervals and length of irrigation. In the second year, the irrigation was stepped back to once every five days with a staggered irrigation cycle (on for a minute, off for 1/2-hour, on for a minute, off for 1/2-hour and on for one minute). This fancy scheduling was necessary to deal with their clay soil, which quickly puddles under most schedules or types of irrigation.
“BUY FROM THE SOURCE TO HELP KEEP WRITERS WRITING”Let me know what you think.Visit my web site to learn about my new book on drip irrigation and other gardening books.NOTE: The comments section at the bottom of the post has disappeared. Click on the "___ Comments" button or the title under the "Blog Archives". Thanks, Robert
To test your soil and to understand how emitters work, buy a single emitter, use an emitter punch (which cuts a tiny circular hole) to pierce a hole near the bottom of an empty plastic one-gallon milk jug, and insert the emitter's barb. Fill the jug with water and place it on a dry spot in the garden. After the milk jug is empty (which may take 24 hours or more because it's not under pressure), dig a small trench next to the emitter to see the shape of the wet spot in your soil. This is the most graphic way to understand how the water from each emitter you install will move in your soil. (The shape of the wet spot will change, usually both wider and deeper, when the emitter is used under pressure.) The shape of the moist area is affected differently by each type of garden soil and ranges from long and carrot-like to squat and beet-shaped. Emitters with a higher rating, such as 2 or 4 gph, will cause the wet spot to be wider than 1/2 or 1 gph emitters. 
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